glyphosate resistant weeds

TERMINATION REPORT

MISSISSIPPI SOYBEAN PROMOTION BOARD

FY 2007

Title: Strategies to Monitor and Control Glyphosate-resistant Weeds

Principal

Investigators: Daniel H. Poston, Delta Research and Extension Center

Clifford H. Koger, USDA-ARS Crop Production and Genetics Research Unit

David R. Shaw, Plant and Soil Sciences Department

Objectives:

1. Monitor and screen weed populations suspected of being glyphosate-resistant.

2. Develop management strategies for glyphosate-resistant and -susceptible

horseweed.

3. Determine the optimum time to control horseweed by documenting emergence

and growth patterns for horseweed at various locations in Mississippi.

4. Disseminate information to producers in a timely fashion.

Objective 1: Plant materials and seed of weeds suspected of being resistant to glyphosate

continue to be collected from various locations around the state. Glyphosate titrations in

the field and greenhouse as well as assay techniques, reported on in earlier reports, have

been used successfully to determine resistance in horseweed populations from across the

state. Glyphosate resistant horseweed has been documented as far south as Greenville,

MS and is suspected of being more widely disseminated throughout the state. More

complaints of glyphosate failing to control horseweed were reported this past growing

season and more alternative chemistries, mostly cloransulam, were used to control this

weed in soybean. Italian ryegrass populations with 3-fold tolerance to glyphosate have

been confirmed and alternative control methods are being evaluated. Glyphosate tolerant

Italian ryegrass could potentially pose a larger threat to crop production than glyphosate-

resistant horseweed because there are few affordable control alternatives. This raises

great concern because many herbicides that are added to glyphosate to control resistant

horseweed reduce the control of ryegrass.

Pigweed plants and seed were collected from several locations, but no confirmed cases of

pigweed or waterhemp resistance have been documented to date. Giant ragweed is

another weed that we suspect of developing increased tolerance to glyphosate in

Mississippi, however in many cases it may represent the encroachment of this weed into

fields from field borders and this weed was never effective controlled by glyphosate.

Reports of poor Johnsongrass control with glyphosate raised perhaps the most concern

among our research group. We investigate on location where glyphosate failed to control

Johnsongrass. Coincidentally, this occurred about the time that glyphosate-resistant

Johnsongrass was confirmed in Argentina. Rhizomes were collected from this location,

but we have been unable to confirm resistance at this location as of yet. Plans are in

place to closely monitor this location in 2007. Glyphosate-resistant Johnsongrass poses

perhaps the largest threat to date for production systems in Mississippi.

SEE ALSO ATTACHED MANUSCRIPT REPRINT.

Objectives 2

SEE ATTACHED DOCUMENTS.

Objective 3:

SEE ATTACHED DOCUMENTS.

Objective 4: To date, two journal articles associated with this project have been

accepted. In addition, one M.S. thesis has been completed and at least two articles will be

submitted within the next month. Findings from this project have been presented at

numerous meetings around the state, region, and nation including the DREC field day,

grower meetings, The Southern Weed Science Society annual meeting, The Weed

Science Society of America annual Meeting, The Beltwide Cotton Conference, The

Mississippi Joint Pest Management Conference, The Delta Ag Expo, and the Mississippi

Crop College. Findings have also been featured in the Delta Business Journal and other

popular press articles.

Appendix A

FIELD EMERGENCE OF HORSEWEED [Conyza canadensis (L.) Cronq.] AND

CONTROL UTILIZING TILLAGE AND HERBICIDES

By

Thomas William Eubank III

A Thesis

Submitted to the Faculty of

Mississippi State University

in Partial Fulfillment of the Requirements

for the Degree of Master of Science

in Weed Science

in the Department of Plant and Soil Sciences

Mississippi State, Mississippi

December 2006

FIELD EMERGENCE OF HORSEWEED [Conyza canadensis (L.) Cronq.] AND

CONTROL UTILIZING TILLAGE AND HERBICIDES

By

Thomas William Eubank III

Approved:

Daniel H. Poston David R. Shaw

Associate/Extension Professor of Giles Distinguished Professor of

Weed Science Weed Science

(Co-Major Professor) (Co-Major Professor)

D. Wayne Wells Daniel B. Reynolds.

Extension Professor Professor of Weed Science

(Minor Professor) (Committee Member)

Clifford H. Koger William Kingery

USDA Research Agronomist Graduate Coordinator of

(Committee Member) the Department of Plant & Soil

Sciences

Vance H. Watson

Dean of the College of Agriculture

and Life Sciences

Name: Thomas William Eubank III

Date of Degree: December 8, 2006

Institution: Mississippi State University

Major Field: Weed Science

Major Professor: Dr. Daniel H. Poston

Title of Study: FIELD EMERGENCE OF HORSEWEED [Conyza canadensis (L.)

Cronq.] AND CONTROL UTILIZING TILLAGE AND HERBICIDES

Pages in Study: 49

Candidate for Degree of Master of Science

Horseweed has been documented in many crops around the world and is

listed as being a problem weed in no-till production systems. Horseweed has developed

resistance to many herbicides including glyphosate. Field experiments were conducted

from 2004 to 2006 in the Mississippi Delta to evaluate the field emergence of horseweed

and various treatment programs for its control.

Field emergence of horseweed was observed occurring primarily in the fall of the

year, September through November, when temperatures were between 16 to 23 C with

later emergence occurring in January through April when temperatures were from 5 to 16

C. Tillage in September followed by herbicide in March gave 100% control of horseweed

across all locations. Glyphosate + 2,4-D and glyphosate + dicamba provided 90% or

better horseweed control 4 WAT both years. Glufosinate-based burndowns provided 81

to 97% horseweed control, and soybean yields were generally similar with all

glufosinate-based treatments.

ii

DEDICATION

I would like to dedicate this work to my wife (Beth Eubank) and children (Adelle

and Will Eubank). Your love, patience and support have allowed me to pursue my

dreams, and I thank you.

iii

ACKNOWLEDGEMENTS

I would like to express my sincere appreciation to Dr. Daniel Poston for allowing

me to further my education. Your patience and mentoring have enabled me to broaden

my horizons and brighten my future. I would also like to thank Drs. David Shaw, Dan

Reynolds, Trey Koger and Wayne Wells for their time in serving as members of my

graduate committee.

A special thanks to Russel Coleman, Brewer Blessitt, Hunter Doty, Troy Gibson,

Beth Graves, and Robert Wells who have all contributed to my research efforts in the

field, lab and office. I appreciate all your hard work.

iv

TABLE OF CONTENTS

Page

DEDICATION ......................................................................................................... ii

ACKNOWLEDGEMENTS...................................................................................... iii

LIST OF TABLES ................................................................................................... v

LIST OF FIGURES.................................................................................................. vi

CHAPTER

I. INTRODUCTION......................................................................................... 1

LITERATURE CITED ................................................................................. 9

II. FIELD EMERGENCE OF HORSEWEED AND IMPACT OF

HERBICIDE AND TILLAGE TIMING ON CONTROL.............................. 13

Abstract ........................................................................................................ 13

Introduction .................................................................................................. 14

Materials and Methods .................................................................................. 17

Horseweed Emergence ............................................................................ 18

Tillage and Herbicide Controls................................................................ 18

Results and Discussion.................................................................................. 19

Horseweed Emergence ............................................................................ 19

Tillage and Herbicide Controls................................................................ 21


III. GLYPHOSATE-RESISTANT HORSEWEED CONTROL USING

GLYPHOSATE-, PARAQUAT-, AND GLUFOSINATE-BASED

HERBICIDE PROGRAMS........................................................................... 31

Abstract ........................................................................................................ 31

Introduction .................................................................................................. 32

Materials and Methods .................................................................................. 35

Results and Discussion.................................................................................. 37


v

LIST OF TABLES

TABLE Page

2.1 Impact of tillage and herbicide applied at various timings on horseweed

control in May at 3 locations in the Mississippi Delta.................................... 30

3.1 Air temperature, relative humidity, and precipitation for 7 d prior and 7 d

after postemergence herbicide applications in 2005 and 2006........................ 46

3.2 Glyphosate-resistant horseweed control 2 and 4 WAT and plant density 7

WAT with glyphosate-based burndown programs ......................................... 47


WAT with paraquat-based burndown programs............................................... 48


WAT with glufosinate-based burndown programs......................................... 49

vi

LIST OF FIGURES

FIGURE Page

2.1 Horseweed emergence and average air temperature at Stoneville, MS from

September 2004 to May 2005. Mean horseweed emergence is plotted for the

first and last half of each month and the standard error of each mean is

affixed as an indicator of variability for each observation period.. ................. 26

2.2 Horseweed emergence and average air temperature at Lamont, MS (Lamont

location I) from September 2004 to May 2005. Mean horseweed emergence

is plotted for the first and last half of each month and the standard error of

each mean is affixed as an indicator of variability for each observation

period................................................................................................................ 27

2.3 Horseweed emergence and average air temperature at Lamont, MS (Lamont

location II) from October 2004 to May 2005. Mean horseweed emergence is

plotted for the first and last half of each month and the standard error of

each mean is affixed as an indicator of variability for each observation

period................................................................................................................ 28

2.4 Horseweed emergence and average air temperature averaged across 3

locations in the Mississippi Delta from September 2004 to May 2005. Mean

horseweed emergence is plotted for the first and last half of each month and

the standard error of each mean is affixed as an indicator of variability for

each observation period......................................................................................................... 29

1

CHAPTER I

INTRODUCTION

Glyphosate-resistant (GR) crops, such as cotton (Gossypium hirsutum L.),

soybean (Glycine max. (L.) Merr.), and corn (Zea mays L.), have revolutionized the

agricultural landscape allowing producers to control a broad spectrum of weed species

with in-season applications of glyphosate (Baylis 2000). The widespread adoption of GR

crops has made glyphosate the worlds leading agrochemical (EPA 2004) and has led to

the decreased utilization of tillage and alternative herbicides for the control of problem

weeds (Young 2006).

Horseweed, mares tail, Canada fleabane, hogweed, mule tail, and butterweed are

listed as being common names for Conyza canadensis (L.) Cronq. also listed as Erigeron

canadensis (Holm et al. 1997; Steyermark 1963; SWSS 2003). Erigeron comes from the

Greek language er, meaning spring, and geron for old man old man in spring which

refers to the plants hairy covering of plant parts (Bailey, 1951; Holm et al. 1997).

Horseweed was moved from the genus Erigeron to the genus Conyza (which is mainly

tropical) in recent years and the change has met with some reservations and thusly both

genus names are still used (Holm et al. 1997). Horseweed is in the family Asteraceae

(Compositae) with the genus and species being Conyza (syn. Erigeron) canadensis (L)

Cronq. and the species being separated into 3 varieties:

2

1) Var. canadensis is most abundant in the north and central U.S., and is

distinguished by stems with spreading pubescence.

2) Var. glabrata (Gray) Cronq. is most common in the southwest portion of the U.S.,

and is distinguished by nearly glabrous stems and stramineous phyllaries.

3) Var. pusilla (Nutt) Cronq. is most common in the southeast portion of the U.S.,

and is distinguished by nearly glabrous stems and tips of the phyllaries stained a

purplish-red (Correll and Johnston 1970; Radford et al. 1968).

Described as an erect winter or summer annual or biennial from 1 to 3 meters in height, a

solid stem nearly smooth or bristly hairy, unbranching at the base, branching near the

apex; leaves are alternate and numerous often appearing opposite, light to dark green in

color, sessile (without petioles), simple linear to oblanceolate, 2.5-10 cm long and 1-15

mm wide, entire, toothed or slightly lobed, more or less pubescent; roots are described as

a long taproot or fibrous; flowers are numerous small heads arranged in an elongated

panicle, white to lavender ray flowers 1-2 mm long, disk flowers yellow; fruit is an

achene, cylindrical, elongate, broadest above the middle, surface longitudinally grooved,

somewhat tapering from base to apex with numerous slender white bristles, yellowish

brown in color with scattered white hairs, pappus of 10-25 bristles, 2-2.3 mm long;

seedling cotyledons are green, lacking evident veins, smooth, early leaves usually entire

and later leaves toothed with a short apical projection, blades green on upper surface and

light green on lower smooth surface; crushed leaves and other plant parts are said to have

the faint odor of carrots (Correll and Johnston 1970; Holm et al. 1997; Radford et al.

1968; SWSS 2003; Uva et al. 1997). Horseweed reproduces only via seed production

3

and in the northern temperate zone flowering begins in July and August and ends in

October to early November (Holm et al. 1997). Horseweed can produce many wind-

disseminated seeds, Estimates of seed production for horseweed average 50,000

seeds/plant, with some plants producing over 200,000 seeds (Bhowmik and Bekech 1993;

Holm et al. 1997). Horseweed is capable of traveling great distances and is aided in

dispersal by means of a small achene and pappus (Dauer et al. 2006; Bhowmik and

Bekech 1993). Some research has shown allelophathic-like properties of horseweed

where poor seedling growth being reported where residue of previous horseweed plants

were decaying in the soil, with the effects lasting up to 2 months (Holm et al. 1997).

Horseweed has been reported as being used as a tonic, astringent, and a diuretic

(Steyermark 1963). The whole plant of horseweed is used as a folk medicine in China

(Rustaiyan 2004). Horseweed decoction is indicated as an inhibitor of the growth of

bacteria and common mold (Rustaiyan 2004). The oil from horseweed can prevent

allergic diarrhea of children to milk (Rustaiyan 2004).

Horseweed has been documented in 70 countries and 40 different crops around

the world (Holm et al. 1997) and is listed as being a problematic weed in cotton, grain

sorghum (Sorghum bicolor (L.) Moench), corn, and soybean (Brown and Whitwell 1988;

Buhler and Owen 1997; Mueller 2003; Vencill and Banks 1994; Wiese et al 1997). Since

1993, horseweed in North America has developed resistance to photosystem II inhibitors,

bipyridiliums, acetolactate synthase (ALS) inhibitors, ureas, amides, and most recently,

glycines (Heap 2006; Koger et al. 2004; Smisek et al. 1998; Trainer et al. 2005;

VanGessel 2001; Weaver et al. 2004). Smisek et al. (1998) reported on the discovery of

4

paraquat-resistant horseweed in Ontario, Canada, where paraquat had been applied 4 to 5

times per year for a 10-year period and exhibited resistance levels 25 to 35 times higher

than susceptible populations (Smisek et al. 1998). These paraquat-resistant plants were

also found to have a 7-fold resistance to diquat and a 3-fold resistance to linuron (Weaver

et al. 2004). In Delaware, after three years of a glyphosate-only no-till production system,

glyphosate failed to control horseweed (VanGessel 2001). Seeds were collected from

suspected resistant plants and grown in a green house and were found to exhibit a 8- to

13-fold glyphosate resistance compared to susceptible horseweed populations (VanGessel

2001). Currently glyphosate-resistant populations of horseweed have been reported in

several states across the United States including Arkansas, Tennessee, and Mississippi

(Heap 2006; Koger et al. 2004; VanGessel 2001). Horseweed is the first annual broadleaf

documented as resistant to glyphosate (VanGessel 2001), and is considered one of the top

10 worst weeds among herbicide-resistant biotypes (Heap 2006). Research focusing on

the mechanism of resistance has shown that resistance does not appear to be based on the

differential uptake of glyphosate, metabolism, differential gene expression of a specific

5-enolpyruvylshikimate-3-phosphonate synthase (EPSP), or amplification EPSP synthase

(Feng et al. 2004). However, a difference was noted between resistant and susceptible

plants in that similar amounts of glyphosate were taken up into the leaf by each plant but

the susceptible plants translocated approximately 2-fold more glyphosate into the roots

(Feng et al. 2004; Koger and Reddy 2005).

Horseweed is a robust, tap-rooted plant, is classified as a winter annual (Brown

and Whitwell 1988), and is a problem weed in no-tillage production systems (Barnes et al

5

2004; Bruce and Kells 1990). Ecologists consider horseweed a successional winter

annual that can rapidly inhabit abandoned fields (Regehr and Bazzaz 1979). Winter

annuals typically germinate in the fall, remain vegetative throughout the winter months,

and produce seed in the spring to early summer. Depending upon the region, research has

shown that horseweed may emerge from early fall through late spring (Bhowmik and

Bekech 1993; Buhler and Owen 1997). Saphangthong and Witt (2006) reported

germination occurring throughout the year when seed were planted. Horseweed also can

germinate over a wide range of temperature and environmental extremes (Nandula et al.

2006). VanGessel (2001) notes that understanding the biology and ecology of horseweed

is essential for developing effective management strategies.

Horseweed is considered one of the most common and troublesome winter annual

weeds in no-tillage production systems (Bruce and Kells 1990). Horseweed is commonly

found in undisturbed areas such as abandoned fields, right of ways, and fallow areas and

has been listed as a problem weed in conservation and no-tillage systems (Brown and

Whitwell 1988; Bruce and Kells, 1990; Keeling et al. 1989). The elimination of tillage

practices in agricultural fields creates an environment similar to abandoned fields, where

horseweed can thrive, because of the lack of annual soil disturbance (Buhler and Owen

1997; Vencill and Banks 1994). Reduced-till production systems are well suited for the

growth and development of horseweed. Kapusta (1979) reports a field being observed as

a solid stand of horseweed in the first year of a no-tillage system following 20 years of a

conventional tillage system free of horseweed. Much of the resistant populations

identified to date occur in areas of the country where no-till practices have been widely

6

adopted (Koger et al. 2004; Main et al. 2004; VanGessel 2001). Studies conducted in

Alabama found that shallow fall tillage, in the form of disking 7 to 10 cm in depth,

sufficiently disturbed soil enough to prevent horseweed survival in each year of a 3 year

study (Brown and Whitwell 1988). Tillage can be effective in the removal of horseweed

in the fall (Brown and Whitwell 1988), however the potential for germination after a

tillage event is possible (based on current findings), and a single tillage event in the fall

may not be sufficient in controlling later-emerging horseweed that emerge from seed

already in the seedbank or that blow in from other locations. No-tillage systems are not

widely embraced in Mississippi, especially in the Delta, where tillage is often required in

the fall of the year to remove implement ruts and prepare fields for the upcoming

production season. However, reduced tillage practices are gaining popularity in the

Midsouth and will likely continue to increase as fuel and production costs continue to

rise.

Chemical control of horseweed had been successful with glyphosate prior to

2001. Studies by Wilson and Worsham (1988) found that 0.6 kg ae/ha of glyphosate gave

75% control of 15 to 46 cm tall horseweed and 1.1 kg ae/ha gave 94% control 4 WAT.

Similar findings have been observed where 0.4 kg ae/ha glyphosate gave 83 to 100%

control of 10 to 15 cm tall horseweed 4 WAT (Keeling et al. 1989) and 87 to 93% control

of horseweed 1 to 6 cm in height 6 WAT (Bruce and Kells 1990). Scott et al. (1998)

stated that at least 1.12 kg ae/ha glyphosate was required to control horseweed at least

92% when it was 20 to 25 cm in height, and VanGessel et al. (2001) observed 100%

control of 15 to 30 cm horseweed with 0.8 kg ae/ha glyphosate in 1998. Reduced

7

glyphosate absorption associated with mixing contact herbicides with glyphosate has

been well documented (Hydrick and Shaw 1994; Norris et al. 2001; Starke and Oliver

1998), and research is needed to determine the most efficacious tank mix partners for

glyphosate-based burndowns for the control of horseweed. Control of horseweed with

paraquat alone has proven difficult. Bruce and Kells (1990) reported paraquat at 0.56 kg

ai/ha with a carrier volume of 210 L/ha gave only 15 and 60% control when applied to

horseweed 6 to 8 cm in height. Similar findings were reported in a study by Keeling et al.

(1989) where 0.6 kg ai/ha paraquat in 140 L/ha gave 67 and 53% control of 3 to 4 cm

diameter horseweed 4 WAT; however, control dropped to 40 and 30% 12 WAT. A

broadleaf herbicide that has shown excellent activity on horseweed is 2,4-D (Bruce and

Kells 1990; Keeling et al. 1989; Moseley and Hagood 1990). A study by Wiese et al.

(1995) found that 2,4-D ester at 0.56 kg ai/ha gave 100% control of 30 cm tall

horseweed, but only 47% control of plants in rosette stage at 5 cm in height. The addition

of 2,4-D to paraquat to aid in the control of horseweed has proven erratic. Wilson and

Worsham (1988) found 0.6 kg ai/ha paraquat gave only 74% control of 18 cm tall

horseweed, but the addition of 0.6 kg ai/ha 2,4-D improved control to 92%. When the

same treatments were applied to 15 to 46 cm tall horseweed, however, control was only

58% and 61%, respectively. Moseley and Hagood (1990) found that the addition of 2,4-D

amine did not significantly improve control over paraquat alone. Glufosinate is a contact

herbicide with limited systemic activity (WSSA 2002). Glufosinate has shown promise as

an alternative to glyphosate for the control of horseweed, but control has been

inconsistent (Talbert et al. 2004). Some studies have indicated poor control with

8

glufosinate when high plant densities are present at application (Steckel et al. 1997;

Tharp and Kells 2002). Horseweed, especially glyphosate-resistant horseweed, pose a

threat to production agricultural in the Midsouth and control options need to be

developed to prevent potential losses in yield and increased production costs.

9

LITERATURE CITED

Bailey, L. H. 1951. Compositae, Erigeron in Manual of Cultivated Plants. Macmillan.

1007 p.

Barnes, J., B. Johnson, K. Gibson, and S. Weller. 2004. Crop rotation and tillage system

influence late-season incidence of giant ragweed and horseweed in Indiana soybean.

Online. Crop Management doi:10.1094/CM-2004-0923-02-BR.

Baylis, A. D. 2000. Why glyphosate is a global herbicide: strengths, weaknesses, and

prospects. Pest Manag. Sci. 56:299-308.

Bhowmik, P. C. and M. M. Bekech. 1993. Horseweed (Conyza canadensis) seed

production, emergence, and distribution in no-tillage and conventional-tillage corn

(Zea mays). Agronomy (Trends in Agricultural Science) 1:67-71.

Brown, S. M. and T. Whitwell. 1988. Influence of tillage on horseweed, Conyza

canadensis. Weed Technol. 2:269-270.

Bruce, J. A. and J. J. Kells. 1990. Horseweed (Conyza canadensis) control in no-tillage

soybeans (Glycine max) with preplant and preemergence herbicides. Weed Technol.

4:642-647.

Buhler, D. D. and M. D. K. Owen. 1997. Emergence and survival of horseweed (Conyza

canadensis). Weed Sci. 45:98-101.

Correll, D. S., and M. C. Johnston. 1970. Conyza L. in Manual of the Vascular Plants of

Texas. Texas Research Foundation. 160-1608 p.

Dauer, J. T., D. A. Mortensen, and R. Humston. 2006. Controlled experiments to predict

horseweed (Conyza canadensis) dispersal distances. Weed Sci. 54:484-489.

Feng, P. C., M. Tran, T. Chiu, R. D. Sammons, G. R. Heck, and C. A. CaJacob. 2004.

Investigation into glyphosate-resistant horseweed (Conyza canadensis): retention,

uptake, translocation, and metabolism. Weed Sci. 52:498-505.

Heap, I. 2006. The International Survey of Herbicide Resistant Weeds.

www.weedscience.com

Holm, L., J. Doll, E. Holm, J. Pancho, and J. Herberger. 1997. Conyza canadensis (L.)

Cronq. (syn. Erigeron canadensis L.) in World Weeds: Natural Histories and

Distributions. John Wiley & Sons, Inc. 226-235 p.

10

Hydrick, D. E. and D. R. Shaw. 1994. Effects of tank-mix combinations of non-selective

foliar and selective soil-applied herbicides on three weed species. Weed Technol.

8:129-133.

Kapusta, G. 1979. Seedbed tillage and herbicide influence on soybean (Glycine max)

weed control and yield. Weed Sci. 27:520-526.

Keeling, J. W., C. G. Henniger, and J. R. Abernathy. 1989. Horseweed (Conyza

canadensis) control in conservation tillage cotton (Gossypium hirsutum). Weed

Technol. 3:399-401.

Koger, C. H. and K. N. Reddy. 2005. Role of absorption and translocation in the

mechanism of glyphosate resistance in horseweed (Conyza canadensis). Weed Sci.

53:84-89.

Koger, C. H., D. H. Poston, R. M. Hayes, and R. F. Montgomery. 2004. Glyphosate-

resistant horseweed (Conyza canadensis) in Mississippi. Weed Technol. 18:820-825.

Main, C. L., T. C. Mueller, R. M. Hayes, and J. B. Wilkerson. 2004. Response of selected

horseweed (Conyza canadensis (L.) Cronq.) populations to glyphosate. J. Agric. Food

Chem. 52:879-883.

Moseley, C. M., and E. S. Hagood. 1990. Horseweed (Conyza canadensis) control in full-

season no-till soybeans (Glycine max). Weed Technol. 4:814-818.

Mueller, T. C., J. H. Massey, R. M. Hayes, C. L. Main, and C. N. Stewart. 2003.

Shikimate accumulates in both glyphosate-sensitive and glyphosate-resistant

horseweed (Conyza canadensis L. Cronq.). J. Agric. Food Chem. 51:680-684.

Nandula, V. K., T. W. Eubank, D. H. Poston, C. H. Koger, and K. N. Reddy. 2006.

Factors affecting germination of horseweed (Conyza canadensis). Weed Sci. 54:898-

902.

Norris, J. L. D. R. Shaw, and C. E. Snipes. 2001. Weed control from herbicide

combinations with three formulations of glyphosate. Weed Technol. 15:552-558.

Radford, A. E., H. E. Ahles, and C. R. Bell. 1968. Erigeron: in Manual of the Vascular

Flora of the Carolinas. University of North Carolina Press. 1067-1070 p.

Regehr, D. L. and F. A. Bazzaz.1979. The population dynamics of Erigeron canadensis,

a successional winter annual. J. Ecol. 67:923-933.

Rustaiyan, A., P. A. Azar, M. Moradlizadeh, S. Masoudi, and N. Ameri. 2004. Volatile

constituents of three Compositae herbs: Anthemis altissima L. var. altissima, Conyza

11

canadensis (L.) Cronq. and Grantina aucheri Boiss. growing wild in Iran. Journal of

Essential Oil Research. 16:579-581.

Saphangthong, T. and W. W. Witt. 2006. Germination of horseweed (Conyza canadensis

L.) under field conditions. In W. Vencill (ed.) 59th Proc. South. Weed Sci. Soc., San

Antonio, TX. 23 25 Jan. 2006. South. Weed Sci. Soc., Champaign, IL.

Scott, R., D. R. Shaw, and W. L. Barrentine. 1998. Glyphosate tank mixtures with SAN

582 for burndown or postemergence applications in glyphosate-tolerant soybean

(Glycine max). Weed Technol. 12:23-26.

Smisek, A., C. Doucet, M. Jones, and S. Weaver. 1998. Paraquat resistance in horseweed

(Conyza canadensis) and Virginia pepperweed (Lepidium virginicum) from Essex

County, Ontario. Weed Sci. 46:200-204.

Starke, R. J. and L. R. Oliver. 1998. Interaction of glyphosate with chlorimuron,

fomesafen, imazethapyr, and sulfentrazone. Weed Sci. 46:652-660.

Steckel, G. J., L. M. Wax, F. W. Simmons, and W. H. Phillips. 1997. Glufosinate

efficacy on annual weeds is influenced by rate and growth stage. Weed Technol.

11:484-488.

Steyermark, J. A. 1963. Compositae, Erigeron in Flora of Missouri. Iowa State

University Press. 1529 p.

[SWSS] Southern Weed Science Society. 2003. Weed Identification Guide. Champaign,

IL: Southern Weed Science Society.

Talbert, R. E., M. R. McClelland, J. L. Barrentine, K. L. Smith, and M. B. Kelley. 2004.

Managing glyphosate-resistant horseweed in Arkansas cotton. Fayetteville, AR.

University of Arkansas Division of Agriculture. Research Series 530.

Tharp, B. E., and J. J. Kells. 2002. Residual herbicides used in combination with

glyphosate and glufosinate in corn (Zea mays). Weed Technol. 16:274-281.

Trainer, G. D., M. M. Loux, S. K. Harrison, and E. Regnier. 2005. Response of

horseweed biotypes to foliar applications of cloransulam-methyl and glyphosate.

Weed Technol. 19:231-236.

[EPA] United States Environmental Protection Agency. 2004. Office of Prevention,

Pesticides, and Toxic Substances (7503C) EPA-733-R-04-001.

www.epa.gov/pesticides. May.

12

Uva, R. H., J. C. Neal, and J. M. DiTomaso. 1997. Dicots: Asteraceae = Compositae:

Horseweed in Weeds of the Northeast. Cornell Univ. Press. 136-137 p.

VanGessel, M. J., A. O. Ayeni, and B. A. Majek. 2001. Glyphosate in double-crop no-till

glyphosate-resistant soybean: Role of preplant applications and residual herbicides.


VanGessel, M. J. 2001. Rapid Publication. Glyphosate-resistant horseweed from

Delaware. Weed Sci. 49:703-705.

Vencill, W. K. and P. A. Banks. 1994. Effects of tillage systems and weed management

on weed populations in grain sorghum (Sorghum bicolor). Weed Sci. 42:541-547.

Weaver, S., M. Downs, and B. Neufeld. 2004. Response of paraquat-resistant and

susceptible horseweed (Conyza canadensis) to diquat, linuron, and oxyfluorfen. Weed

Sci. 52:549-553.

Weed Science Society of America. 2002. Herbicide Handbook. 8th ed. W. K. Vencill, ed.

Lawrence, KS: Weed Science Society of America. 229 p.

Wiese, A. F., C. D. Salisbury, and B. W. Bean. 1995. Downy brome (Bromus tectorum),

jointed goatgrass (Aegilops cylindrica) and horseweed (Conyza canadensis) control in

fallow. Weed Technol. 9:249-254.

Wilson, J. S. and A. D. Worsham. 1988. Combinations of nonselective herbicides for

difficult to control weeds in no-till corn, Zea mays, and soybeans, Glycine max. Weed

Sci. 36:648-652.

Young, B. G. 2006. Changes in herbicide use patterns and production practices resulting

from glyphosate-resistant crops. Weed Sci. 20:301-307.

13

CHAPTER II

FIELD EMERGENCE OF HORSEWEED AND IMPACT OF HERBICIDE AND

TILLAGE TIMING ON CONTROL

Abstract

Experiments were conducted at 3 locations in the Mississippi Delta to determine

time of field emergence for horseweed in relation to temperature. Studies were initiated

in September 2004 and terminated in May 2005. Averaged across locations, horseweed

emerged primarily in the fall of the year, September through early November, when

temperatures ranged from 16 to 23 C with subsequent emergence occurring from late

January through early April when temperatures ranged from 5 to 16 C. No emergence to

very low emergence was observed from late November through early March. Limited

emergence could possibly be due to the lower temperatures observed during this time

from a high of 13 C to a low of 3 C. Experiments were also conducted to evaluate

horseweed control utilizing tillage or glufosinate applied in September, November, or

March. Tillage in September followed by (fb) glufosinate in March was included as a

comparison treatment because fall tillage fb chemical weed removal in the spring is a

common practice in the Mississippi Delta. Tillage in September fb glufosinate in March

provided 100% control of horseweed across all locations. Horseweed control with

glufosinate in March and tillage in March was comparable. Percent horseweed

groundcover in May averaged across locations was 0, 4, and 8% for tillage in September

14

fb glufosinate in March, glufosinate in March, and tillage in March, respectively.

Horseweed control with tillage in November and glufosinate in November was

comparable to tillage in September fb glufosinate in March at only 1 and 2 of 3 locations,

respectively. Tillage in November failed to completely kill emerged horseweed at one

location and new emergence following November tillage occurred at all locations.

Nomenclature: Glyphosate, glufosinate; Conyza canadensis (L.) Cronq. #1 ERICA.

Additional index words: glyphosate resistance, weed resistance, burndown, herbicide

mixtures, herbicide efficacy, yield reduction.

Introduction

Glyphosate-resistant (GR) crops have revolutionized the agricultural landscape,

allowing producers to control a broad spectrum of weed species with in-season

applications of glyphosate. The widespread adoption of GR crops has led to increased use

of glyphosate and decreased use of tillage and alternative herbicides for the control of

problem weeds (Young 2006). Tillage practices have been utilized for decades for the

control of troublesome weeds in production agriculture, however, concerns over soil

erosion and increased production costs associated with tillage have prompted many

producers to adopt no-till practices. Reduced- and no-tillage production systems may

promote many weed species such as hemp dogbane (Apocynum cannabinum) (Webster et

al. 2000), redvine (Brunnichia ovata) (Elmore et al. 1995), green foxtail (Setaria viridis

(L.) Beauv.), redroot pigweed (Amaranthus retroflexus L.), and horseweed (Conyza

canadensis (L.) Cronq.) (Buhler, 1992). Glyphosate has been useful in the control of

1 Letters following this symbol are a WSSA-approved computer code from Composite List of Weeds,

Revised 1989. Available only on computer disk from WSSA, 810 East 10th

Street, KS 66044-8897.

15

many weeds associated with no-tillage practices, however, after three years of a

glyphosate-only no-till production system, glyphosate failed to control horseweed in

Delaware (VanGessel 2001). Currently glyphosate-resistant populations of horseweed

have been reported in several states across the United States including Arkansas,

Tennessee, and Mississippi (Heap 2006; Koger et al. 2004; VanGessel 2001).

Horseweed is a robust, tap-rooted member of the Asteraceae (Compositae) family

and is classified as a winter annual (Brown and Whitwell 1988). Winter annuals typically

germinate in the fall (August through November), remain vegetative throughout the

winter (December through February), and produce seed in the spring (March through

May) to early summer (June and July). Depending upon the region, horseweed typically

emerges in the fall and to a lesser extent in the spring (Bhowmik and Bekech 1993;

Buhler and Owen 1997). Saphangthong and Witt (2006) reported germination occurring

throughout the year when seed were planted in Kentucky. Horseweed can produce many

wind-disseminated seeds. Estimates of seed production for horseweed average 50,000

seeds/plant, with some plants producing over 200,000 seeds (Bhowmik and Bekech 1993;

Holm et al. 1997). Horseweed also can germinate over a wide range of temperature and

environmental extremes (Nandula et al. 2006). VanGessel (2001) noted that

understanding the biology and ecology of horseweed is essential for developing effective

management strategies. Determining when horseweed emerges in the Mississippi Delta is

imperative in determining the best timings for control.

Horseweed is considered one of the most common and troublesome winter annual

weeds in no-tillage production systems (Bruce and Kells 1990). Horseweed is commonly

16

found in undisturbed areas such as abandoned fields, right of ways, and fallow areas and

has been listed as a problem weed in conservation and no-tillage systems (Brown and

Whitwell 1988; Bruce and Kells, 1990; Keeling et al. 1989). The elimination of tillage

practices in agricultural fields creates an environment similar to abandoned fields, where

horseweed can thrive, because of the lack of annual soil disturbance (Buhler and Owen

1997; Vencill and Banks 1994). Reduced-till production systems are well suited for the

growth and development of horseweed. Kapusta (1979) reported a field being observed

as a solid stand of horseweed in the first year of a no-tillage system following 20 years of

a conventional tillage system free of horseweed. Much of the resistant populations

identified to date occur in areas of the country where no-till practices have been widely

adopted (Koger et al. 2004; Main et al. 2004; VanGessel 2001). Tillage is effective in the

removal of horseweed in the fall (Brown and Whitwell 1988), however the potential for

germination after a tillage event is possible (based on current findings), and a single

tillage event in the fall may not be sufficient in controlling later-emerging horseweed that

emerge from seed already in the seed bank or that blow in from other locations. No-

tillage systems are not widely utilized in Mississippi, especially in the flat, alluvial flood

plain of the Mississippi Delta, where tillage is often required in the fall of the year to

remove implement ruts and prepare fields for the upcoming production season. However,

reduced tillage practices are gaining popularity in Mississippi and will likely increase as

the use of herbicide-resistant crops increases. Research is needed to determine emergence

patterns and the optimum timing for effective control of horseweed by either tillage or

herbicide means in preventing this weed from competing with subsequent crops.

17

The objectives of this study were to observe field emergence of horseweed in

Mississippi and to evaluate the influence of tillage and herbicide timing on control of

horseweed. Little data are available relative to emergence and control of horseweed in the

mid-South.

Materials and Methods

Experiments were conducted at 3 locations in the Mississippi Delta to monitor

horseweed emergence and evaluate the impact of tillage and herbicides applied at various

timings on horseweed control. One study was conducted at the Delta Research and

Extension Center in Stoneville, Mississippi and two studies were conducted near Lamont

(Lamont I and Lamont II), Mississippi. Studies at Stoneville and Lamont I were initiated

in early September 2004, and Lamont II was initiated in early October 2004. The soil at

Stoneville was a Sharkey clay (very-fine, smectitic, thermic Chromic Epiaquerts) with a

pH of 7.1 and 2.8% organic matter content. The soil at both sites in Lamont was a

Dundee very fine sandy loam (fine-silty, mixed, active, thermic Typic Endoaqualfs) with

a pH of 6.2 and organic matter content of 1.2%. Plots at all locations were established

following no-tillage soybean and were naturally infested with horseweed. No crops were

planted for the duration of the study.

Horseweed Emergence. Emergence of horseweed was monitored every two weeks by

counting all emerged plants in a 1-m2 plot with eight replications per location. Horseweed

emergence was monitored from the time plots were initiated until June the following

18

year. After emergence counts were recorded, emerged plants in each replication were

controlled with 0.56 kg ai/ha glufosinate. Glufosinate was applied with a CO2-pressurized

sprayer calibrated to deliver 140 L/ha at 207 kPa using flat fan nozzles2. Daily averaged

air temperature data were obtained from weather stations maintained by the Delta

Agricultural Weather Center located at the Delta Research and Extension Center.

Weather stations were located approximately eight km from Lamont and one km from

Stoneville. Mean horseweed emergence was plotted for the first and last half of each

month. Standard errors of means were affixed around each mean as an indicator of

variability for a particular emergence period.

2 TeeJet XR 11002VS. Spraying Systems Co., Wheaton, IL 60189.

19

Tillage and Herbicide Controls. Tillage (disk) or herbicide (glufosinate) was applied in

September, November, or March. Tillage in September fb glufosinate in March was

included as a comparison treatment because fall tillage fb chemical weed removal in the

spring is a commonly use practice in the Mississippi Delta. A nontreated control was also

included. Tilled plots were disked once. Herbicide plots were treated with 0.56 kg ai/ha

glufosinate applied with a CO2-pressurized sprayer calibrated to deliver 140 L/ha at 207

kPa using regular flat fan nozzles. Plots were 3 x 12 m with 4 replications. The

experimental design was a randomized complete block with four replications. Visual

ratings were made in May 2005 and expressed as percent horseweed groundcover. Plant

densities were taken by counting all plants in a 1-m2 grid and expressed as plants/m

2.

Data were subjected to analyses of variance and means separated using Fishers LSD (P =

0.05). Data were pooled across locations only when appropriate.

Results and Discussion

Horseweed Emergence. Horseweed emergence at Stoneville was greatest in late October

(90 plants/m2) through early November (20 plants/m

2), with some emergence noted in

late January (1 plant/m2), and early and late February (5.5 plants/m

2 and 4 plants/m

2,

respectively) (Figure 2.1). Emergence occurred within a temperature range from 5 to 23

C although emergence was greatest during the higher temperatures of late October and

early November when temperatures ranged from 15 to 23 C. Horseweed emergence at

20

Lamont I was greatest from late September (97 plants/m2) through late October (58

plants/m2), with lesser emergence occurring in late March (10 plants/m

2) and early April

(5 plants/m2)(Figure 2.2). Temperatures during observed emergence ranged from 13 to 21

C with greatest emergence being seen when temperatures were highest in late September

and late October. Peak emergence occurred at Lamont II in early and late October 2004

(130 and 51 plants/m2) and some horseweed emergence occurring in late March and early

April (2 and 1 plants/m2)(Figure 2.3). Temperature ranges for emergence were from 13 to

21 C with greatest emergence occurring in early to late October 2004 with corresponding

temperatures from 18 to 21 C, respectively. Averaged across all locations, horseweed

emerged primarily in the fall of the year, September 2004 through early November 2004,

when temperatures were between 16 to 23 C with subsequent emergence occurring from

late January through early April of 2005 with temperatures ranging from 5 to 16 C

(Figure 2.4). These field emergence findings compared well with the findings of Nandula

et al. (2006), where peak germination of horseweed occurred when air temperatures were

from 20 to 24 C in growth chamber studies. No emergence to very low emergence was

observed from late November 2004 through early March 2005 at all locations and could

possibly be due to the lower temperatures observed during this time from a high of 13 C

to a low of 4 C, which supports the findings of Nandula et al. (2006) where no

germination of horseweed was observed when temperatures were below 12 C. The

extreme variance in emergence in the months of September and October are thought to be

due to the availability of moisture during these typically drier months (8.6 and 8.0 cm

precipitation, respectively) in the Mississippi Delta3.

3 30-year average rainfall for the months of September and October in Stoneville MS

21

These results support the findings of others (Bhowmik and Bekech 1993; Buhler

and Owen 1997), where horseweed germinated primarily in the fall and to a lesser extent

in spring; however, the densities observed in Mississippi are much lower than those

found in Massachusetts, Minnesota, and Iowa that approached 1,000 plants/m2.

Saphangthong and Witt (2006) found that horseweed could emerge at any time of year

given suitable environmental conditions. In Mississippi, the majority of viable horseweed

seed emerges in the fall (August through November) when moisture and temperature

requirements are suitable for germination. Spring emergence of horseweed may occur

given there are viable seed remaining near the soil surface and environmental conditions

are suitable for emergence. Based on personal observations in producer fields, spring

emergence tends to be greater in years when rainfall is limited the previous fall. Under

these conditions, few seed germinate in the fall due to lack of moisture, and a large

percentage of seed germinate in the spring when temperatures reach optimal levels for

emergence.

Tillage and Herbicide Controls. Location by treatment interactions were detected for

percent groundcover (Table 2.1). Therefore, these data are presented separately by

location. No such interactions were detected for horseweed density. Therefore, these data

were pooled over location.

Tillage in September fb glufosinate in March provided 100% control of

horseweed at all locations (Table 2.1). Horseweed control with glufosinate in March and

tillage in March was comparable. Percent horseweed groundcover at Stoneville was 5, 6,

22

and 0% for glufosinate in March, tillage in March, and tillage in September fb glufosinate

in March, respectively. Horseweed control with tillage in November was comparable to

glufosinate or tillage in March, but not as effective as November tillage fb glufosinate in

March. Glufosinate in September, glufosinate in November, and tillage in September

were the least effective treatments at Stoneville. Percent horseweed groundcover for these

treatments was 70, 43, and 33%, respectively, compared to 73% with the nontreated

control. Stoneville was the only site where fall tillage had a distinct advantage over fall

herbicide applications. Horseweed emerged later in the fall at this site, probably due to

lack of rainfall, and tillage effectively removed any horseweed plants that were present

when tillage was applied. The soil at Stoneville was a heavy clay soil compared to

lighter-textured soils at the other two sites. Therefore, burial of horseweed seed by tillage

may be more effective on heavier-textured soil types where emergence may be more

restricted. Nandula et al. (2006) reported no horseweed emergence from a depth of 0.5

cm in a Bosket sandy loam, fine-loamy, mixed, thermic Mollic Hapludalfs soil. Tillage in

September fb glufosinate in March was the only treatment at Lamont I that reduced

percent horseweed groundcover significantly compared to the nontreated control.

Groundcover ranged from 0 to 11% at the Lamont II location with the best treatments,

which were September tillage fb glufosinate in March, glufosinate in March, glufosinate

in November, and March tillage. The least effective treatments at Lamont II were tillage

or glufosinate in September and tillage in November, all of which did not reduce

groundcover significantly compared to the nontreated control Glufosinate applied in

November was more effective than November tillage at Lamont II. November tillage, at

23

Lamont II, did not effectively kill all horseweed plants and many plants recovered

subsequent to treatment. In contrast, glufosinate provided essentially complete control of

emerged plants. Horseweed emerged earlier at Lamont locations compared to Stoneville

and these older plants were more difficult to control completely with tillage.

Based on plant counts, September tillage fb glufosinate in March removed100%

of horseweed plants. March tillage, glufosinate in March, glufosinate in November, and

November tillage treatments all reduced plant populations to levels similar to September

tillage fb glufosinate in March.

Tillage or non-residual herbicides like glufosinate applied in the fall will likely

not provide consistent control of spring-emerging horseweed. However, fall herbicide

programs that include residual herbicides to prevent subsequent horseweed flushes may

be effective (data not presented). Spring-applied herbicide treatments or fall tillage fb

spring herbicides are likely to provide more effective control in the absence of residual

herbicides. Although tillage in March was one of the most effective horseweed control

options in this study, it is unlikely that spring tillage will be widely adopted in the

Mississippi Delta where stale seedbed production systems that rely on spring applied

herbicides for preplant weed control are most commonly used. These results differed

from the findings of Brown and Whitwell (1988) where fall tillage effectively controlled

horseweed for the subsequent crop. While studies have shown burial events of horseweed

seed greater than 0.5 cm can effectively negate future emergence (Nandula et al. 2006),

this does not explain why fall tillage events were inconsistent in the control of

horseweed. One hypothesis may be that the wind-dispersed seed of horseweed, which

24

have a small achene and pappus to aid in dispersal (Dauer et al. 2006), may continue to

move with winds throughout the winter months into new areas for re-introduction. In

addition, tillage may not completely eliminate all plants present in the field, allowing

plants to subsequently recover.

25

LITERATURE CITED








4:642-647.

Buhler, D. D. 1992. Population dynamics and control of annual weeds in corn (Zea mays)

as influenced by tillage systems. Weed Sci. 40:241-248.



Dauer, J. T., D. A. Mortensen, and R. Humston. 2006. Controlled experiments to predict

horseweed (Conyza canadensis) dispersal distances. Weed Sci. 54:484-489.

Elmore, C. D., L. G. Heatherly, R. A. Wesley. 1995. Weed control in no-till doublecrop

soybean (Glycine max) following winter wheat (Triticum aestivum) on a clay soil.



www.weedscience.com




Kapusta, G. 1979. Seedbed tillage and herbicide influence on soybean (Glycine max)

weed control and yield. Weed Sci. 27:520-526.



Technol. 3:399-401.



26

Main, C. L., T. C. Mueller, R. M. Hayes, and J. B. Wilkerson. 2004. Response of selected

horseweed (Conyza canadensis (L.) Cronq.) populations to glyphosate. J. Agric. Food

Chem. 52:879-883.

Nandula, V. K., T. W. Eubank, D. H. Poston, C. H. Koger, and K. N. Reddy. 2006.

Factors affecting germination of horseweed (Conyza canadensis). Weed Sci. 54:898-

902.

Saphangthong, T. and W. W. Witt. 2006. Germination of horseweed (Conyza canadensis

L.) under field conditions. In W. Vencill (ed.) 59th Proc. South. Weed Sci. Soc., San

Antonio, TX. 23 25 Jan. 2006. South. Weed Sci. Soc., Champaign, IL.

VanGessel, M. J. 2001. Rapid Publication. Glyphosate-resistant horseweed from

Delaware. Weed Sci. 49:703-705.

Vencill, W. K. and P. A. Banks. 1994. Effects of tillage systems and weed management

on weed populations in grain sorghum (Sorghum bicolor). Weed Sci. 42:541-547.

Webster, T. M., J. Cardina, and S. J. Woods. Apocynum cannabinum interference in no-

till Glycine max. Weed Sci. 48:716-719.

Young, B. G. 2006. Changes in herbicide use patterns and production practices resulting

from glyphosate-resistant crops. Weed Sci. 20:301-307.

27

0

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40

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ture

(C)

Emergence

Temperature

Figure 2.1. Horseweed emergence and average air temperature at Stoneville, MS from

September 2004 to May 2005. Mean horseweed emergence is plotted for the

first and last half of each month and the standard error of each mean is affixed

as an indicator of variability for each observation period.

28

0

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Temperature

Figure 2.2. Horseweed emergence and average air temperature at Lamont, MS (Lamont

location I) from September 2004 to May 2005. Mean horseweed emergence is

plotted for the first and last half of each month and the standard error of each

mean is affixed as an indicator of variability for each observation period.

29

0

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Figure 2.3. Horseweed emergence and average air temperature at Lamont, MS (Lamont

location II) from October 2004 to May 2005. Mean horseweed emergence is

plotted for the first and last half of each month and the standard error of each

mean is affixed as an indicator of variability for each observation period.

30

0.0

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Figure 2.4. Horseweed emergence and average air temperature averaged across 3

locations in the Mississippi Delta from September 2004 to May 2005. Mean

horseweed emergence is plotted for the first and last half of each month and

the standard error of each mean is affixed as an indicator of variability for

each observation period.

31

Table 2.1. Impact of tillage and herbicide applied at various timings on horseweed control in

May at 3 locations in the Mississippi Delta.

Groundcoverb

Treatment/Timinga

Stoneville Lamont I Lamont II Average Densityc

_______________________________

%______________________________

___

Plants/m2 ___

Tillage/September 33 51 53 45 20

Tillage/November 18 19 58 31 15

Tillage/March 6 5 11 8 6

Herbicide/September 70 29 73 57 43

Herbicide/November 43 13 9 21 10

Herbicide/March 5 8 0 4 11

Tillage/September fb

Herbicide/March 0

0 0

0 0

Nontreated 73 23 73 56 33

LSD (0.05) 15 20 26 12 16 a Tilled plots were disked once. Herbicide treatment was 0.56 kg ai/ha glufosinate. fb =

followed by. Studies were initiated in September 2004 and concluded May 2005. b Groundcover was visually assess using a rating scale of 0=no horseweed present to 100=soil

in plots completely covered with horseweed plants. c Data pooled because no treatment by location interactions occurred.

32

CHAPTER III

GLYPHOSATE-RESISTANT HORSEWEED CONTROL USING GLYPHOSATE-,

PARAQUAT-, AND GLUFOSINATE-BASED HERBICIDE PROGRAMS

Abstract

Field studies were conducted in 2005 and 2006 to determine the most effective

chemical control options utilizing glyphosate-, paraquat- and glufosinate-based

burndowns for glyphosate-resistant horseweed in Mississippi. Burndown treatments were

applied April 5, 2005, and March 15, 2006, to horseweed plants 15 to 30 cm in height.

Glyphosate at 0.86 kg ae/ha alone provided 60 to 65% horseweed control 4 WAT.

Control 4 WAT ranged from 73 to 74% when the glyphosate rate was increased to 1.25

kg ae/ha. Glyphosate + 2,4-D and glyphosate + dicamba were the best glyphosate-based

treatments and provided 90% or better horseweed control 4 WAT both years. Soybean

yields with glyphosate + 2,4-D and glyphosate + dicamba were equal to the best

treatments both years. Control 4 WAT with paraquat alone ranged from 55 to 63% and

control was not improved by increasing the rate from 0.84 to 0.98 kg ai/ha. In 2005

paraquat + metribuzin provided 94% horseweed control 4 WAT and reduced horseweed

plant populations 96%. Horseweed control 4 WAT in 2006 with paraquat + 2,4-D and

paraquat + dicamba was 88 and 89%, respectively. Control with these treatments was

significantly better than with paraquat + metribuzin, which provided only 79% control in

2006 4 WAT. Glufosinate-based burndowns provided 81 to 97% horseweed control and

33

soybean yields were generally similar with all glufosinate-based programs. Soybean

yields were generally best with herbicide programs that provided that highest level of

horseweed control.

Nomenclature: Glyphosate, paraquat, glufosinate, 2,4-D, dicamba, metribuzin; Conyza

canadensis. #4 ERICA; soybean, Glycine max. (L.) Merr. DK 4763RR, AG 4801RR.

Additional index words: glyphosate resistance, weed resistance, burndown, herbicide

mixtures, herbicide efficacy, yield reduction.

Abbreviations: Weeks after treatment, (WAT).

Introduction

Horseweed is a robust, tap-rooted member of the Asteraceae (Compositae) family.

Horseweed is a winter annual (Brown and Whitwell 1988), and is a problem weed in no-

tillage production systems (Barnes et al 2004; Bruce and Kells 1990). Most winter

annuals emerge in the fall of the year; however, research has shown that horseweed may

emerge from early fall through late spring (Bhowmik and Bekech 1993; Buhler and

Owen 1997) However, Saphangthong and Witt (2006) reported germination occurring

throughout the year. Horseweed has been documented in 70 countries, 40 different crops

around the world (Holm et al. 1997), and is listed as being a problematic weed in cotton

(Gossypium hirsutum L.), grain sorghum (Sorghum bicolor (L.) Moench), corn (Zea mays

L.), and soybean (Glycine max. (L.) Merr.) (Brown and Whitwell 1988; Buhler and Owen

1997; Mueller 2003; Vencill and Banks 1994; Wiese et al 1997). Since 1993, horseweed

4 Letters following this symbol are a WSSA-approved computer code from Composite List of Weeds,

Revised 1989. Available only on computer disk from WSSA, 810 East 10th

Street, KS 66044-8897.

34

in North America has developed resistance to photosystem II inhibitors, bipyridiliums,

acetolactate synthase (ALS) inhibitors, ureas, amides, and most recently, glycines (Heap

2006; Koger et al. 2004; Smisek et al. 1998; Trainer et al. 2005; VanGessel 2001;

Weaver et al. 2004). Horseweed is the first annual broadleaf documented as resistant to

glyphosate (VanGessel 2001), and is considered one of the top 10 worst weeds among

herbicide-resistant weeds (Heap 2006). Currently, glyphosate-resistant populations of

horseweed have been reported in several states across the United States, including

Mississippi (Heap 2006; Koger et al. 2004; VanGessel 2001).

Glyphosate-resistant (GR) crops, such as cotton, soybean, and corn, allow

producers to control a broad spectrum of weed species with in-season applications of

glyphosate (Baylis 2000). The widespread adoption of GR crops has made glyphosate the

most utilized agrochemical worldwide with amounts ranging from 38 to 40 million kg

annually (EPA 2004). Chemical control of horseweed had been successful with

glyphosate prior to 2001. Studies by Wilson and Worsham (1988) found that 0.6 kg ae/ha

of glyphosate provided 75% control of 15 to 46 cm tall horseweed and 1.1 kg/ha provided

94% control 4 WAT. Similarly 0.4 kg/ha glyphosate controlled 10 to 15 cm tall

horseweed 83 to 100% 4 WAT (Keeling et al. 1989) and 87 to 93% control of 1 to 6 cm

tall horseweed 6 WAT (Bruce and Kells 1990 Scott et al. (1998) reported a minimum of

1.12 kg/ha glyphosate was required to control horseweed 20 to 25 cm tall horseweed

greater than 92%, and VanGessel et al. (2001) observed 100% control of 15 to 30 cm

horseweed with 0.8 kg ae/ha glyphosate in 1998.

35

Control of horseweed with paraquat alone has proven difficult. Bruce and Kells

(1990) reported paraquat at 0.56 kg ai/ha with a carrier volume of 210 L/ha controlled 6

to 8 cm tall horseweed 15 and 60% years one and two, respectively. Similar findings

were reported in a study by Keeling et al. (1989) where 0.6 kg/ha paraquat in 140 L/ha

provided controlled 3 to 4 cm diameter horseweed 67 and 53% 4 WAT, years one and

two, respectively; however, control dropped to 40 and 30% at 12 WAT.

A broadleaf herbicide that has shown excellent activity on horseweed is 2,4-D

(Bruce and Kells 1990; Keeling et al. 1989; Moseley and Hagood 1990). A study by

Wiese et al. (1995) found that 2,4-D ester at 0.56 kg ai/ha provided 100% control of 30

cm tall horseweed, but only 47% control of plants in rosette stage at 5 cm in height. The

addition of 2,4-D to paraquat to aid in the control of horseweed has proven erratic.

Wilson and Worsham (1988) found 0.6 kg/ha paraquat controlled 18 cm tall horseweed

74%, but the addition of 0.6 kg/ha 2,4-D improved control to 92%. When the same

treatments were applied to 15 to 46 cm tall horseweed, controls were 58% and 61%,

respectively. Moseley and Hagood (1990) found that the addition of 2,4-D amine did not

improve control over paraquat alone.

Glufosinate is a non-selective herbicide that has shown potential as an alternative

to glyphosate for the control of horseweed. Glufosinate is considered a contact herbicide

with limited systemic activity and typically performs better in warmer temperatures

(WSSA 2002). Control of horseweed with glufosinate has been inconsistent (Talbert et al.

2004) and little data is available relative to the control of horseweed with glufosinate.

36

The objective of this study was to determine the most effective chemical control

options for glyphosate-resistant horseweed populations in Mississippi using glyphosate-,

paraquat-,and glufosinate-based herbicide programs.

Materials and Methods

Experiments were conducted at two locations in the Mississippi Delta at sites

known to have horseweed populations that were no longer controlled with glyphosate.

Studies were conducted near Lamont, MS, in 2005 and near Leland, MS, in 2006. Sites

were located approximately 29 kilometers apart. The soil type at both locations was a

Dundee very fine sandy loam (fine-silty, mixed, active, thermic Typic Endoaqualfs) with

the Lamont location having a pH of 6.2 and organic matter content of 1.2% and Leland

location having a pH of 6.1 and organic matter content of 1.2%. Plots at Lamont were

established following three years of no-tillage glyphosate-resistant soybean and were

naturally infested with glyphosate-resistant horseweed as documented by Koger et al.

(2004). Plots at Leland were established following five years of no-tillage glyphosate-

resistant soybean and were naturally infested with horseweed which the grower had

reported having trouble controlling with glyphosate the previous season. The

experimental design was a randomized complete block design with four replications.

Plots were 3 x 12 m, and herbicide treatments were applied with a tractor-mounted

compressed-air sprayer calibrated to deliver 140 L/ha at a pressure of 207 kPa using flat

fan nozzles5. Treatments at Lamont were applied to 25 to 30 cm tall horseweed on April

5, 2005, with an ambient air temperature of 19 C, 53% relative humidity, and adequate

5 TeeJet XR 11002VS. Spraying Systems Co., Wheaton, IL 60189.

37

soil moisture. Ambient air temperature increased to 27 C the day of application. For the

interval of seven days prior to application to seven days after application in 2005,

maximum and minimum air temperatures averaged 22 and 11 C, respectively, with the

application date being one of the three warmest days of the 15-day interval (Table 3.1).

Relative humidity averaged 61% for this 15-day interval. The period prior to application

was marked by mostly sunny skies and adequate soil moisture. Conditions following

applications were marked by average relative humidity of 82%, partly cloudy skies, and

excessive soil moisture. Treatments at the Leland location in 2006 were applied to 15 to

20 cm tall horseweed on March 15 with an ambient air temperature of 17 C, 55% relative

humidity and adequate soil moisture. For the interval of seven days prior to application to

seven days after application in 2006, maximum and minimum air temperatures averaged

21 and 9 C, respectively, with the application date being one of the three coolest days of

the 15-day interval (Table 3.1). Air temperature did not increase further the day of

application. Leading up to application, relative humidity averaged 58%. Mostly sunny

skies prevailed, and adequate soil moisture was noted. Following application cold and

wet conditions occurred with 16 C for an average high and 6 C for an average low.

Relative humidity averaged 74%. Skies were mostly cloudy and soil moisture was

excessive (Table 3.1).

Glyphosate-, paraquat-, and glufosinate-based herbicide programs were evaluated

in separate but adjacent experiments. Non-selective herbicides rates were glyphosate,

0.86 kg/ha and 1.25 kg/ha; paraquat, 0.84 kg/ha and 0.98 kg/ha; and glufosinate, 0.47 and

0.59 kg/ha, applied alone and in combination with various tank-mix partners. Tank-mix

38

herbicides and their respective rates were 2,4-D ester, 0.84 kg/ha; dicamba, 0.28 kg/ha;

flumioxazin, 0.07 kg/ha; oxyfluorfen, 0.28 kg/ha; linuron, 0.56 kg/ha; metribuzin, 0.42

kg/ha; carfentrazone, 0.014 kg/ha; sulfentrazone, 0.36 kg/ha; sulfentrazone +

chlorimuron, 0.13 and 0.03 kg/ha, respectively; and thifensulfuron + tribenuron, 0.013

and 0.006 kg/ha, respectively. A nontreated control was also included. A nonionic

surfactant6 (NIS) was added to all paraquat-based treatments at 0.5% v/v. No at-planting

burndown applications were applied. Delta King DK 4763RR soybean was seeded April

25, 2005, and Asgrow AG 4801 soybean was seeded April 12, 2006. Soybean was

allowed to compete with weeds that remained following preplant burndowns for

approximately four weeks before in-season herbicides were applied. For in-season weed

management, plots received postemergence applications of 2.2 kg ai/ha Sequence and

0.86 kg/ha glyphosate 4 and 7 wk after planting, respectively. Visual control ratings were

collected at 2 and 4 WAT and horseweed plant counts were recorded 7 WAT,

approximately 3 wk after soybean planting. Yields were determined at each location by

harvesting the interior five rows of a 7-row plot using a small-plot combine. Yields were

adjusted for 13% moisture. All data were subjected to ANOVA and means separated

using Fishers Protected LSD at the alpha = 0.05 level.

Results and Discussion

Significant treatment by year interactions were detected for all variables;

therefore, data are presented separately by year. Glyphosate at 0.86 kg/ha alone provided

60 and 65% horseweed control 4 WAT in 2005 and 2006, respectively (Table 3.2).

6 Induce. Helena Chemical Co., Collierville, TN 38017.

39

Control ratings increased to only 74 and 73% 4 WAT in 2005 and 2006, respectively,

when glyphosate rates were increased to 1.25 kg ae/ha. However, horseweed density was

reduced 91% with 1.25 kg ae/ha glyphosate in 2006, compared to only 67% in 2005

which may suggest a higher percentage of glyphosate-susceptible plants at the Leland

location or a slightly lower level of tolerance to glyphosate. Further studies are needed to

confirm differential tolerance to glyphosate between these locations. Based on visual

ratings and reductions in plant density, glyphosate + 2,4-D and glyphosate + dicamba

were the only treatments that provided 90% horseweed control 4 WAT both years.

Control levels, based on visual ratings, were higher with the addition of 2,4-D or dicamba

to glyphosate than with any other glyphosate-based program.

Horseweed control was reduced both years when carfentrazone was mixed with

glyphosate. Similar reductions in plant density occurred with glyphosate alone and

glyphosate + carfentrazone compared to glyphosate alone. However, plants treated with

glyphosate + carfentrazone recovered faster following treatment compared to plants

treated with only glyphosate. The reduction in control of horseweed with glyphosate +

carfentrazone may be the result of reduced glyphosate absorption compared to glyphosate

alone. Reduced glyphosate absorption associated with mixing contact herbicides with

glyphosate is well documented (Hydrick and Shaw 1994; Norris et al. 2001; Starke and

Oliver 1998), but further studies are needed to determine if reduced glyphosate

absorption is occurring with the glyphosate + carfentrazone mixture.

Soybean yields with glyphosate + 2,4-D and glyphosate + dicamba were equal to

the best treatments both years. In 2005, however, soybean yields with four other

40

treatments produced similar yields. Yields with 0.86 and 1.25 kg/ha glyphosate were

equal to yields with glyphosate + 2,4-D and glyphosate + dicamba, which provided the

highest levels of horseweed control. Although glyphosate alone resulted in only 59 to

74% control based on visual ratings, plants remained stunted and chlorotic for several

weeks following treatment and were apparently not substantially competitive with

narrow-row soybean planted at the Lamont location in 2005.

Glyphosate + flumioxazin and glyphosate + chlorimuron + sulfentrazone also had

yields similar to the glyphosate + 2,4-D and glyphosate + dicamba in 2005. In 2006,

soybean yields with glyphosate + 2,4-D were 3121 kg/ha and only glyphosate +

oxyfluorfen, and glyphosate + dicamba were the only other treatments to produce similar

yields. Glyphosate alone failed to produce soybean yields similar to the best treatments

despite the fact that glyphosate at 1.25 kg/ha reduced horseweed density 91% compared

to the nontreated control. However, horseweed densities were much higher at Leland in

2006 than at Lamont in 2005, and 26 plants/m2 still remained following treatment with

1.25 kg/ha glyphosate compared to only 12 plants/m2 following the same treatment in

2005. In addition, drier soil conditions were more prevalent in 2006 than in 2005.

Consequently, weed competition likely had a more detrimental effect on soybean yields.

These data suggests that narrow-row soybean yields might be sustained with glyphosate-

only weed control systems providing horseweed plant densities are relatively low, and

moisture is not extremely limited. However, further selection of glyphosate-resistant

horseweed will only lead to higher plant densities in subsequent years, and other

management strategies will have to be utilized for long-term sustainability.

41

Control 4 WAT with paraquat alone ranged from 55 to 63% regardless of year,

and control was not improved by increasing the rate from 0.84 to 0.98 kg ai/ha (Table

3.3). Paraquat + metribuzin provided 94% horseweed control 4 WAT in 2005. Control

with paraquat + 2,4-D and paraquat + dicamba was better than with paraquat alone but

not equal to paraquat + metribuzin. Horseweed control 4 WAT in 2006 with paraquat +

2,4-D and paraquat + dicamba was 88 and 89%, respectively. Control with paraquat +

2,4-D and paraquat + dicamba treatments were better than with paraquat + metribuzin,

which provided only 79% control 4 WAT in 2006. Paraquat + metribuzin reduced

horseweed plant populations 96% in 2005 and paraquat + dicamba was the only other

treatment in 2005 that produced similar yields. Soybean yield in 2006 with paraquat +

dicamba was 1774 kg/ha. Paraquat + 2,4-D, paraquat + metribuzin, and paraquat +

chlorimuron + sulfentrazone produced comparable yields. These four treatments reduced

horseweed populations 40 to 58%. Paraquat + 2,4-D, paraquat + dicamba, paraquat +

metribuzin, and paraquat + chlorimuron + sulfentrazone were the only treatments in 2006

to provide greater than 70% horseweed control 4 WAT. Because the apical meristem of

horseweed is densely covered with pubescence and leaf tissues, control of horseweed

with a contact herbicide such as paraquat may be limited due to poor coverage. Bruce and

Kells (1990) noted that paraquat-treated horseweed plants, although completely

desiccated, would recover and reinitiate new terminal growth.

The addition of metribuzin to paraquat improved control over paraquat alone both

years. Initially plants appeared unaffected by paraquat + metribuzin treatments and only

after several days did plants begin to show signs of treatment resulting in nearly complete

42

desiccation. Cool and cloudy conditions in 2006 may have contributed to reduction in the

activity of paraquat + metribuzin compared to 2005, when conditions where warm and

sunny. The synergistic interactions between photosystem I and photosystem II

chemistries have long been accepted by the weed science community. However, current

literature on these interactions is limited, and additional research is needed to further

evaluate the physiological and biochemical processes that may be involved in this

phenomenon. .

Horseweed control 4 WAT ranged from 81 to 97% with all glufosinate-based

treatments both years (Table 3.4). All treatments in 2005 reduced horseweed populations

compared to the nontreated control, and yields were similar with all treatments except

glufosinate + flumioxazin. Most glufosinate treatments in 2006 reduced horseweed

populations by at least 87% except for glufosinate alone. Plant populations were reduced

61 and 77% with 0.47 and 0.59 kg/ha glufosinate, respectively. Horseweed densities were

very high at Leland in 2006, with 175 plants/m2 in nontreated plots. Glufosinate is

considered a contact herbicide with limited systemic activity (WSSA 2002). The higher

plant densities in 2006 may have resulted in a lack of control due to poor spray coverage

by the glufosinate alone treatments. Steckel et al. (1997) and Tharp and Kells (2002) have

also reported similar findings where glufosinate activity was reduced due to high plant

populations at application. An average of 28 plants/m2 were allowed to escape when

horseweed control ratings were between 88 and 90% 4 WAT with glufosinate-based

treatments in 2006. However, soybean yields were similar in all herbicide treated plots.

43

The tank-mix additions of 2,4-D or dicamba were the only treatments that

improved horseweed control with glyphosate-based burndowns both years. Control of

horseweed was reduced with the addition of carfentrazone to glyphosate. The addition of

2,4-D, dicamba, or metribuzin to paraquat improved control of horseweed both years, as

did the tank-mix of paraquat + sulfentrazone + chlorimuron in 2006. Control of

horseweed was reduced with the addition of carfentrazone to paraquat in 2005.

Glufosinate alone provided 88 to 96% horseweed control. Tank mixtures were no better

than glufosinate alone in 2005 when applications were made when warm temperatures

prevailed. When cool and cloudy conditions prevailed in 2006, however, horseweed

control with glufosinate alone 4 WAT was improved with the addition of 2,4-D, dicamba,

metribuzin, sulfentrazone, and sulfentrazone + chlorimuron.

44

LITERATURE CITED

Barnes, J., B. Johnson, K. Gibson, and S. Weller. 2004. Crop rotation and tillage system

influence late-season incidence of giant ragweed and horseweed in Indiana soybean.

Online. Crop Management doi:10.1094/CM-2004-0923-02-BR.

Baylis, A. D. 2000. Why glyphosate is a global herbicide: strengths, weaknesses, and

prospects. Pest Manag. Sci. 56:299-308.








4:642-647.




www.weedscience.com




Hydrick, D. E. and D. R. Shaw. 1994. Effects of tank-mix combinations of non-selective

foliar and selective soil-applied herbicides on three weed species. Weed Technol.

8:129-133.



Technol. 3:399-401.



Moseley, C. M., and E. S. Hagood. 1990. Horseweed (Conyza canadensis) control in full-

season no-till soybeans (Glycine max). Weed Technol. 4:814-818.

45

Mueller, T. C., J. H. Massey, R. M. Hayes, C. L. Main, and C. N. Stewart. 2003.

Shikimate accumulates in both glyphosate-sensitive and glyphosate-resistant

horseweed (Conyza canadensis L. Cronq.). J. Agric. Food Chem. 51:680-684.

Norris, J. L. D. R.

glyphosate resistant weeds

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