comparing organic lettuce production practices: crop performance and weed...

COMPARING ORGANIC LETTUCE PRODUCTION PRACTICES: CROP PERFORMANCE AND WEED CONTROL

By

YUSHEN HUANG

A THESIS PRESENTED TO THE GRADUATE SCHOOL

OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF

MASTER OF SCIENCE

UNIVERSITY OF FLORIDA

2016

To my family who always support me with their love

4

ACKNOWLEDGMENTS

My advisor Dr. Xin Zhao provided tremendous help and patience during this

journey. Beyond a superior professor, she is kind as a friend. Her attitude, enthusiasm

and dedication towards research, work and life deeply impressed me.

I want to thank both of my committee members Dr. Carlene Chase and Dr.

Jeffrey K. Brecht who offered considerable guidance for this project. The project would

have not been successful without their help and advice.

In addition I would like to thank the following Zhao lab members: Zack Black,

Jason Neumann, Caroline Hamilton, Wenjing Guan and Michelle Caibio. I would like to

thank Buck Nelson and Leonard Novinger for providing me with advice and assistance

at the Plant Science Research and Education Unit in Citra, FL.

My wife Rong Yang, my parents Kedi Huang and Yuee Li, my aunt Yaru Huang

offered me numerous support to overcome all the challenges I encountered during my

study at the University of Florida.

I would also like to thank the Horticultural Sciences Department for providing the

pleasant environment for my graduate program.

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TABLE OF CONTENTS page

ACKNOWLEDGMENTS .................................................................................................. 4

LIST OF TABLES ............................................................................................................ 7

LIST OF FIGURES .......................................................................................................... 9

LIST OF ABBREVIATIONS ........................................................................................... 10

ABSTRACT ................................................................................................................... 11

CHAPTER

1 LITERATURE REVIEW .......................................................................................... 13

Organic Crop Production Overview ........................................................................ 13

Organic Crop Production Practices ......................................................................... 14

Weed Management ................................................................................................ 19

Lettuce Quality ........................................................................................................ 23

Objectives and Hypotheses .................................................................................... 24

2 EFFECTS OF DIFFERENT ORGANIC SYSTEMS ON LETTUCE PERFORMANCE .................................................................................................... 26

Introduction ............................................................................................................. 26

Materials and Methods............................................................................................ 29

Plant Materials and Research Site ................................................................... 29

Experimental Design ........................................................................................ 29

Cover Crops and Field Preparation .................................................................. 30

Lettuce Transplant Production .......................................................................... 31

Field Planting, Harvest, and Data Collection .................................................... 31

Statistical Analysis ............................................................................................ 32

Results and Discussion........................................................................................... 32

Lettuce Yield ..................................................................................................... 32

Lettuce Aboveground Nutrient Concentrations and Accumulation ................... 35

Conclusions ............................................................................................................ 38

3 EFFICACY OF CULTRUAL AND PHYSICAL WEED MANAGEMENT IN ORGANIC LETTUCE PRODUCTION ..................................................................... 50

Introduction ............................................................................................................. 50


Experimental Design and Field Trial Establishment ......................................... 53

Weed Aboveground Coverage and Biomass .................................................... 54


6


Weed Community in the Experimental Field..................................................... 55

Annual Grasses ................................................................................................ 56

Perennial Grasses ............................................................................................ 57

Broadleaf Weeds .............................................................................................. 58

Nutsedge .......................................................................................................... 60

Conclusions ............................................................................................................ 61

4 THE INFLUENCE OF DIFFERENT ORGANIC PRODUCTION SYSTEMS ON THE LEVELS OF ASCORBIC ACID, TOTAL PHENOLICS, AND CHLOROPHYLL IN LETTUCE ............................................................................... 73

Introduction ............................................................................................................. 73


Field Experimental Design ................................................................................ 75

Plant Materials and Determination of Levels of Ascorbic Acid, Total Phenolics, and Chlorophyll ............................................................................ 76



Ascorbic Acid Content ...................................................................................... 78

Total Phenolic Content ..................................................................................... 79

Chlorophyll Content .......................................................................................... 80

Conclusions ............................................................................................................ 81

5 SUMMARY ............................................................................................................. 91

LIST OF REFERENCES ............................................................................................... 93

BIOGRAPHICAL SKETCH .......................................................................................... 108

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LIST OF TABLES

Table page 2-1 Effects of cultivar (C) and production system (P) on lettuce yield, dry matter

content, nitrogen, phosphorus, sulfur, potassium, calcium, and magnesium concentrations in the 2011 and 2012 field trials in Citra, FL ............................... 40

2-2 Impact of different management practices on lettuce marketable yield (t ha-1) ... 41

2-3 Impact of different management practices on lettuce aboveground dry weight (g/head) .............................................................................................................. 42

2-4 Impact of different management practices on lettuce aboveground dry matter content (%) ......................................................................................................... 43

2-5 Lettuce aboveground dry weight (g/head) in 2011 and 2012 trials ..................... 44

2-6 Lettuce aboveground dry matter content (%) in 2011 and 2012 trials ................ 45

2-7 Lettuce macronutrient concentrations (g/kg) in different production system treatments in the 2011 and 2012 trials in Citra, FL ............................................. 46

2-8 Macronutrient concentrations (g/kg) in different lettuce cultivars ........................ 47

2-9 Impact of different production systems on accumulation of macronutrients in lettuce (mg/head) ................................................................................................ 48

2-10 Accumulation of macronutrients in different lettuce cultivars (mg/head) ............. 49

3-1 Impact of different organic production systems on annual grass biomass and weed aboveground coverage in the 2011 field trial ............................................ 65

3-2 Impact of different organic production systems on annual grass biomass and weed aboveground coverage in the 2012 field trial ............................................ 66

3-3 Impact of different organic production systems on perennial grass biomass and weed aboveground coverage in the 2011 field trial ..................................... 67

3-4 Impact of different organic production systems on perennial grass biomass and weed aboveground coverage in the 2012 field trial ..................................... 68

3-5 Impact of different organic production systems on broadleaf weeds biomass and weed aboveground coverage in the 2011 field trial ..................................... 69

3-6 Impact of different organic production systems on annual broadleaf weeds biomass and weed aboveground coverage in the 2012 field trial ....................... 70

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3-7 Impact of different organic production systems on nutsedge biomass and weed aboveground coverage in the 2011 field trial ............................................ 71

3-8 Impact of different organic production systems on nutsedge biomass and weed aboveground coverage in the 2012 field trial ............................................ 72

4-1 Impact of different organic production systems on lettuce chlorophyll content (μg/mg FW) in the 2011 trial ............................................................................... 89

4-2 Impact of different organic production systems on lettuce chlorophyll content (μg/mg FW) in the 2012 trial ............................................................................... 90

9

LIST OF FIGURES

Figure page 2-1 Temperature and precipitation during lettuce production period in 2011 and

2012. .................................................................................................................. 39

3-1 Weed community in 2011. .................................................................................. 63

3-2 Weed community in 2012. .................................................................................. 64

4-1 Effects of different organic production systems on lettuce ascorbic acid content in the 2011 trial. ..................................................................................... 82

4-2 Ascorbic acid content in different lettuce cultivars in the 2011 trial. .................... 83

4-3 Effects of different organic production systems on lettuce ascorbic acid content (mg/100g FW) in the 2012 trial. ............................................................. 84

4-4 Effects of different organic production systems on lettuce total phenolic content in the 2011 trial. ..................................................................................... 85

4-5 Total phenolic content in different lettuce cultivars in the 2011 trial. ................... 86

4-6 Effects of different organic production systems on lettuce total phenolic content in the 2012 trial. ..................................................................................... 87

4-7 Total phenolic content in different lettuce cultivars in the 2012 trial. ................... 88

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LIST OF ABBREVIATIONS

AA Ascorbic acid

BMP Best management practices

CC Cover crop

CC-C-CT Treatment with cover crop and conservation tillage

CC-CT Treatment with cover crop, conservation tillage, and higher organic fertilizer input

CC-T Treatment with cover crop and conventional tillage but without polyethylene mulch

CC-T-M Treatment with cover crop, conventional tillage, and polyethylene mulch

DNPH Dinitrophenylhydrazine

GAE Gallic acid equivalent

NCC-T Treatment without cover crop and polyethylene mulch but with conventional tillage

NCC-T-M Treatment without cover crop but with conventional tillage and polyethylene mulch

PSREU Plant Science Research and Education Unit

SH

SS

Sunn hemp

Sorghum-sudangrass

TKN

UV

Total Kjeldahl nitrogen

Ultraviolet

VWC

WAT

Volumetric water content

Week after transplanting

11

Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the

Requirements for the Degree of Master of Science

COMPARING ORGANIC LETTUCE PRODUCTION PRACTICES: CROP PERFORMANCE AND WEED CONTROL

By

Yushen Huang

December 2016

Chair: Xin Zhao Major: Horticultural Sciences

Organic vegetable production systems can vary considerably depending upon

the crops, soil and climatic conditions, farming inputs, and geographic regions. The

impact of different management practices on crop performance and quality is also

affected by site-specific conditions. A two-year field study was conducted at the

University of Florida Plant Science Research and Education Unit in Citra, FL to assess

the effect of summer cover crops, conservation tillage and polyethylene mulch on

organic lettuce performance and weed suppression in fall 2011 and fall 2012. Sunn

hemp and sorghum-sudangrass were planted as a biculture in Aug. and terminated in

Oct., 15 days prior to lettuce transplanting. Six production systems were evaluated

including: 1) incorporated cover crops followed by lettuce on raised beds with

polyethylene mulch, 2) incorporated cover crops followed by lettuce on raised beds

without polyethylene mulch, 3) no cover crops, lettuce on raised beds with polyethylene

mulch, 4) no cover crops, lettuce on raised beds without polyethylene mulch, 5)

conservation tillage with cover crops retained as organic mulch and 6) conservation

tillage with cover crops retained as organic mulch and higher organic fertilizer input.

Production systems were arranged in a randomized complete block design with 4

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replications. Two loose leaf lettuce cultivars, ‘Tropicana’ and ‘New Red Fire’ were

transplanted in Nov. and harvested after 7 weeks. Overall, lettuce grown with

polyethylene mulch had higher yields than in systems without polyethylene mulch.

Conservation tillage management methods need to be improved to enhance soil fertility

management and crop yield. Weed density and biomass assessment results showed

the effectiveness of using summer cover crops in nutsedge management. Weeds were

most effectively controlled by the combination of black polyethylene mulch and bicultural

cover crops of sunn hemp and sorghum-sudangrass. Conservation tillage with cover

crop treatments could be used to effectively control broadleaf weeds and nutsedge. The

postharvest results showed that lettuce ascorbic acid and total phenolic contents were

significantly higher in the conservation tillage treatments as compared with the

treatments with black polyethylene mulch in the 2011 trial but not in 2012. Production

system did not affect lettuce chlorophyll content. The red leaf lettuce ‘New Red Fire’

consistently exhibited higher levels of total phenolics than the green leaf lettuce

‘Tropicana’.

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CHAPTER 1 LITERATURE REVIEW

Organic Crop Production Overview

Organic production systems that utilize practices to minimize off-farm inputs,

enhance biodiversity, and promote soil biological activity have been emphasized for

several decades. Organically Produced Commodities in the United States has been

growing rapidly, reaching sales of $6.2 billion in 2015, an 12.7 % increase compared

with 2014 and a 100% increase compared with 2008 (USDA, 2010; 2015; 2016).

Even with the steady growth of the organic industry, the demand for organic

vegetables still exceeds supply (Badgley, 2007; Jensen and Zanoli, 2011). With the

increasing consumer demand for organic food and the higher profits and greater market

opportunities for the organic industry, more producers are adopting organic farming

(Dimitri et al., 2002; Smukler et al., 2008). This trend, which resulted in total national

acreage of certified organic farms increasing from 3.67 million acres in 2014 to 4.36

million acres in 2015 (USDA, 2015; 2016), has not occurred in Florida, where the total

acreage of certified organic farms dropping from 19,364 in 2014 to 12,757 in 2015

(USDA, 2015; 2016). This may be because, compared with conventional agriculture,

organic agriculture is more susceptible to weeds and nutrient deficiencies (Akinyemi,

2007). To reverse the stagnation in the development of organic vegetable farming in

Florida, site-specific management strategies are needed in this region.

In organic agricultural systems, practices such as conservation tillage, cover

crops, animal manure, compost and mulch inputs are generally used to enhance fertility

and to manage weeds (Bengtsson et al., 2005; Gomiero et al., 2011; Lockeretz et al.,

2007). Different soil types, environmental conditions, and the selection of inputs largely

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impact management practices (Gomiero et al., 2011). The situation in Florida is

challenging in terms of weed management and maintenance of soil fertility. This is partly

due to Florida’s sandy soils, consistently high year-round temperatures, and frequent

rainfall that may promote soil erosion. These issues lead to lower soil organic matter

content and nutrient leaching (Crews et al., 2004). Increased public concern about

environmental contamination (such as eutrophication) increases demand for an

ecologically friendly management system specifically for Florida organic agriculture

(Liebman and Davis, 2000). As a result, growers in Florida are seeking an efficient and

integrated approach to soil, nutrient, water, and other input management in organic

farming.

Organic Crop Production Practices

Growing concerns about the negative impact of high input conventional

agriculture on human health and the environment have contributed to the development

of organic vegetable production (Liebman et al., 2000; Worthington et al., 1998).

Growers who rely on synthetic chemical fertilizer and other intensive conventional

practices are wary about the performance of organic vegetable production. Hence, there

is an increasing demand for information on the actual performance of organically

produced vegetables (Gomiero et al., 2011).

Previous studies have yielded contradictory results in terms of crop yield under

production, with some indicating that organic production practices may lower crop yields

(Giller et al., 2011; Pang et al., 2000; Pfiffner et al., 1992; Ponti et al., 2012; Seufert et

al., 2012), while others concluding that organic production could match the yields from

conventional production (Colla et al., 2000; Larson et al., 2000; Pimentel et al., 2005;

Poudel et al., 2002; Seufert et al., 2012). Thus it appears that the crop yields vary

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widely based on different characteristics of organic production systems. It is possible

that integrated farming practices can be adopted to enhance vegetable yield and

improve soil quality given the site-specific conditions.

Among the common practices, the use of cover crops, conservation tillage, and

polyethylene mulch appear to contribute to effective organic vegetable production

systems.

Cover crops are crops planted primarily to manage soil fertility, enhance soil

quality, conserve soil water, suppress weeds, control pests, and increase biodiversity

and wildlife in agroecosystems (Lu et al., 2000; Munawar et al., 1990; Reicosky et al.,

1998; Wayman et al., 2015). Cover crops offer different benefits depending on which

crops are planted. For instance, cover crops in the Fabaceae family (legumes) are

chosen to improve soil fertility due to their symbiotic nitrogen fixation ability (Hartwig et

al., 2002). Utilizing nitrogen-fixation capacity will improve soil nitrogen fertility and

reduce the need for extra N fertilizer. Cover crops in the Poaceae family (grasses) are

chosen to increase onsite biomass in order to increase soil organic matter after

decomposition and to maintain soil surface moisture when retained as mulch (Mirsky et

al., 2012). Similarly, non-legume cover crops can promote soil fertility by serving as

green manure (LaRue et al., 1981). Before cash crops are planted, living cover crops

can significantly reduce soil erosion (Hartwig et al., 2002). During the cash crop season,

either living mulch or cover crop residues help maintain and preserve the soil. This

benefit is important for growers in Florida due to the sandy soil which is susceptible to

erosion (water- or wind-borne), and water runoff (Hall et al., 1984). With less surface

water runoff, nutrient loss is reduced while improving soil fertility. Meanwhile,

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environmental pollutants are also reduced, thus increasing long-term ecosystem

benefits (Altieri et al., 1999).

Because the potential benefit for a specific cover crop is relatively unique,

growers and researchers tend to use mixed cover crop systems rather than

monoculture systems. Multiple cover crops can offer impressive benefits to the organic

production system. The non-legume cover crop system could stimulate the process of

nitrogen fixation (Creamer et al., 1997) while the mixed cover crop system could adjust

the C:N ratio which is favorable to fertility mineralization (Power et al., 1988). It should

be noted that the use of multiple cover crops requires extra management and increases

the requirement for resource-use efficiency (Tilman et al., 1997). Meanwhile, there is

competition among different cover crops for abiotic factors such as light, water, and

nutrients. Some species may induce unexpected results in production systems

depending on the location and other abiotic and biotic factors. For example, hairy vetch

and crimson clovers may encourage root-knot nematodes, which can be problematic in

sandy soils (McSorley and Dickson 1989). In organic production systems, cover crops

are always terminated mechanically, with different methods and different termination

times impacting mineralization in the field (Gaskell et al., 2007; Schomberg et al., 2002;

Snapp et al., 2005). Due to high temperatures and humidity, terminating cover crops

with roller–crimper is used in conservation tillage organic production systems in the

southeastern United States (Reberg-Horton et al., 2012). Given the important role of

cover crops in organic systems, selecting the best management strategies of cover

crops need careful considerations.

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In organic production systems, conventional tillage induces quick release of

nutrients which increases the loss of soil nutrients and lowers the efficiency of soil C

and N utilization as compared with the conservation tillage (Mirsky et al., 2012; West et

al., 2002). Another difference between conventional tillage and conservation tillage or

no-tillage is related to soil organic matter (Wander et al., 1998). Based on soil texture,

there are different loss rates for soil organic matter, with the loss of soil organic matter

being a principal problem in sandy soils (Hassink et al., 1995). The disadvantage of

conventional tillage is significant for sandy soil areas because soil disturbance caused

by tillage is a major source of the reduction of soil aggregates in sandy soils (Six et al.,

1999). Long-term research has indicated that reduced tillage systems can help preserve

and promote soil organic matter due to physical homogenization, and soil organic

matter has much slower cycling in reduced tillage systems (Chivenge et al., 2007).

Previous research has also demonstrated that while conservation tillage systems

preserve a much higher level of soil organic matter than conventional tillage systems

(Six et al., 1999), they may cause greater crop yield reductions (Franzluebbers et al.,

2004). One of the reasons for the higher crop yield reduction is that plant pathogens can

survive in reduced tillage systems over the season, which is not a problem for

conventional tillage systems (Gallandt et al., 2004; Weller et al., 1986). As stated before,

some legume cover crops are ideal hosts for root-knot nematodes. Without sufficient

tillage, the possibility of the nematodes surviving the winter is higher, which may affect

the next cash crop, induce yield loss or lower crop performance. Research shows that

the impact of conservation tillage systems on crop yield is still uncertain whereas some

research indicates there is no significant difference between conservation tillage

18

systems and conventional tillage systems in terms of crop yield (Delate et al., 2012;

Maltas et al., 2013; Mupangwa et al., 2012; Mitchell et al., 2015;), other studies have

shown that conservation tillage systems can result in crop yield reductions (Alliaume et

al., 2014; Arvidsson et al., 2014; Cooper et al., 2014; Zhao et al., 2013). Thus, the

influence of conservation tillage systems on vegetable crop yields still need to be further

evaluated.

Polyethylene mulch is known to promote vegetable production by altering the

microclimate of vegetables, thereby affecting plant growth and yield (Cooper et al., 1973;

Jenni et al., 2002; Moreno et al., 2008). Polyethylene mulch acts as a barrier between

the soil and the atmosphere which modifies the microclimate around vegetables. Using

solar radiation as the source of heating the soil, polyethylene mulch could increase the

soil temperatures under the mulch by absorbing heat from solar radiation and decrease

heat transfer by lowering soil evaporation (Ham et al., 1993; Tindall et al., 1991). The

microclimate around the mulched vegetables is thus altered by changing the

temperature and the fluctuations around the root zone (Ham et al., 1993) which

increases nutrient availability and uptake (Liu et al., 2003), leading to favorable root and

foliage growth. In addition, mulch can effectively preserve soil moisture by reducing

evaporation (Liakatas et al., 1986). Although the soil surface is constantly exchanging

water with the atmosphere, the mulch barrier could lower the exchange rate to preserve

the soil moisture to the benefit of mulched crops. Polyethylene mulch is critical for

production systems utilizing drip irrigation because it reduces the evaporation from the

wetted surface and increases the efficiency of irrigation. For vegetables,

evapotranspiration is a major factor that affects crop yield, with the linear relationship

19

between evapotranspiration and crop yield being well-established (Zhang et al., 2004).

Besides the benefits that polyethylene mulch offers to soil properties, it also provides an

environment that favors microbial populations that influence soil fertility (Hankin et al.,

1982; Marinari et al., 2007). Less irrigation demand, less leaching, and less nutrient loss

occur with polyethylene mulch which would promote crop production. Moreover, it also

protects the soil from rainfall and wind erosion. Mulching is also an effective method for

weed management by providing shade effects and better nutrient management thus

significantly affecting vegetable performance (Chandra et al., 2015; Huang et al., 2005;

Liu et al., 2003; Singh et al., 2012).

Conversely, polyethylene is considered to be an environmental pollutant if

incorrect disposal is conducted onsite (Briassoulis et al., 2006; Halley et al., 2001). In

organic crop production, the polyethylene mulch needs to be removed from the

production field at the end of the production season. The removal and installation of

polyethylene mulch is expensive considering the costs for labor, machinery, materials

(McCraw et al., 1991). Additional costs may occur because polyethylene mulching is

often combined with drip irrigation systems, and raised vegetable beds to ensure a

higher mulching efficiency.

Weed Management

Weeds are widely acknowledged to be a key constraint for organic agriculture

production. Furthermore, the absence of synthetic herbicides and fumigation makes the

situation even tougher. Weeds can cause severe yield loss if managed inadequately

(Clark et al., 1999; Hutchinson et al., 2000). Due to the resiliency of weeds, weed

management is important for organic vegetable production where the weed control

measures may be limited (Baker et al., 1983; Willer et al., 1999). Research on weed

20

management in organic agriculture is still limited, with less than 20 papers published

from 1995 to 2000 (Bàrberi et al., 2002).

Weeds can negatively affect vegetable production in different ways such as

competition for nutrients and water and for space at different stages. For example,

vegetables are susceptible to weed infestation due to their early growth pattern and

smaller canopy (Kropff et al., 2000). A wide range of weed management practices from

pre- to post-emergence is available to producers. Most weed control methods used in

organic agriculture are labor intensive and greatly affect profit margins (Clements et al.,

1995). For areas with high temperatures and high humidity like Florida, weeds tend to

be a more crucial factor for crop production. With the limited access to herbicides in

organic agriculture, approaches to weed management, especially in places like Florida,

can be expensive. Common methods against weeds besides hand weeding include

cover crop systems, tillage, and mulching.

Cover crops have been adopted as a common practice by many producers. With

respect to weed control, cover crops can be planted before cash crops as an indirect

approach to weed management by suppressing the weed seed bank as well as weed

emergence and weed seed germination (Teasdale et al., 1993). Rapidly growing cover

crops produce a considerable amount of biomass, which contributes to the resource

competition with weeds. For instance, the cover crop works as living mulch to reduce

the sun light that weeds receive, modify soil property to suppress weeds, and capture

nutrients (Gallandt et al., 2006; Teasdale et al., 1996). Cover crops help improve soil

fertility and structure (Lu, 2000), and provide habitats for weed seed predators before

their termination which assists with weed management (Sullivan et al., 2001).

21

Besides these factors, cover crops also provide chemical effects (allelopathy)

which suppress weed emergence, growth, and development (Liebman et al., 2000).

When cover crops are terminated, decomposing residues release allelochemicals to

further impact weeds (Liebman et al., 2000). Some cover crops in the Brassicaceae

family contain glucosinolates which have a phytotoxic effect that is useful for

biofumigation in weed control (Haramoto et al., 2004). In addition, cover crops are

successfully used in weed management via conservation tillage systems (Mäder et al.,

2012; Mirsky et al., 2012)

However, there are limitations for using cover crops. For example, perennial

weed control is relatively unsuccessful using cover crops. The cost of cover crop seeds,

additional labor, machinery, and the management of cover crops before and after

termination are expensive for growers. Also, the allelopathy effect introduced by cover

crops could potentially inhibit the cash crops grown following the cover crops harvested

(Singh et al., 1996). Thus a better system to optimize the benefits and minimize the

limitations of cover crops needs to be developed.

Tillage is the direct manipulation of soil to remove weeds, redistribute soil, and

aid irrigation systems to provide a favorable environment for the growth of cash crops,

especially in conventional agriculture (McKyes et al., 1985). However, tillage may lead

to some unfavorable outcomes like loss of moisture and organic matter, soil erosion,

soil compaction, degradation of soil aggregates, and death or disruption of soil microbes

and other organisms (Hamblin et al., 1987; Baker et al., 1983, Dieckow et al., 2009). In

terms of weed management, frequent tillage may introduce factors that help with weed

seed propagation by bringing the buried, dormant weed seeds to the soil surface, thus

https://scholar.google.com/citations?user=pJeyRicAAAAJ&hl=zh-CN&oi=sra

22

exposing them to oxygen content and providing light stimuli to facilitate germination

(Mulugeta et al., 1997; Yenish et al., 1992). Organic matter plays a critical role in soil

fertility management. Organic matter releases nutrients at a slower rate which controls

weed growth by limiting available nutrients. This is due to the fact that at the early stage

of cash crop growth, weeds are far more competitive and more efficient than the cash

crops in terms of nutrient uptake (Liebman et al., 2000). Thus intensive tillage which

rapidly breaks down the organic matter may have a negative effect on weed control.

Conservation tillage practices decelerate the speed at which nutrients are leached from

soil organic matter to conserve soil organic matter, reduce mineralization, and suppress

weeds (Benech-Arnold et al., 2000).

Conservation tillage is good for organic agriculture because it balances soil

modification, soil preservation, and weed management. However, the efficacy on weed

suppression is uncertain. On the one hand, annual weeds are suppressed by

conservation tillage which keeps weed seeds deep in the soil as opposed to

conventional tillage which can move the weed seeds to the soil surface (Fawcett et al.,

1987). On the other hand, conservation tillage may exacerbate a shift toward perennial

weed species (Buhler et al., 1994; Légère et al., 2011). Still, the effect of tillage is

strongly influenced by the site-specific practices employed. For example, some

research found that conservation tillage systems could efficiently manage onsite weeds

in Washington (Collins et al., 2011), while other research indicated that conservation

tillage systems failed to control weeds (Kainz et al., 2005; Mitchell et al., 2015). Hence,

the effects of conservation tillage on weed management in organic production systems

need further study.

23

Another method for weed control is the use of mulch. Mulching, natural or

synthetic, is widely used worldwide as an efficient method to suppress weeds (Bond et

al., 2003; Gupta et al., 1991). Mulching suppresses weeds by blocking

photosynthetically active radiation to inhibit weed germination and growth. Furthermore,

because soil moisture and temperature are the two key factors that influence weed seed

germination, mulch is used in weed management to conserve soil moisture and modify

soil temperature which suppresses weeds (Edwards et al., 2000).

Organic mulch has several disadvantages compared to synthetic mulch. Light

organic material could be unevenly distributed by wind or water; seeds in the mulch

could be another source of weed infestation; moreover, the transport cost to employ a

well-established organic mulch system is not negligible (Merwin et al., 1995).

As for synthetic mulch, some research has found that black polyethylene mulch

controls weeds better than other types of mulch (Marks et al., 1993). However, there are

some limitations of polyethylene mulch. Purple nutsedge (Cyperus rotundus) and yellow

nutsedge (Cyperus esculentus), perennial weeds, have the ability to grow through

polyethylene mulch (Bangarwa et al., 2014). At the end of the cropping season, the

removal of polyethylene mulch might be a problem both environmentally and

economically. The effectiveness of using polyethylene mulch for weed management is

inconsistent and varies by site.

Lettuce Quality

Besides the postharvest storage conditions and harvest techniques, quality of

vegetables are influenced by several factors such as microorganisms and cultivation

methods (Li et al., 2001; Liu et al., 2006). The growth of microorganisms such as the

one causing microbial decay may have different localized microclimate due to different

24

cultivation practices which can induce abnormal microbial behavior (Ponce et al., 2008;

Zagory et al., 1999). For instance, polyethylene mulch could directly affect microflora by

increasing and maintaining soil moisture and soil temperature (Li et al., 2004; Verdial et

al., 2001). Optimal soil temperature and moisture provide a more stable microclimate for

microorganisms to grow. Different practices may also influence the beneficial

compounds in crops, even though the results have been inconclusive. For example,

while some research has found that the application of organic fertilizer could increase

the total chlorophyll content, other research has indicated the exact opposite

(Amujoyegbe et al., 2007; Ouda et al., 2008). The antioxidant content in vegetables can

also be affected by processing and storage methods (Kalt et al., 1999). Secondary plant

metabolites such as phenolics may play an important role in promoting human health.

Some research demonstrates that organic vegetables have higher concentrations of

total phenolics and ascorbic acid compared with conventionally produced vegetables

(Asami et al., 2003). However, few reports are available on research related to organic

practices such as cover crops and conservation tillage systems and their effect on

quality attributes of vegetables in organic production systems.

Objectives and Hypotheses

Focused on organic lettuce (Lactuca sativa) production, this project was aimed at

developing site-specific organic vegetable production systems for growers in Florida,

especially in north Florida, that integrate multiple organic management practices.

The first objective was to assess the effects of organic production systems using

cover crops, polyethylene mulch, and conservation tillage on lettuce performance such

as yield, dry matter, and mineral content. All three practices have been reported to

enhance vegetable yield by modifying the microclimate of the planted crop. The black

25

polyethylene mulch-cover cropping system was expected to have the most positive

effect on lettuce growth and yield.

The second objective was to compare the efficacy of different systems on weed

control. As common weed control practices, the effect of polyethylene mulch, cover crop

biculture, and conservation tillage on weed control may vary by site. It is important to

evaluate whether each practice is individually necessary in a multiple-practice adopted

organic production system in terms of yield difference and weed control effectiveness. It

is hypothesized that a combined polyethylene mulching and cover crop bicultural

system will provide the most effective weed management. It is also expected that the

conservation tillage system will provide as an acceptable level of weed control.

The third objective was to determine whether cover crop biculture, black

polyethylene mulch, and conservation tillage could enhance antioxidant compounds in

organically-grown lettuce. As previous research has indicated, different levels of soil

fertility and other biotic and abiotic factors may lead to differences in phytochemical

content. Therefore, soil fertility management methods, cover crops, and conservation

tillage might exhibit impacts on ascorbic acid and total phenolic contents of lettuce

under organic production. It is hypothesized that the organic production systems

examined in this study will show different effects on lettuce ascorbic acid and total

phenolic contents.

26

CHAPTER 2 EFFECTS OF DIFFERENT ORGANIC SYSTEMS ON LETTUCE PERFORMANCE

Introduction

Organic farming as an alternative to conventional farming is one of the most

rapidly growing segments of the U.S. agriculture. The total sales of organic foods have

more than doubled between 2008 and 2014 (USDA, 2010; 2015). Following California,

Florida ranked second nationwide in total vegetable harvested area, production quantity,

and production value in 2015 (USDA, 2016). While based on the USDA report, Florida

has stagnated its organic production acreage since 2008 (2,566 acres in 2008, 2,334

acres in 2011, 2,376 acres in 2014, and 2589 acres in 2015, respectively) (USDA, 2010;

2012; 2015; 2016), Florida ranked 5th in organic lettuce (Lactuca sativa) production in

acreage nationwide in 2015 (USDA, 2016). Lettuce is a leading vegetable crop in

organic production, and the sales of organic lettuce reached $262 million in 2015 in the

U.S. (USDA, 2016). Due to the subtropical and tropical climate in Florida, lettuce

production in the state is limited to the period between October and May (Lu, 2011).

According to the USDA 2014 Organic Survey, green manure, organic

mulch/compost, and no-till or minimum till cropping were among the production

practices used by over 40% of organic farms surveyed (USDA, 2015). In Florida, many

organic growers have integrated cover cropping into their crop management systems,

and some of them tried or adopted conservation tillage for specific crops. Organic mulch

(e.g., hay) and polyethylene (plastic) mulch are commonly used for weed suppression

by organic vegetable growers.

Cover crops are crops that have been planted for many millennia to manage soil

fertility (Gaskell et al., 2007), enhance soil quality (Jagadamma et al., 2008), conserve

27

soil water (Utomo et al., 1990), suppress weeds (Creamer et al., 1996; Vollmer et al.,

2010), control pests (Carrera et al., 2004), and increase biodiversity and wildlife in

agroecosystems (Lu et al., 2000). Cover crops are planted between regular cash crop

seasons and may affect nutrient availability in subsequent cash crops (Cookson et al.,

1998). Residue decomposition modeling has been useful for establishing patterns of

nutrient release and plant demand (Palm et al., 1997). Carefully scheduled cover crops

can enhance N uptake and increase the yield of cash crops (Thorup-Kristensen et al.,

2006). Cover cropping is also an essential practice used in organic conservation tillage

systems to control weeds.

Bicultural cover crop systems combine one green manure crop, usually

leguminous, with one catch crop, commonly a fast-growing annual cereal (Snapp et al.,

2005). Choosing proper cover crops varies from field to field based on the carbon-to-

nitrogen (C:N) and nutrient availability in the field (Masiunas et al., 1998). The most

widely-known and extensively researched organic conservation tillage systems are

those based on hardy winter annual cover crops, mostly combinations of cereal grain

rye and hairy vetch (Kuo et al., 1997; Rosecrance et al., 2000). Sunn hemp (Crotalaria

juncea) and sorghum-sudangrass (Sorghum bicolor x S. bicolor var. sudanense) grow

well in sandy soils and can accumulate large amount of biomass, and both are widely

used in the southern states as summer cover crops (Wang et al., 2003).

Tillage as a practice to eradicate weeds and redistribute soil for crops has been

utilized by humans for thousands of years (Lal et al., 2007). However, tillage sometimes

leads to unfavorable outcomes like loss of water and organic matter, soil erosion, soil

compaction, degradation of soil aggregates, and the death or disruption of soil microbes

28

and other organisms (Baker et al., 1983; Dieckow et al., 2009; Hamblin et al., 1987).

The conservation tillage system, where at least 30% of the plant residues are preserved

on the soil surface at the time when the cash crop is grown minimizes negative

outcomes (Hoyt et al. 1994). This reduced-energy input system can help retain

favorable soil property with low soil disturbance and offers great coverage to maintain

soil moisture and temperature, increase soil organic matter, and reduce soil erosion,

while potentially improving weed management in organic production. However, the

effects of conservation tillage on organic vegetable performance may vary greatly with

site-specific conditions.

Mulch is commonly used in agriculture systems to retain soil moisture, warm the

soil underneath, and suppress weeds (Greer and Dole, 2003). Mulch can effectively

preserve soil moisture loss to evaporation by acting as a barrier between the

atmosphere and soil and increases water-use efficiency (Liakatas et al., 1986). Despite

the disposal and environmental concerns, polyethylene mulch is still widely used by

many organic vegetable growers. Black polyethylene mulch can absorb more solar

radiation than any other colored synthetic mulch (Kelly et al., 1995). The heat retention

of black polyethylene mulch is favorable for vegetable growing because it increases

nutrient availability (Gough, 2001; Liu et al., 2009). Organic mulch has less capacity for

heating the soil and temperature retention (Teasdale, 1996), but it is preferred over

synthetic mulch for increasing soil organic matter content via decomposition, an

advantage synthetic mulch could not offer. Organic mulch can be grown as a cover crop

prior to the cash crop thus utilizing the many benefits of cover crops, another advantage

over synthetic mulch application. Organic mulch can be used in the form of a living

29

mulch; however, living mulches can be difficult to manage in vegetable cropping

systems because of their competitiveness with crops for sunlight and water (Masiunas

et al., 1998).

This 2-year study involved the use of different organic production practices in

lettuce production including cover cropping, conservation tillage, and the use of

polyethylene mulch. The main objective was to examine the effects of different organic

production systems on crop yield and plant nutrient uptake.

Materials and Methods

Plant Materials and Research Site

The lettuce (Lactuca sativa L.) evaluated in this study consisted of two cultivars,

organic green loose leaf lettuce ‘Tropicana’ and organic red loose leaf lettuce ‘New Red

Fire’ (Johnny’s Selected Seeds, Winslow, ME). The organic lettuce trials were

conducted at the University of Florida Plant Science Research and Education Unit

(PSREU) in Citra, Florida, in 2011 and 2012. The soil texture at the site is loamy sand.

Experimental Design

Six organic production system treatments, consisting of different management

practices related to cover cropping, tillage, and mulching were tested. The six

treatments were as follows:

1. No cover crop + conventional tillage + polyethylene mulch = NCC-T-M

2. No cover crops + conventional tillage + no mulch = NCC -T

3. Cover crop + conventional tillage + polyethylene mulch = CC-T-M

4. Cover crop + conventional tillage + no mulch = CC-T

5. Cover crop + conservation tillage = CC-C-CT

6. Cover crop + conservation tillage + high organic fertilizer input = CC-CT

30

The production systems were arranged in a split plot design with four replications.

The whole plot treatments were six production systems, and the subplot treatments

were two loose leaf lettuce cultivars. There were 48 plants of each cultivar in each of the

production system plots per replication. The subplot size was 6.1 m x 2.3 m. The

production system treatment plots were maintained in the same locations during this

two-year study.

Cover Crops and Field Preparation

For the plots with the cover crop treatment, cover crops were dill-seeded for

establishment. A roller chopper was used to terminate cover crops and the cover crop

residues were left as an organic mulch in the conservation tillage treatments, while the

cover crop residues were tilled into the soil in the conventional tillage treatments.

A cover crop biculture of sunn hemp (Kauffman Seeds, Haven, KS) and

sorghum-sudangrass (High Mowing Organic Seeds, Wolcott, VT) was used in the

treatments with cover crops planted. The sunn hemp seeds were mixed with Guard-N

Seed Inoculant (NE Seed Growers, Hartford, CT) before planting. Cover crops were

drill-seeded at a rate of 33.5 kg/ha for sunn hemp and 22.4 kg/ha for sorghum-sudan

grass on 8 Aug. 2011 and 30 July 2012, respectively. Cover crops were terminated by

roller chopper on 14 Oct. 2011 and 19 Oct. 2012. After termination, all the plots with

conventional tillage were tilled twice for bed formation (24 and 26 Oct. 2011 and 23 and

26 Oct. 2012, respectively).

Prior to lettuce transplanting, two types of organic fertilizer products were applied

to each production system plot, including 10N-0.9P-6.6K NatureSafe organic fertilizer

(Darling Ingredients Irving, TX) and 3N-0.9P-2.5K processed poultry manure (Perdue

AgriRecycle, Seaford, DE). Each organic fertilizer was applied at the N application rate

31

of 84 kg/ha, assuming 60% of N availability during the lettuce production season (Hartz

and Johnstone, 2006). Specifically, 8.5 kg of processed poultry manure and 2.5 kg of

NatureSafe organic fertilizer were applied to each of the production system treatment

plots with cover crops, while 9.8 kg of processed poultry manure and 3.0 kg of

NatureSafe organic fertilizer were applied to each of the production system treatment

plots without cover crops. In addition, in the conservation tillage treatment receiving high

organic fertilizer input, 16.9 kg of processed poultry manure and 2.5 kg of NatureSafe

organic fertilizer were applied. Following organic fertilizer application, raised planting

beds, 76.2 cm wide and 22.9 cm high, were made in the conventional tillage treatments,

while flat beds were used in the conservation tillage treatments. A single drip irrigation

tape was laid in each bed. Black polyethylene mulch was also applied using a mulch

layer in the production system treatment plots with polyethylene mulch.

Lettuce Transplant Production

‘New Red Fire’ and ‘Tropicana’ lettuce seeds were sown on 23 Sept. 2011 and

28 Sept. 2012, respectively, into 128-cell polystyrene foam flats (Speedling, Inc., Sun

City, FL) filled with peat-based Natural & Organic 10 potting soil (Fafard, Agawam, MA).

The organic fish and seaweed fertilizer, 2N-1.3P-0.8K (Neptune’s Harvest, Gloucester,

MA) was used to fertilize the lettuce transplants every other day.

Field Planting, Harvest, and Data Collection

Lettuce seedlings were transplanted to the organic field at PSREU (when lettuce

had 4-5 true leaves) on 27 Oct. 2011 and 1 Nov. 2012, respectively. Plants were grown

in double-row beds with 30 cm within-row spacing and 46 cm between-row spacing.

After transplanting the lettuce, there were 16 plants of each cultivar per bed resulting in

48 plants of each cultivar in production system treatment per replication. Plants were

32

drip irrigated twice a day from 7:00 to 7:30 a.m. and from 4:00 to 4:30 p.m. Lettuce

heads were harvested on 14 Dec. 2011 and 15 Dec. 2012. All the lettuce heads

harvested were considered marketable and they were weighed and counted to record

the marketable yield.

The aboveground lettuce dry weight and dry matter content were determined by

drying at 65°C until constant weight. Leaf tissue analysis was performed to determine

the concentrations of macronutrients including N, P, K, Calcium (Ca), Magnesium (Mg),

and Sulfur (S). Aboveground nutrient accumulation was estimated by multiplying the

aboveground dry weight with the nutrient concentration.

Statistical Analysis

Data analysis was performed using the PROC GLIMMIX procedure of SAS

program (Version 9.3 for Windows; SAS Institute, Cary, NC). Fisher’s LSD test was

used for multiple comparisons of measurements among different treatments (P ≤ 0.05).

Results and Discussion

Lettuce Yield

In both 2011 and 2012 trials, cultivar and production system as well as their

interactions showed significant impacts lettuce marketable yield performance (Table 2-

1). The lettuce yield was significantly higher in polyethylene mulched conventional

tillage treatments than other production system treatments, regardless of cover cropping

and cultivar (Table 2-2). In 2011, ‘Tropicana’ had significantly higher yields than ‘New

Red Fire’ in polyethylene mulched treatments and conservation tillage treatment with

higher organic fertilizer input, while such cultivar difference was also found in the bare

soil conventional tillage treatment without cover crops. In 2011, for ‘Tropicana’ lettuce,

polyethylene mulched treatment regardless of cover crops had the highest yield,

33

followed by bare soil with conventional tillage treatments and the treatment with cover

crops, conservation tillage, and high fertilizer input (Table 2-2). The conservation tillage

treatment with standard fertilizer input yielded the least. However, in the case of ‘New

Red Fire’ lettuce, lettuce yield did not differ significantly between the two conservation

tillage treatments. In 2012, lettuce yield was comparable between bare soil tilled

treatments and conservation tillage treatments for both ‘Tropicana’ and ‘New Red Fire’

(Table 2-2). Interestingly, the reduce tillage treatment with higher organic fertilizer input

led to a significant increase in marketable yield compared with the other conservation

tillage treatment and the bare soil tilled treatment with cover crops in the case of

‘Tropicana’ lettuce. Qin et al. (2016) reported that polyethylene mulch could significantly

improve soil fertility, such as available N and available P, by enhancing the soil enzyme

activities and soil properties. There was no difference between the two polyethylene

mulched systems in terms of yield for either cultivar which indicated that cover crops did

not significantly affect lettuce yield in polyethylene mulch systems. Moreover, within

bare soil treatments, no significant yield difference was found between treatments with

or without cover crops. Our findings indicated that the cover crop treatments did not

influence lettuce yield when sunn hemp and sorghum-sudangrass were planted and

incorporated into the soil prior to lettuce planting. In this study, N input to lettuce

production from cover crop incorporation was considered and therefore the organic

fertilization rate was reduced in the tilled treatments with cover crops. Previous research

reported that if a cover crop produces large amounts of biomass and is not given

sufficient time to decompose, nutrients immobilized in the cover crop residue may not

be available at the early growth stages of the following cash crops, and that the

34

following cash crop yield could also be reduced due to an allelopathic effect of cover

crop residues (Kessavalou, 1997; Weston, 1989). The ‘Tropicana’ lettuce had a positive

response to additional fertilizer application in conservation tillage systems; however,

such a response was absent in ‘New Red Fire’. It implied that the two lettuce cultivars

might have differential responses to N in yield development. Stirzaker (1995) and

Ferreira (2009) reported that with cover crops, the yield of lettuce grown using

conservation tillage systems could match or exceed that for lettuce grown using

conventional tillage systems. Similar crop yields between conservation tillage and

conventional tillage production systems were also reported by others (Delate, 2012;

Maltas, 2013; Mitchell, 2015). On the other hand, it was pointed out that yield may fall if

the conservation tillage system is not managed correctly (Cooper, 2014; Zhao,

2013).Previous research has shown contrary results indicating that

During the 2011 trial which was characterized by higher rainfall in mid-November

(Figure 2-1), significantly higher lettuce aboveground dry weight and significantly lower

dry matter content were observed for polyethylene mulch treatments (CC-T-M and

NCC-T-M) compared with other treatments (NCC-T, CC-T, CC-C-CT, and CC-CT)

(Table 2-3 and 2-4). As a rapid grown vegetables lettuce requires a considerate amount

of water during the season, the polyethylene mulch preserved soil moisture which may

benefit lettuce growth and result in higher aboveground dry weight (Ferreira et al. 2009).

No significant difference was observed among treatments without polyethylene mulch

(NCC-T, CC-T, CC-C-CT, and CC-CT) (Table 2-3 and 2-4).

During the 2012 trial, which was characterized by higher rainfall at the end of the

production season (Figure 2-1), lettuce in conservation tillage treatments had

35

significantly lower aboveground dry weight than other treatments (Table 2-3). The dry

matter content was significantly higher in the conservation tillage treatment with

standard fertilization as compared with other production system treatments (Table 2-4).

No significant difference in lettuce aboveground dry weight or dry matter content was

found among the 4 conventional tillage treatments (Table 2-3 and 2-4).

For both 2011 and 2012 trials, ‘Tropicana’ lettuce had significantly higher

aboveground dry weight compared with ‘New Red Fire’ lettuce (Table 2-5). However, no

significant difference was observed for lettuce aboveground dry matter content between

the two cultivars in both years (Table 2-6).

Lettuce Aboveground Nutrient Concentrations and Accumulation

Lettuce N and P concentrations were not significantly different among production

system treatments or between lettuce cultivars in the 2011 trial (Table 2-1). However, in

the 2012 trial, the bare soil tilled resulted in a significantly lower concentration of N in

lettuce compared with the conservation tillage treatments, while lettuce P concentration

was the highest in the conservation tillage treatment with standard fertilization (Table 2-

7). The results indicated that conservation tillage may increase lettuce nitrogen and

phosphorus concentration. The report by García-Marco (2014) supported our findings.

Meanwhile, Sharma et al. (2016) reported that organic agricultural practices such as

minimum tillage and surface application of mulch could significantly influence nitrogen

content by increasing the soil N level.

In the 2011 trial, lettuce had higher sulfur and potassium concentrations in

conventional tillage systems especially in the systems with polyethylene mulch (as

compared with conservation tillage systems (Table 2-1, 2-7, and 2-8). Kraska et al.

(2011) reported that wheat grain in conservation tillage had a significantly lower

36

potassium content compared with conventional tillage systems due to the availability of

potassium. However, in the 2012 trial, the similar trend did not repeat. Lettuce in the

conservation tillage treatment with high organic fertilizer input had the highest sulfur

concentration compared with conventional tillage treatments without polyethylene mulch.

Lettuce in the conservation tillage systems (CC-C-CT and CC-CT) also had significantly

higher potassium concentrations compared with three other conventional tillage

systems (CC-T-M, NCC-T, and CC-T) (Table 2-7). The different results over the two

seasons might be related to the difference in weed competition with the lettuce crop

which will be discussed in the next chapter.

Lettuce had a significantly higher calcium concentration in NCC-T-M treatment

compared with conservation tillage treatments (CC-C-CT and CC-CT) in 2011, whereas

no significant difference in lettuce calcium concentration was observed in 2012 (Table

2-1 and 2-7). Different management practices did not affect lettuce magnesium

concentration in either year (Table 2-1).

‘Tropicana’ had significantly higher concentrations of nitrogen, sulfur, and

calcium but a significantly lower phosphorus concentration than that of ‘New Red Fire’

lettuce in the 2012 trial (Table 2-8).

In the 2011 trial, lettuce in conventional tillage systems (CC-T-M, NCC-T-M,

NCC-T, and CC-T) had significantly higher levels of nitrogen, sulfur, calcium and

magnesium accumulation compared with conservation tillage systems (CC-C-CT and

CC-CT) (Table 2-9). Similar results were observed for phosphorus accumulation in

lettuce in 2011. In the 2012 trial, lettuce in polyethylene mulched treatments (CC-T-M

and NCC-T-M) in general had significantly higher levels of nitrogen, phosphorus, sulfur,

37

calcium, and magnesium accumulation than lettuce in other treatments (NCC-T, CC-T,

CC-C-T, and CC-CT) (Table 2-9). The results indicated that polyethylene mulch may

increase nitrogen, phosphorus, sulfur, calcium, and magnesium uptake by lettuce plants.

It is possible that due to the improved microclimate created by polyethylene mulch

(Moreno et al., 2008) the accumulation of above mentioned macronutrients increased.

Potassium accumulation was significantly higher in polyethylene mulched treatments

compared with the conservation tillage treatments in 2011; however, there was no

difference in lettuce potassium accumulation among production systems in 2012 (Table

2-9). As reported by Goverdarica-Lucic (2015), covering material was shown to have a

significant impact on lettuce potassium content.

In both 2011 and 2012 trials, we found significant differences between the two

lettuce cultivars in terms of macronutrient accumulation. ‘Tropicana’ lettuce had

significantly higher levels of nitrogen, phosphorus, sulfur, potassium, calcium and

magnesium accumulation than ‘New Red Fire’ lettuce (Table 2-10).

The results of nutrient accumulation in aboveground lettuce biomass showed that

different organic practices could show differential effects on plant nutrient uptake in

lettuce production. Except for potassium, most of the macronutrients uptake results in

2011 showed that the lettuce nutrient accumulation in the conservation tillage systems

was significantly lower than the conventional tillage systems. After one year, however,

no significant difference in lettuce nutrient accumulation was found between

conservation tillage systems and conventional tillage systems without polyethylene

mulch. This result indicated that the response of lettuce to conservation tillage may be

improved over time. No clear impact from cover crops on lettuce nutrient uptake was

38

found in this study. Even so, the two-year period may not be long enough to fully

elucidate the influence of different organic production systems on crop performance.

Further research especially longer-term studies are needed to obtain a better

understanding of optimizing management practices in organic production systems.

Conclusions

In this study, a bicultural cover crop system (sunn hemp and sorghum-sudan

grass) did not show a significant influence on lettuce yield performance. Polyethylene

mulch significantly improved lettuce yield and nutrient uptake. Conservation tillage

management methods need to be improved to enhance soil fertility management and

crop yield. Overall, ‘Tropicana’ lettuce had higher yield, aboveground dry weight, and

nutrient accumulation than ‘New Red Fire’ despite the production system used. Long-

term studies are needed to better understand the integration of different management

practices in organic production systems to optimize crop yield, plant nutrient uptake,

and soil fertility and quality.

39

Figure 2-1. Daily air temperature and daily precipitation during lettuce production

seasons in 2011 and 2012. Daily average temperature at 60 cm from the

soil in 2011 and 2012 and daily precipitation data were collected from

FAWN (http://fawn.ifas.ufl.edu).

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0

5

10

15

20

25

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45

2M

TO

TA

L R

AIN

FA

LL (

CM

)

60

CM

DA

ILY

AV

ER

AG

E T

EM

P (

C)

DAYS AFTER TRANSPLANTING

2012 rainfall 2011 rainfall 2011 temperature 2012 temperature

40

Table 2-1. Effects of cultivar (C) and production system (P) on lettuce yield, dry matter content, nitrogen, phosphorus, sulfur,

potassium, calcium, and magnesium concentrations in the 2011 and 2012 field trials in Citra, FL

NS, *,**,*** Nonsignificant or significant at P ≤ 0.05, 0.01, or 0.001, respectively.

Yield Dry matter content

Nitrogen Phosphorus Sulfur Potassium Calcium Magnesium

Effect df 2011 2012 2011 2012 2011 2012 2011 2012 2011 2012 2011 2012 2011 2012 2011 2012

C 1 *** *** NS * NS *** NS * NS * NS NS * *** NS NS

P 5 *** *** *** *** NS ** NS *** * * * * * NS NS NS

C×P 5 *** ** NS NS NS NS NS NS NS NS NS NS NS NS NS NS

41

Table 2-2. Impact of different management practices on lettuce marketable yield (t

ha-1)

Means within the same trial followed by the same letter do not differ significantly at P

≤ 0.05 by Fisher’s LSD test.

G = ‘Tropicana’; R = ‘New Red Fire’.

CC-T-M = ‘treatment with cover crop, conventional tillage and plastic mulch’; NCC-T-

M = ‘treatment without cover crop, but with conventional tillage and plastic mulch’;

NCC-T = ‘treatment without cover crop, but with conventional tillage’; CC-T =

‘treatment with cover crop and conventional tillage’; CC-C-CT = ‘treatment with cover

crop, standard fertilizer input, and conservation tillage’; CC-CT = ‘treatment with

cover crop conservation tillage and high fertilizer.

Cultivar CC-T-M NCC-T-M NCC-T CC-T CC-C-CT CC-CT

2011 G 20.0 a 20.8 a 11.2 bc 10.7 cd 7.7 ef 8.6 cd R 13.6 b 12.1 b 9.1 cd 8.8 ed 6.7 ef 5.6 f 2012 G 10.8 a 11.8 a 5.6 bc 4.4 cd 3.6 cd 6.8 b R 6.5 b 7.1 b 3.5 d 3.0 , d 2.9 ,d 4.3 cd

42

Table 2-3. Impact of different management practices on lettuce aboveground dry

weight (g/head)

Year CC-T-M NCC-T-M NCC-T CC-T CC-C-CT CC-CT

2011 34.2 a 30.6 a 22.8 b 23.2 b 19.5 b 18.1 b

2012 41.3 a 39.7 a 40.0 a 40.81 a 26.8 b 30.8 b

Means within a row and a treatment category followed by the same letter do not

differ significantly at P ≤ 0.05 by Fisher's LSD test.

43

Table 2-4. Impact of different management practices on lettuce aboveground

dry matter content (%)


2011 3.64 b 3.46 b 6.29 a 6.09 a 5.93 a 5.32 a

2012 5.28 b 5.28 b 5.42 b 5.54 b 7.97 a 6.38 b

Means within a row and a treatment category followed by the same letter do

not differ significantly at P ≤ 0.05 by Fisher's LSD test.

44

Table 2-5. Lettuce aboveground dry weight (g/head) in 2011 and 2012 trials

Means within a column followed by the same letter do not differ significantly at

P ≤ 0.05 by Fisher's LSD test.


Cultivar 2011 2012

G 30.5 a 42.6 a

R 19.0 b 30.6 b

45

Table 2-6. Lettuce aboveground dry matter content (%) in 2011 and 2012

trials

Means within a column followed by the same letter do not differ significantly at P ≤ 0.05 by Fisher's LSD test.


Cultivar 2011 2012

G 5.4 a 6.4 a

R 4.9 a 5.6 a

46

Table 2-7. Lettuce macronutrient concentrations (g/kg) in different production

system treatments in the 2011 and 2012 trials in Citra, FL

Means within a row followed by the same letter do not differ significantly at P ≤ 0.05

by Fisher's LSD test.

CC-T-M = treatment with cover crop, conventional tillage and plastic mulch; NCC-T-

M = treatment without cover crop, but with conventional tillage and plastic mulch;

NCC-T = treatment without cover crop, but with conventional tillage; CC-T =

treatment with cover crop and conventional tillage; CC-C-CT = treatment with cover

crop, standard fertilizer input, and conservation tillage; CC-CT = treatment with cover

crop, conservation tillage, and high organic fertilizer input.


Nitrogen 2011 36.8 39.1 35.3 32.6 33.0 31.6 2012 39.3 ab 39.5 ab 34.0 c 36.8 bc 40.9 a 42.4 a Phosphorus 2011 5.5 6.0 6.2 5.9 5.2 5.8 2012 3.5 b 3.9 ab 3.4 b 3.5 b 4.5 a 4.1 ab Sulfur 2011 3.0 ab 3.5 a 2.9 ab 2.7 bc 2.6 bc 2.4 c 2012 2.5 ab 2.6 ab 2.3 b 2.4 b 2.6 ab 2.7 a Potassium 2011 43.5 a 40.0 ab 35.6 abc 31.3 c 33.7 bc 30.7 c 2012 30.9 b 33.5 ab 31.1 b 27.1 b 44.2 a 44.6 a Calcium 2011 16.3 ab 16.7 a 16.0 ab 15.4 ab 14.3 bc 13.4 c 2012 21.5 20.9 19.5 21.4 19.4 20.2 Magnesium 2011 7.7 7.8 7.9 7.6 6.8 6.7 2012 9.4 9.5 8.8 9.5 8.7 9.1

47

Table 2-8. Macronutrient concentrations (g/kg) in different lettuce cultivars

Means within a column followed by the same letter do not differ significantly at P ≤

0.05 by Fisher’s LSD test.

G = Tropicana; R = New Red Fire.


Cultivar 2011 2012 2011 2012 2011 2012 2011 2012 2011 2012 2011 2012

G 33.8 40.5 a 6.0 3.5 b 2.7 2.6 a 34.7 33.9 15.9 a 23.5 a 7.3 9.2

R 35.6 37.2 b 5.6 4.1 a 2.9 2.4 b 37.0 36.6 14.4 b 17.6 b 7.5 9.1

48

Table 2-9. Impact of different production systems on accumulation of macronutrients

in lettuce (mg/head)


Nitrogen

2011 1473.0 a 1542.5 a 1415.7 a 1319.3 a 876.6 b 955.9 b

2012 1676.7 a 1628.3 a 964.4 b 1107.3 b 1033.3 b 1015.8 b

Phosphorus

2011 227.3 ab 234.6 a 246.6 a 241.7 a 138.2 c 176.6 bc

2012 148.7 a 160.2 a 100.0 b 104.9 b 111.0 b 97.2 b

Sulfur

2011 120.3 a 123.1 a 115.8 a 110.6 a 70.8 b 74.4 b

2012 108.6 a 105.4 a 69.8 b 74.1 b 64.7 b 67.2 b

Potassium

2011 1785.8 a 1593.0 ab 1426.8 abc 1258.0 bcd 895.0 d 962.1 cd

2012 1293.2 1326.8 914.7 781.8 1106.0 1151.6

Calcium

2011 661.9 a 675.2 a 643.8 a 645.8 a 382.1 b 415.5 b

2012 949.6 a 892.0 ab 602.1 c 668.7 bc 515.7 c 475.7 c

Magnesium

2011 297.6 a 307.5 a 318.1 a 306.6 a 182.3 b 202.7 b

2012 408.5 a 391.9 a 260.0 b 289.3 b 219.6 b 212.8 b


by Fisher’s LSD test.







49

Table 2-10. Accumulation of macronutrients in different lettuce cultivars (mg/head)

Means within a column followed by the same letter do not differ significantly at P ≤ 0.05 by Fisher’s LSD test.

G = Tropicana; R = New Red Fire.


Cultivar 2011 2012 2011 2012 2011 2012 2011 2012 2011 2012 2011 2012

G 1433.8 a 1417.9 a 238.3 a 134.3 a 117.1 a 99.6 a 1496.4 a 1269.7 a 677.5 a 905.4 a 307.9 a 354.4 a

R 1093.9 b 1057.4 b 183.3 b 106.3 b 87.9 b 63.7 b 1143.9 b 921.7 b 463.9 b 462.6 b 230.3 b 239.5 b

50

CHAPTER 3

EFFICACY OF CULTRUAL AND PHYSICAL WEED MANAGEMENT IN ORGANIC

LETTUCE PRODUCTION

Introduction

The United States produced more than $262 million of organic lettuce

(Lactuca sativa) on approximately 37,916 acres in 2015 (USDA, 2016). Among the

ubiquitous pests, competition from weeds remains the key hurdle for lettuce

productivity and quality in organic production systems, especially for Florida growers.

The absence of synthetic herbicides makes organic weed management crucial to the

adoption and development of different cultural practices in Florida.

Cover crops are recognized as a means of management to enhance soil

fertility, reduce soil erosion, increase soil organic matter, and suppress weeds

(Gaskell et al., 2007; Teasdale et al., 2007; Vollmer et al., 2010). In Florida, many

growers cover crops in hot, humid summers in rotation with vegetable crops.

Sorghum-sudangrass (Sorghum bicolor x S. bicolor var. sudanense), known for its

high biomass yield, heat tolerance, and weed suppression ability, is a widely used

cover crop in Florida. From a weed control perspective, sorghum–sudangrass is

reported to reduce weeds species by 25% to 40% (Smith et al., 2015). Sorghum-

sudangrass is also effective in reducing weed density (Ngouajio et al., 2005). Fast

growing sorghum-sudangrass is relatively competitive with weeds. Moreover, the

allelopathic effects of sorghum-sudangrass is well demonstrated (Weston et al.,

1989). Sunn hemp (Crotalaria juncea), a tropical legume, is commonly used as a

summer cover crop in Florida. Research interest in sunn hemp is growing due to its

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high biomass production, efficient biological nitrogen fixation, and weed suppression

abilities. The allelopathic properties and vigorous establishment of sunn hemp are

helpful for suppressing weeds during both germination and growth (Sangakkara et

al., 2006; Skinner et al., 2012). The benefit of cover crop bicultural systems are well-

documented as maintaining appropriate C:N ratio and increasing weed suppression

capacity (Creamer et al., 1996). However, cover crop management issues may

create yield losses of cash crops (Singh et al., 1996).

By incorporating soil surface residues and surface soil, tillage offers numerous

advantages such as weed control, seed bed preparation, soil fertility, and soil quality

management (Busari et al., 2015). Because tillage can change weed seed depth in

soil, weed seed bank redistribution keeps them from readily germination conditions

that inhibit weed seedlings from emerging (Ball, 1992). Conventional tillage which

buries weed seeds extends the longevity of weed seeds in the soil by persisting

them in deeper soil (Ball, 1992). Meanwhile, soil redistribution also leads to weed

seed exposure to favorable conditions that encourages weed seed germination

(Liebman et al., 2001). At the same time, conventional tillage creates unsustainable

conditions for organic agriculture by increasing soil erosion and decreasing soil

organic matter. In order to maintain sustainability, conservation tillage has been

developed that maintains more than 30% soil coverage (Hoyt, 1999). The maintained

soil coverage, usually from previous cover crop residues, act as a barrier between

the soil and sunlight to prevent weed seed germination and limit the weed

establishment (Teasdale et al., 2000). It has been reported that conservation tillage

52

could decrease weed longevity and weed seed survival over years in contrast to

conventional tillage (Anderson, 2008; Wicks et al., 1988). However, the inconsistent

results showed that the weed population may increase during the early years after

transiting from conventional tillage (Bàrberi et al., 2001; Gruber et al., 2012).

Polyethylene mulch is widely used to create a favorable environment for

growing cash crops and nutrient management (Ham et al., 1993). As an effective

alternative for weed suppression, polyethylene mulch intercepts the photosynthesis

of weeds from sunlight and confines weed emergence by up to 98% (Egley, 1983).

Polyethylene mulch also creates a beneficial environment for cash crops to improve

crop establishment and plant growth and to promote nutrient competition after the

early growth stage. Successful elimination of weeds by use of black polyethylene

mulch has been shown in previous research (Singh et al., 2009). Polyethylene mulch

provides substantial control of broadleaf weeds, but not for nutsedge, especially

Purple Nutsedge (Webster, 2005). On the downside, polyethylene mulch is

expensive to use and increases agricultural chemical runoff and disposal issues.

The overall goal of this study was to provide suggestions to Florida organic

lettuce growers for controlling weeds without sacrificing yields. The objectives of this

study were to (1) evaluate the feasibility of the implementation of sorghum-

sudangrass and sunn hemp cover crop bicultural system to suppress weeds in

organic lettuce production and (2) determine whether conservation tillage is an

effective substitute for conventional tillage to control weeds; and (3) compare black

polyethylene mulch system and conservation tillage system with cover crop residues

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as organic mulch for weed suppression. This study tested the following hypotheses

associated with different cultural and physical practices. First, weed pressure in

cover cropping systems will be significantly lower than in systems with no cover

crops. Second, conservation tillage systems with cover crop residues as organic

mulch will have similar abilities to control weeds as do conventional tillage systems.

Third, polyethylene mulch and conservation tillage systems will result in differences

in weed competition in organic lettuce production.


Experimental Design and Field Trial Establishment

Field trials were conducted at the University of Florida Plant Science

Research and Education Unit (PSREU) in Citra, FL, in the 2011 and 2012 fall lettuce

seasons. The soil type at the research site was loamy sand. The field trials were

arranged in a randomized complete block design using four replications. Plots were

6.1 m x 2.3 m. Six production system treatments consisting of three factors (the

adoption of cover corps with sunn hemp and sorghum-sudangrass, the use of both

conventional and conservation tillage systems; and the application of polyethylene

much) were examined. The six treatments were arranged as follows:

1. No cover crop + conventional tillage + polyethylene mulch = NCC-T-M

2. No cover crops + conventional tillage + no mulch = NCC -T

3. Cover crop + conventional tillage + polyethylene mulch = CC-T-M

4. Cover crop + conventional tillage + no mulch = CC-T

5. Cover crop + conservation tillage = CC-C-CT

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6. Cover crop + conservation tillage + high organic fertilizer input = CC-CT

For the field trials, sunn hemp at the rate of 33.5 kg/ha and sorghum-

sudangrass at the rate of 22.4 kg/ha were mixed and drill-seeded to the plots with

cover crop treatments on 8 Aug. 2011 and 30 July 2012. Cover crop stand and

growth were evaluated every week after emergence to determine harvest time

between flowering and the initiation of seed set. For conservation tillage treatments,

cover crops were terminated using a roller chopper on 14 Oct. 2011 and 19 Oct.

2012. For conventional tillage treatments, cover crop residues were tilled into the soil.

Black polyethylene mulch was applied for plastic mulch treatments.

Detailed information regarding field trial establishment and lettuce production

and harvest can be found in Chapter 2. During the lettuce growing season, no hand

weeding was performed.

Weed Aboveground Coverage and Biomass

Weed aboveground coverage and biomass were evaluated four times: at

cover crop termination, 2 weeks after transplanting (WAT) of lettuce, 4 WAT, and at

lettuce harvest. The cover crops were collected separately by species to evaluate

their growth by measuring their biomass. Weed aboveground coverage was

determined and recorded by the percentage of emerged weed coverage (separated

by weed category) located within 0.33 m2 quadrats. Quadrats were placed on two

randomly selected spots from each plot at cover crop termination and on two of three

beds during the lettuce production season. Within each quadrat, weeds were

classified into five categories: annual grass, perennial grass, annual broadleaf,

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perennial broadleaf, and nutsedge. Emerged weeds were then identified by species

and recorded. Weed community were recorded based on species frequency during

the whole production season.

After the weed aboveground coverage was recorded, weed biomass samples

were harvested by cutting weeds at the soil surface within the quadrat and then

collected into individual paper bags by weed category. Weed biomass samples were

dried at 70°C until constant weight and weighed to determine plant biomass on a

dry-weight basis.


Data analysis was performed using SAS 9.3 (SAS Institute Inc. Cary, NC) with

the PROC GLIMMIX procedure. Fisher’s LSD test (P ≤ 0.05) was used for multiple

comparisons of measurements among treatments.


Weed Community in the Experimental Field

Weed community varied by year. In the 2011 field trial, the prevalent weeds

were smooth crabgrass (Digitaria ischaemum), Florida pusley (Richardia scabra),

and nutsedge including purple nutsedge (Cyperus rotundus) and yellow nutsedge

(Cyperus esculentus) (Figure 3-1). In the 2012 field trial, the primary weeds were

hairy indigo (Indigofera hirsuta), Florida pusley (Richardia scabra), bermudagrass

(Cynodon dactylon), nutsedge including purple nutsedge (Cyperus rotundus) and

yellow nutsedge (Cyperus esculentus) (Figure 3-2). Crabgrass is an annual grass,

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Florida pusley and hairy indigo are annual broadleaf weeds, while bermudagrass is a

perennial grass.

Annual Grasses

In the 2011 (first year) trial, the main problem was annual grass weeds that

infested the whole site. No significant difference among the production system

treatments was observed at the time the cover crop was terminated (Table 3-1).

During the lettuce production season, except for treatments with black polyethylene

mulch, all other treatments were heavily infested by annual grass.

In the 2012 (second year) trial, at the time the cover crops were terminated,

overall annual grass weed biomass and weed aboveground coverage were

significantly higher in plots with no cover crops than in plots with cover crops (Table

3-2). Moreover, among the two treatments with no cover crop implemented,

polyethylene mulch treatment established since 2011 had higher annual grass

infestations. Results from a previous study showed that sunn hemp and other cover

crops had the ability to inhibit up to 80% of goosegrass germination (Adler et al.,

2007). Sorghum-sudangrass was reported to be effective in the early stages of weed

management (Schoofs et al., 2000). Other than the influence of cover crops, the

results showed no influence of tillage on grass weed biomass (Table 3-2). From two

weeks onwards after the lettuce was transplanted, there was an insufficient amount

of annual grass weeds to determine the efficacy of different cultural and physical

controls over annual grass. The results showed that annual grass weeds were

successfully eliminated at the study site. Future studies will be necessary to assess

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weed aboveground coverage and weed biomass after cover crop termination and

before the lettuce is transplanted. Because few reports have determined the factors

that extend weed biomass control after cover crop termination, the weed biomass

study should be conducted during the time of cover crop decomposition.

Perennial Grasses

In the 2011 field study, at the time the cover crop was terminated, the

perennial grass biomass and aboveground coverage were not significantly different

among the treatments. However, results of weed assessment during the lettuce

growing season revealed that the conservation tillage treatments tended to have

higher infestation levels in terms of weed biomass and weed aboveground coverage

compared to the conventional tillage treatments (Table 3-3). At the end of the 2011

season, there was no significant difference among all the production system

treatments regarding perennial grass control.

In the 2012 trial, the prevalent perennial grass was bermudagrass. At the time

the cover crop was terminated, the conservation tillage treatments had the highest

perennial grass biomass and aboveground coverage across all the production

system treatments, while there was no significant difference among the conventional

tillage treatments (Table 3-4). According to Hassan et al. (2016), perennial weeds

were the main problem in conservation tillage systems. At 2 WAT during the lettuce

growing season the treatment difference disappeared in terms of weed aboveground

coverage, whereas the conservation tillage treatment with higher organic fertilizer

input showed significantly higher perennial grass biomass than the polyethylene

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much treatments and the bare soil tilled treatments without cover crops (Table 3-4).

Two weeks later, the conservation tillage treatments had significantly higher

perennial grass biomass and aboveground coverage compared with the

conventional tillage treatments (Table 3-4). Nevertheless, the trend disappeared at

the end of the lettuce cropping season. Overall, summer cover crops showed little

influence on perennial grass biomass and aboveground coverage during the fall

lettuce planning in the conventional tillage treatments regardless of the use of

polyethylene much.

Broadleaf Weeds

In the 2011 trial, treatments with no cover crops planted were significantly

infested by annual broadleaf weeds at the time cover crops were terminated. Two

weeks after the lettuce was transplanted, the conventional tillage treatment without

cover crops and black polyethylene mulch had a significantly higher annual broadleaf

weed infestation than the other treatments (Table 3-5). This indicated that the

treatment that combined cover crops with black polyethylene mulch could

significantly suppress annual broadleaf weeds at the early stage of lettuce

production. At the harvest of lettuce, there was no significant difference among all

the treatments in terms of annual broadleaf weed biomass, while the conventional

tillage without cover crops and black polyethylene mulch treatment showed a

significantly higher level of broadleaf weed aboveground coverage than other

treatments (Table 3-5).

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In the 2012 trial, annual broadleaf weed biomass and weed aboveground

coverage were significantly reduced by cover crop implementation (Table 3-6).

Cover crops such as sorghum-sudangrass could effectively suppress broadleaf

weeds by the release of allelochemicals (Barnes et al., 1983; Weston et al., 1989).

Moreover, previous research has shown that the large amount of biomass produced

by cover crops is tremendously competitive with broadleaf weeds (Creamer et al.,

2000; Rolston et al., 2003). Two weeks after the lettuce was transplanted, no

significant difference in annual broadleaf weed infestation was noticed among all the

treatments, whereas four weeks after the lettuce was transplanted, the conventional

tillage treatment without cover crops and black polyethylene mulch had a

significantly higher annual broadleaf weed infestation (Table 3-6). These results

might be due to the ability of cover crops and polyethylene mulch to inhibit broadleaf

weed germination. At the end of the lettuce production season, both bare soil

treatments with conventional tillage showed significantly higher levels of annual

broadleaf weed biomass and aboveground coverage than the polyethylene mulched

treatments and the conservation tillage systems (Table 3-6). The differences in

broadleaf weed biomass among the tillage systems observed in this study were

similar to the broadleaf weed biomass increase resulting from conventional tillage as

reported by Vijaya et al. (2014). Meanwhile, conservation tillage was reported to be

effective in the suppression of broadleaf weeds (Tabaglio et al., 2013). It is also

possible that the allelopathic effect of cover crops was not effective enough in the

later stage of the lettuce production season. Unsurprisingly, polyethylene mulch

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eliminated the emergence of annual broadleaf weeds throughout the whole

production season of fall lettuce.

Few perennial broadleaf weeds infested the site to determine the effects of

each production system treatment in this two-year study (data not shown).

Nutsedge

In both 2011 and 2012 trials, the results from weeds harvested at the time

cover crops were terminated revealed that in general cover crops substantially

reduced nutsedge biomass and aboveground coverage (Table 3-7 and 3-8). Two

weeks after the lettuce was transplanted, nutsedge biomass and aboveground

coverage were significantly higher in the bare soil tilled treatment with no cover crop

incorporated as compared with the conventional tillage treatments with cover crop

incorporation and the conservation tillage treatments. It is possible that the reduction

of nutsedge was due to the allelochemicals present in sorghum-sudangrass

(Cheema et al. 2004). In both years, four weeks after the lettuce was transplanted,

nutsedge infestation levels remained the highest in the conventional tillage

treatments without cover crop incorporation. The results indicated that black

polyethylene mulch alone may be unable to reduce nutsedge infestation, similar to

the results reported by Webster et al. (2005). In the 2011 trial, at the end of the

production season, nutsedge infestation was not significantly different among all the

treatments (Table 3-7). In the 2012 trial, the treatment without cover crops and

polyethylene mulch had a significantly higher level of nutsedge biomass and

aboveground coverage than the treatment with polyethylene mulch and cover crops

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as well as the conservation tillage treatments (Table 3-8). It is possible that the

difference in cover crop establishment and biomass in the two production seasons

resulted in the difference in nutsedge control between 2011 and 2012 trials. The

combination of cover crop and black polyethylene mulch significantly suppressed

nutsedge infestation. Interestingly, in both years, the conservation tillage treatments

successfully controlled nutsedge during the lettuce production season.

Conclusions

Overall, using polyethylene mulch and growing sunn hemp and sorghum-

sudangrass in biculture prior to fall lettuce planting showed the most effective

suppression of weeds in organic lettuce production in this two-year study. Bicultural

cover crops used in this study could help suppress annual grass and broadleaf

weeds. Moreover, weed aboveground coverage and biomass assessment results

showed the effectiveness of using summer cover crops in nutsedge management.

Given that cover crop aboveground coverage plays an important role in reducing

weed biomass, future studies need to include different cover crop aboveground

coverage treatments to better understand the optimal weed control effects

associated with cover crops in Florida. To achieve sufficient weed control levels,

efforts also need to be made to optimize cover crop establishment and production.

Regrowth of sorghum-sudangrass was observed in the conservation tillage systems

in this study. While conservation tillage was not as effective as conventional tillage

for perennial grass control, conservation tillage with cover crop residues as organic

mulch could be used to effectively control broadleaf weeds and nutsedges. Using

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cover crops could also help reduce the nutsedge infestation level when using black

polyethylene mulch during the lettuce production season. It should be noted that no

effort was made in this study to separate nutsedges by species, so future studies are

needed to better understand the effects of different production systems on yellow

nutsedge and purple nutsedge management.

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Figure 3-1. Weed community in 2011. Weed emergence was assessed by species

within a 0.33 m2 quadrat at the time of cover crop termination, 2 weeks

and 4 weeks after lettuce transplanting, and at the time of lettuce harvest.

Weed species composition averaged over data collection dates.

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Figure 3-2. Weed community in 2012. Weed emergence was assessed by species

within a 0.33 m2 quadrat at the time of cover crop termination, 2 weeks

and 4 weeks after lettuce transplanting, and at the time of lettuce harvest.

Weed species composition averaged over data collection dates.

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Table 3-1. Impact of different organic production systems on annual grass biomass

and weed aboveground coverage in the 2011 field trial

CC-T-M NCC-T-M NCC-T CC-T CC-C-CT CC-CT Significance

Weed biomass (g/m2)

CCT 91.8 104.6 74.7 82.7 123.3 63.5 NS

2 WAT 0 0 1.5 2.9 74.1 87.2 NS

4 WAT 0 0 31.1 38.6 55.9 46.9 NS

Harvest 0 0 14.9 26.8 40.9 51.1 NS

Weed aboveground coverage (%)

CCT 39 32 42 47 43 49 NS

2 WAT 0 0 1 1 16 14 NS

4 WAT 0 0 37 28 41 33 NS

Harvest 0 0 22 17 34 30 NS

CCT: at the time of cover crop termination; WAT: week after transplanting; Harvest:

at the time of lettuce harvest.







NS: Nonsignificant.

.

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Table 3-2. Impact of different organic production systems on annual grass biomass

and weed aboveground coverage in the 2012 field trial


Weed biomass (g/m2)

CCT 1.0 c 96.6 a 54.0 b 4.7 c 6.1 c 25.6 bc ***

2 WAT 0 0 0 0 0 1.3 NS

4 WAT 0 0 0 0 0 3.4 NS

Harvest 0 0 0 0 0 0 NS


CCT 1 c 20 a 10 b 2 c 3 c 4 c ***

2 WAT 0 0 0 0 0 1 NS

4 WAT 0 0 0 0 0 1 NS










NS, *, **, *** Nonsignificant or significant at P ≤ 0.05, 0.01, or 0.001, respectively.


according to Fisher’s LSD test.

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Table 3-3. Impact of different organic production systems on perennial grass

biomass and weed aboveground coverage in the 2011 field trial


Weed biomass (g/m2)

CCT 2.1 25.1 7.5 13.9 59.4 52.5 NS

2 WAT 0 b 0 b 2.8 b 6.1 b 22.0 a 18.5 a *

4 WAT 0 b 0 b 6.8 b 12.5 b 49.6 a 31.5 a *

Harvest 0 1.0 9.4 7.1 9.7 12.8 NS


CCT 5 14 5 3 7 4 NS

2 WAT 0 b 0 b 5 b 7 b 15 a 20 a **

4 WAT 0 0 3 8 25 30 NS

Harvest 0 1 3 5 13 9 NS












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Table 3-4. Impact of different organic production systems on perennial grass


CC-T-M NCC-T-M NCC-T ,,CC-T CC-C-CT CC-CT Significance

Weed biomass (g/m2)

CCT 0.8 b 3.2 b 6.3 b 9.1 b 56.8 a 74.6 a ***

2 WAT 0 b 0 b 0 b 6.1 ab 9.2 ab 18.5 a *

4 WAT 0 b 0 b 0 b 0 b 30.4 a 31.4 a *

Harvest 0 0 0 0 7.6 12.2 NS


CCT 2 b 6 b 3 b 2 b 18 a 23 a ***

2 WAT 0 0 0 4 a 5 a 9 a NS

4 WAT 0 b 0 b 0 b 0 b 13 a 14 a *













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Table 3-5. Impact of different organic production systems on broadleaf weeds


CC-T-M NCC-T-M NCC-T CC-T ,CC-C-CT CC-CT Significance

Weed biomass (g/m2)

CCT 3.9 b 123.7 a 107.9 a 5.6 b 3.8 b 5.7 b *

2 WAT 0 b 0 b 74.6 a 2.6 b 12.4 b 6.3 b **

4 WAT 0 b 0 b 89.9 a 33.4 b 11.6 b 33.3 b *

Harvest 0 0 71.5 50.4 13.2 35.0 NS


CCT 2 b 14 a 17 a 1 b 3 b 3 b *

2 WAT 0 b 0 b 20 a 2 b 5 b 4 b *

4 WAT 0 0 15 10 4 11 NS

Harvest 0 c 0 c 24 a 16 b 3 c 5 c *












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Table 3-6. Impact of different organic production systems on annual broadleaf

weeds biomass and weed aboveground coverage in the 2012 field trial


Weed Biomass (g/m2)

CCT 1.0 b 68.9 a 49.1 a 0.7 b 0.5 b 0.7 b *

2 WAT 0 0 0 0.7 0.7 2.1 NS

4 WAT 0.1 b 0 b 2.6 a 0.9 b 0.3 b 0.4 b *

Harvest 0 b 0 b 128.6 a 119.6 a 0.8 b 4.6 b ***


CCT 3 c 16 a 10 b 3 c 3 c 3 c ***

2 WAT 0 0 0 1 1 1 NS

4 WAT 0 b 0 b 2 3 1 1 NS

Harvest 0 c 0 c 35 a 27 b 3 c 4 c ***












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Table 3-7. Impact of different organic production systems on nutsedge biomass and

weed aboveground coverage in the 2011 field trial













Weed biomass (g/m2)

CCT 8.4 b 200.2 a 162.4 a 29.9 b 35.3 b 15.8 b *

2 WAT 1.1 b 2.3 b 5.0 a 1.7 b 0.5 b 0.9 b *

4 WAT 2.1 b 9.2 a 8.5 a 5.0 ab 1.3 b 0.9 b *

Harvest 4.2,,, 13.3,,, 16.1,,, 12.1,,,,, 2.1 1.7,,, NS


CCT 14 b 33 a 40 a 15 b 15 b 11 b *

2 WAT 4 b 8 ab 12 a 5 b 3 b 3 b *

4 WAT 5 b 13 a 17 a 7 b 3 b 4 b *

Harvest 5 13 12 9 5 3 NS

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Table 3-8. Impact of different organic production systems on nutsedge biomass and

weed aboveground coverage in the 2012 field trial


Weed biomass (g/m2)

CCT 15.4 c 128.7 a 126.9 a 29.5 bc 54.1 bc 72.1 b ***

2 WAT 1.1 b 2.3 b 5.0 a 1.7 b 0.9 b 1.6 b *

4 WAT 2.1 b 9.1 a 8.4 a 4.9 ab 0.5 b 1.9 b **

Harvest 4.6 bc 13.1 ab 16.0 a 12.1 ab 1.7 c 2.0 c *


CCT 8 c 22 ab 27 a 11 c 15 bc 15 bc ***

2 WAT 4 b 11 a 12 a 5 b 2 b 3 b **

4 WAT 6 b 17 a 13 a 7 b 3 b 5 b ***

Harvest 5 bc 13 a 12 a 10 ab 4 bc 3 c **









*, **, *** Significant at P ≤ 0.05, 0.01, or 0.001, respectively.



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CHAPTER 4

THE INFLUENCE OF DIFFERENT ORGANIC PRODUCTION SYSTEMS ON THE

LEVELS OF ASCORBIC ACID, TOTAL PHENOLICS, AND CHLOROPHYLL IN

LETTUCE

Introduction

Lettuce (Lactuca sativa) is the second most consumed vegetable in the

United States, with purchases of romaine and leaf lettuce steadily increasing in

recent years (USDA, 2016). Lettuce is known as a good dietary source of ascorbic

acid. While the level of phenolic compounds in lettuce is not high on the fresh weight

basis, it is important to optimize cultivation conditions to enhance the phenolic

content in lettuce considering the popularity of lettuce in the American diet.

Ascorbic acid (AA) is a water-soluble vitamin with many biological benefits to

humans such as enhancing wound healing rate and improving gum and skin health.

According to the American Cancer Institute and the American Cancer Society,

antioxidants such as ascorbic acid and phytochemicals can reduce the risk of cancer.

It is also reported that ascorbic acid plays a role in preventing diseases such as

heart disease and high blood pressure by scavenging free radicals (Iqbal et al.,

2004).

Phenolic compounds are considered plant secondary metabolites which

consist of phenolic acids and flavonoids. Compared with ascorbic acid, phenolic

compounds have been proven to be more effective as antioxidants (Eberhardt et al.,

2000). Research suggests that phenolic compounds contribute to disease prevention,

including cardiovascular disease and cancer (Nicholson and Hammerschmidt, 1992).

74

Chlorophyll, a key component in photosynthesis, is responsible for the green

color in lettuce. It is also reported to have the potential to prevent cancer (Ferruzzi

and Blakeslee, 2007). Chlorophyll may protect plants from environmental stress such

as light stress and chilling stress, but not nutrient stress which can decrease

chlorophyll content (Kim et al., 2008). The level of chlorophyll is critical to lettuce

quality.

Phenolic compounds have been shown to have a protective role in plants as

inhibitors of harmful ultraviolet (UV) radiation and as a defense against

microorganisms and insects (Iwashina, 2003). Phenolic compounds are secondary

metabolites while ascorbic acid is a major metabolite (Li et al., 2010). The control of

the ascorbic acid biosynthesis pathway is still not well understood. Because both

ascorbic acid and phenolic compounds are associated with plants’ antioxidant and

photo-protective systems, it is possible that they could be increased by biotic and

abiotic stress conditions. Higher levels of ascorbic acid were found in plants with

environmental stress such as wounds and radiation (Conklin et al., 1996). It has

been reported that phenolic compounds are altered by pre-harvest factors such as

stress from neighboring plants and drought (Kim et al., 2007; Ruiz-Lozano et al.,

1996). Moreover, site-specific conditions such as soil quality and fertility influence

phytochemical content, while postharvest processing and storage influence both

ascorbic acid and total phenolic contents (Amarowicz et al., 2009). Organic

production systems involve more complex biotic and abiotic stresses compared with

conventional production systems. Increasing evidence indicates that vegetables and

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fruits produced organically promote the accumulation of greater levels of ascorbic

acid and phenolic compounds when compared with conventionally produced

vegetables and fruits (Asami et al., 2003; Brandt and Mølgaard, 2001). Nevertheless,

other studies indicated that organic production systems did not significantly increase

phytochemicals (You et al., 2011). These inconsistent results did not lead to a

persuasive conclusion toward organic practices positively affecting phytochemical

contents in agricultural produce. Considering that different management practices

are used by organic farmers, it is critical to examine the influence of different organic

production systems on phenolic compound and ascorbic acid levels in vegetables

and fruits at harvest.

The objective of this two-year study on organic lettuce production was to

determine the effects of organic production systems involving polyethylene mulch,

cover crops, and conservation tillage on the levels of ascorbic acid, total phenolics,

and chlorophyll in lettuce.


Field Experimental Design

Three management practice factors were included in establishing the six

production system treatments in the field, including: (1) the use of sunn hemp and

sorghum-sudangrass cover crop biculture, (2) the use of black polyethylene mulch,

and (3) conservation tillage. The green loose leaf lettuce ‘Tropicana’ and the red

loose leaf lettuce ‘New Red Fire’ (Johnny’s Selected Seeds, Winslow, ME) were also

included in this two-year study conducted at the University of Florida Plant Science

76

Research and Education Unit (PSREU) in Citra, FL. The experiment was arranged in

a split-plot design with four replications. The production system was the whole plot

factor, while the lettuce cultivar was the subplot factor. Lettuce seedlings were

transplanted to the organic field at PSREU (when lettuce had 4-5 true leaves) on 27

Oct. 2011 and 1 Nov. 2012, respectively. Plants were grown in double-row beds with

30 cm within-row spacing and 46 cm between-row spacing. After transplanting the

lettuce, there were 16 plants of each cultivar per bed resulting in 48 plants of each

cultivar in production system treatment per replication. Plants were drip irrigated

twice a day from 7:00 to 7:30 a.m. and from 4:00 to 4:30 p.m. Lettuce heads were

harvested on 14 Dec. 2011 and 15 Dec. 2012. All the lettuce heads harvested were

considered marketable and they were weighed and counted to record the marketable

yield.

The detailed information of field trial setup and management can be found in

Chapter 2.

Plant Materials and Determination of Levels of Ascorbic Acid, Total Phenolics,

and Chlorophyll

Lettuce outermost 3 leaves were collected from harvested head and leaves

from harvested lettuce was transferred into a -30ºC freezer within 3 hours of harvest

for further analysis. Samples were homogenized at maximum speed in a 908™

Commercial Bar Blender (HBB908) (Hamilton Beach Brands, Inc., Southern Pines, NC)

before further extraction for measurements of total phenolics, ascorbic acid, and

chlorophyll.

77

Ascorbic acid was determined by the Dinitrophenylhydrazine (DNPH) method

(Terada et al., 1978). Absorbance was measured at 540 nm and concentration was

determined by using a standard curve and reported as mg/100 g FW lettuce.

The total phenolic content of lettuce was determined by using the Folin-

Ciocalteu method (Singleton et al., 1999). The level of total phenolics was calculated

from absorbance measured at 765 nm using a standard curve prepared with gallic

acid. Total phenolic content was expressed as mg gallic acid equivalent (GAE) /100

g FW lettuce.

N,N-Dimethylformamide was used for extraction in the determination of

chlorophyll content (Inskeep and Bloom, 1985). Absorbance values at 664 nm, 647

nm, and 625 nm were measured, and total chlorophyll content was calculated

according to the following formula:

Chlorophyll a = 12.81 x abs.664-2.16 x abs.647+1.44 x abs.625

Chlorophyll b = -4.93 x abs.664+26.01 x abs.647+3.74 x abs.625

Total Chlorophyll Content = Chlorophyll a + Chlorophyll b

All assays were conducted using duplicated samples.


Data was analyzed using SAS 9.3 (SAS Institute Inc. Cary, NC) with the

PROC GLIMMIX procedure. Fisher’s LSD test was used for multiple comparisons of

measurements among treatments (P ≤ 0.05).

78


Ascorbic Acid Content

In the 2011 trial, a significant difference was found in concentration of

ascorbic acid (AA) in lettuce was found between conservation tillage treatments (CC-

C-CT and CC-CT) and polyethylene mulched conventional tillage treatments (NCC-

T-M and CC-T-M). The AA concentration was significantly higher in the conservation

tillage treatments (Figure 4-1). As reported previously, the AA content increased as

a consequence of environmental stress such as wounding (Suza et al., 2010).

Conservation tillage preserved and enhanced biodiversity compared with

conventional tillage which might induce more potential stress, both for biotic stress

such as weed pressure, insects, and diseases and for abiotic stress such as nutrient

competition. In the 2011 (first year) trial, as reported in Chapter 2, the lettuce

produced with black polyethylene mulched treatments were significantly bigger in

size than those produced with conservation tillage treatments, indicating that black

polyethylene mulch created a more favorable environment for lettuce growth. With

less competition and other stress factors, the AA concentration level was significantly

decreased by the application of polyethylene mulch in the production system. No

difference was shown between NCC-T-M and CC-T-M. Similarly, the AA

concentration of lettuce in NCC-T did not differ from that in CC-T (Figure 4-1). It

indicated that the use of cover crops did not have a significant impact on the level of

ascorbic acid in lettuce at harvest in this study. Moreover, there was no difference

between CC-CT and CC-C-CT indicating that the extra amount of organic fertilizer

79

applied did not influence lettuce AA content (Figure 4-1). In the 2011 trial, there was

no difference in AA content between ‘Tropicana’ and ‘New Red Fire’ lettuce (Figure

4-2).

In the 2012 trial, no difference was found in AA content of lettuce among

different production system treatments regardless of the lettuce cultivar (Figure 4-3).

Total Phenolic Content

In the 2011 trial, the results showed that the total phenolic content was

significantly higher in CC-C-CT and CC-CT produced lettuce as compared with

lettuce in the polyethylene mulched treatments (CC-T-M and NCC-T-M) (Figure 4-4).

Conservation tillage has been shown to increase the level of total phenolics in

grapes (Bahar and Yaşasin, 2010). A variety of environmental conditions may affect

the accumulation of phenolics in plants (Liu et al., 2006). It is possible that the

conditions for promoting lettuce growth created by conventional tillage and

polyethylene much reduced total phenolics accumulation in lettuce. Total phenolic

content of lettuce did not differ significantly between CC-T and NCC-T treatments,

while similar levels were also found in CC-T-M and NCC-T-M treatments, suggesting

the little influence of cover crops (Figure 4-4). Nevertheless, cover crops may

improve lettuce growing conditions by suppressing weeds and managing soil fertility.

There have been inconsistent reports of negative impact caused by different

covering materials on the phenolic content in lettuce (Ordidge et al., 2010). This

inconsistency might result from different genotypes and climatic conditions as well as

different levels of biotic stress (Eichholz et al. 2012; Heinäaho et al., 2006). In the

80

2011 trial, significantly higher level of total phenolics was found in ‘New Red Fire’ in

contrast to ‘Tropicana’ (Figure 4-5).

In the 2012 trial, the results showed that different production systems did no

differ significantly in lettuce total phenolic content (Figure 4-6). The trend that higher

level of total phenolics in ‘New Red Fire’ compared with ‘Tropicana’ was repeated in

2012 trial (Figure 4-7). The results showed that genotype played an important role in

lettuce phenolic content level responding to cultivation practices. Further studies are

needed to test multiple lettuce cultivars in order to better understand the effects of

different production systems on phenolic compounds in lettuce.

Chlorophyll Content

In both years, no significant difference in lettuce chlorophyll content was found

among different production systems (Table 4-1 and 4-2). Previous studies showed

that the chlorophyll content in rice could be increased by using a cowpea cover crop

(Marenco and Santos, 1999), and the use of mulch could exhibit different effects on

chlorophyll content depending on the type of mulch material (Ghosh et al., 2006;

Wang et al., 1998). , Future research is needed to test the contribution of different

cover crops and mulches to lettuce growth and determine their effects on lettuce

chlorophyll content. The effects of tillage practices on chlorophyll content reported by

previous studies were also inconsistent. Some showed that zero tillage decreased

chlorophyll level and rotational tillage increased chlorophyll level (Hou et al., 2013;

Monneveux et al., 2006), whereas others demonstrated little influence of the type of

tillage on chlorophyll content (Grant et al., 2003).

81

Conclusions

In this study, the conservation tillage treatments increased the contents of

ascorbic acid and total phenolics in lettuce as compared with the black polyethylene

mulch treatments with conventional tillage during the first growing season. However,

no significant differences were found among different production systems in the

second year. The use of sunn hemp and sorghum-sudangrass bicultural cover crops

did not affect the levels of ascorbic acid and total phenolics in lettuce. ‘New Red Fire’

lettuce had significantly a higher total phenolic content than ‘Tropicana’ in both years.

Chlorophyll content of lettuce was not affected by the production system treatments

in either cultivar in this study. Given the large number of lettuce cultivars from

different types and the difference in production practices used by organic growers,

future studies involving multiple genotypes and site-specific conditions are warranted

to optimize the organic lettuce production systems for optimizing both plant growth

and accumulation of health-promoting phytochemicals.

82

Figure 4-1. Effects of different organic production systems on lettuce ascorbic acid

content in the 2011 trial. Values followed by the same letter do not differ

significantly by Fisher’s least significant difference test at P ≤ 0.05. CC-T-

M = treatment with cover crop, conventional tillage and plastic mulch;

NCC-T-M = treatment without cover crop, but with conventional tillage and

plastic mulch; NCC-T = treatment without cover crop, but with conventional

tillage; CC-T = treatment with cover crop and conventional tillage; CC-C-

CT = treatment with cover crop, standard fertilizer input, and conservation

tillage; CC-CT = treatment with cover crop, conservation tillage, and high

organic fertilizer input.

83

Figure 4-2. Ascorbic acid content in different lettuce cultivars in the 2011 trial.

84

Figure 4-3. Effects of different organic production systems on lettuce ascorbic acid

content (mg/100g FW) in the 2012 trial. CC-T-M = treatment with cover

crop, conventional tillage and plastic mulch; NCC-T-M = treatment without

cover crop, but with conventional tillage and plastic mulch; NCC-T =

treatment without cover crop, but with conventional tillage; CC-T =

treatment with cover crop and conventional tillage; CC-C-CT = treatment

with cover crop, standard fertilizer input, and conservation tillage; CC-CT =

treatment with cover crop, conservation tillage, and high organic fertilizer

input.

85

Figure 4-4. Effects of different organic production systems on lettuce total phenolic

content in the 2011 trial. Values followed by the same letter do not differ

significantly by Fisher’s least significant difference test at P ≤ 0.05. CC-T-

M = treatment with cover crop, conventional tillage and plastic mulch;

NCC-T-M = treatment without cover crop, but with conventional tillage and

plastic mulch; NCC-T = treatment without cover crop, but with conventional

tillage; CC-T = treatment with cover crop and conventional tillage; CC-C-

CT = treatment with cover crop, standard fertilizer input, and conservation

tillage; CC-CT = treatment with cover crop, conservation tillage, and high

organic fertilizer input.

86

Figure 4-5. Total phenolic content in different lettuce cultivars in the 2011 trial.

Values followed by the same letter do not differ significantly by Fisher’s

least significant difference test at P ≤ 0.05.

87

Figure 4-6. Effects of different organic production systems on lettuce total phenolic

content in the 2012 trial. CC-T-M = treatment with cover crop, conventional

tillage and plastic mulch; NCC-T-M = treatment without cover crop, but

with conventional tillage and plastic mulch; NCC-T = treatment without

cover crop, but with conventional tillage; CC-T = treatment with cover crop

and conventional tillage; CC-C-CT = treatment with cover crop, standard

fertilizer input, and conservation tillage; CC-CT = treatment with cover crop,

conservation tillage, and high organic fertilizer input.

88

Figure 4-7. Total phenolic content in different lettuce cultivars in the 2012 trial.

Values followed by the same letter do not differ significantly by Fisher’s

least significant difference test at P ≤ 0.05.

89

Table 4-1. Impact of different organic production systems on lettuce chlorophyll

content (μg/mg FW) in the 2011 trial

Cultivar

Tropicana New Red Fire

Treatment Chl a Chl b Total Chl Chl a Chl b Total Chl

CC-T-M ,,10.7 7.6 18.3 8.3 7.2 16.0

NCC-T-M 9.3 6.3 15.5 7.3 6.5 13.8

NCC-T 8.1 5.5 13.6 8.6 6.9 15.6

CC-T 9.3 6.2 15.5 7.4 6.1 13.4

CC-C-CT ,,10.0 8.4 18.3 9.3 7.4 16.7

CC-CT 9.8 6.8 16.6 8.9 7.0 15.9

Significance NS NS NS NS NS NS







NS: Nonsignificant.

90

Table 4-2. Impact of different organic production systems on lettuce chlorophyll

content (μg/mg FW) in the 2012 trial







NS: Nonsignificant.

Cultivar

Tropicana New Red Fire

Treatment Chl a Chl b Total Chl Chl a Chl b Total Chl

CC-T-M 9.9 5.7 15.6 6.9 5.5 12.5

NCC-T-M 8.9 4.7 13.5 8.2 5.1 13.3

NCC-T 7.7 4.1 11.8 8.4 5.3 13.8

CC-T 8.9 4.6 13.5 7.0 4.7 11.7

CC-C-CT 9.3 6.4 15.8 8.8 5.7 14.6

CC-CT 9.1 5.1 14.2 8.5 5.4 13.9

Significance NS NS NS NS NS NS

91

CHAPTER 5 SUMMARY

This two-year study conducted in north Florida examined the influence of six

different organic production systems on yield performance of lettuce, plant nutrient

accumulation, weed control, and levels of chlorophyll, ascorbic acid, and total phenolics

in lettuce (Lactuca sativa) at harvest. Growing sunn hemp (Crotalaria juncea) and

sorghum-sudangrass (Sorghum bicolor x S. bicolor var. sudanense) bicultural cover

crops prior to the fall lettuce season, use of polyethylene mulch, conventional tillage,

and reduced tillage (conservation tillage) were included in developing the six production

system treatments. The treatment effects were compared in two loose leaf lettuce

cultivars including ‘Tropicana’ (green leaf) and ‘New Red Fire’ (red leaf). Use of cover

crops did not show significant effects on lettuce nutrient accumulation in the

polyethylene mulch or bare soil treatments with conventional tillage. Use of polyethylene

mulch significantly improved lettuce yield in comparison with the bare soil treatments

with conventional tillage and the conservation tillage treatments. Cover crops did not

show significant effects on lettuce yield in the polyethylene mulch or bare soil

treatments with conventional tillage. Lettuce in the conservation tillage treatments

showed lower yield in contrast to the polyethylene mulch treatments. However,

increasing the organic fertilizer input in the conservation tillage system may help

improve lettuce yield. Use of polyethylene mulch may increase nitrogen, phosphorus,

sulfur, calcium, and magnesium accumulation in lettuce. A significant interaction effect

of production system and cultivar on lettuce yield performance was observed.

Cover crops used in this study could successfully suppress nutsedge and

broadleaf weeds. To achieve sufficient weed control, efforts should be made to optimize

92

cover crop establishment and growth. Conventional tillage may help reduce the

perennial grass weeds, while polyethylene mulch can be used to suppress broadleaf

weeds. Conservation tillage systems with cover crop residues retained as organic mulch

could be used to effectively control broadleaf weeds and nutsedge; however, perennial

grass weeds could become a problem in conservation tillage production.

Conservation tillage tended to increase ascorbic acid and total phenolic contents

in lettuce. Either cover crops or polyethylene mulch did not show significant effects on

the levels of ascorbic acid and total phenolics. The production system treatments did

not affect lettuce chlorophyll content. ‘New Red Fire’ tended to show a higher level of

total phenolics than ‘Tropicana’.

Cover crop management needs to be optimized in future studies to further

examine the effects of cover crops on growth and yield of the subsequent vegetable

crops. Further studies are needed to verify the effects of different production practices

and systems on yellow nutsedge and purple nutsedge control. Future studies are also

needed to determine the long-term impacts of different organic production systems on

vegetable production taken into consideration crop genotype, mulch use, and cover

crop mixture and management practices as well as site-specific conditions.

93

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BIOGRAPHICAL SKETCH

Yushen Huang was born in Baoji, China. He was raised by his grandparents until

high school. Both his grandfather and his father are interested in agriculture which

inspired him a lot. Yushen grew up loving the plants, animals and all of the natural

beauty. He graduated from Northwest A&F University in Yangling, China. After receiving

his B.S. in protected agriculture science and engineering, he planned to study in US and

he was accepted by University of Florida M.S. program in horticultural sciences in

Gainesville, FL.

comparing organic lettuce production practices: crop performance and weed...

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