the role of ecology in the development of weed management systems: an outlook

The role of ecology in the development of weedmanagement systems: an outlook

D A MORTENSEN, L BASTIAANS* & M SATTIN Department of Agronomy, University of Nebraska, Lincoln 68583, USA, *Department of Theoretical Production

Ecology, Wageningen Agricultural University, Wageningen, PO Box 430, 6700 AK, The Netherlands, and CentroStudio Biologia e Controllo Piante Infestanti-CNR, 35020 Legnaro (Padua), Italy

Received 28 June 1999

Revised version accepted 13 October 1999

Summary

This paper discusses the extent to which a knowledge of weed biology and ecology can contribute

to the development of weed management strategies. To date, such contributions have been

modest and have been constrained by a number of factors that are discussed in this review. In

contrast to other pest management disciplines, devising integrated weed management strategies

that address a diversity of weed species with a diversity of life history traits is di�cult. Because of

this diversity, robust systems that require ecological insight beyond that of individual species are

needed. Although the contributions have been modest, research ®ndings have helped to shape

weed management strategies in a number of important ways. Approaches directed at weed

population management have revealed important insights into population equilibria, density-

dependent mortality and life stages particularly important in regulating population size. Eco-

physiological research has helped to guide the development of biologically e�ective herbicide

dosage strategies, whereas mechanistic interplant competition modelling coupled with empirical

®eld studies have aided in the identi®cation of weed-suppressive crop phenotypes. Finally, much

has been learned about the in¯uence of control tactics and agronomic measures on the evolution

of herbicide resistance and the development of integrated weed management strategies to address

it. In this paper, examples are reviewed where research in ecology and biology has helped to

shape the practice of integrated weed management. More importantly, characteristics of such

research programmes are identi®ed so that future e�orts in the discipline will have a context in

which the relevance of research questions and approaches can be considered.

Keywords: weed ecology, weed management, systems approach, research impact.

Introduction and background

This is a dynamic time for crop protection in agriculture. Within just a few years, herbicide-

resistant crops have gone from being a curiosity on relatively few hectares to over 60% of the

Correspondence: D A Mortensen, Department of Agronomy, University of Nebraska, Lincoln, NE 68583-0915, USA.

Tel: (+1) 402 472 1543; E-mail: [email protected]

Ó Blackwell Science Ltd Weed Research 2000 40, 49±62 49

soyabean [Glycine max (L.) Merr.] crop planted in the USA. Agrochemical companies promise

that transgenic crops will simplify pest management programmes through the use of singular

chemical tactics. This `silver-bullet' approach has consistently failed and almost certainly will

again. It will do so as a result of fundamental ecological relationships governing population size

and diversity. At the same time, policies regarding pesticides have called for signi®cant use

reduction in many European countries. Initiatives to reduce reliance on herbicides will require a

much fuller understanding of how management practices compliment one another to maintain

weed populations at low equilibrium densities and reduce relative ®tness of weeds (Mortensen

et al., 1998). Without a considerable research e�ort, implementation of alternative weed

management strategies that use various tactics will move at a slower pace, requiring a much

stronger interaction between researcher, agricultural adviser and farmer.

Although progress has been made towards the development, assessment and implementation

of integrated pest and disease management systems (Swanton & Weise, 1991), the discipline of

weed science continues to lag. Weed control was a minor part of agronomy, botany, horticulture

and plant physiology until the 1950s, when synthetic organic herbicides became widely available

for the ®rst time (Timmons, 1970). In his review `A plea for thought', Zimdahl (1991) argues that

a `how to control' technological orientation was shaped early on in the evolution of weed science

as a discipline, and, until recently, this has dominated the science. This is in contrast to the

disciplines of plant pathology and entomology. In plant pathology early work centred on studies

of taxonomy and description of diseases and their causal agents. This was in part because

causality was not obvious. Phytophthora infestans (Mont. De Bary) (potato blight) provides an

excellent example of this. Most studies throughout the Irish potato famine centred on identifying

the cause of the disease in the hope that a method of control would grow out of a basic

understanding of the biology of the disease. Controlling the disease would be extremely di�cult

without knowledge of the cause. In a similar way, the ®eld of entomology had centred on

describing and understanding insects, their taxonomy, life cycles, hosts and the damage they did.

It was not the primary task of entomologists to devise control programmes for insects.

`The herbicide era', a period between the end of the second world war and the late 1970s,

brought about a generally high expectation that this e�ective and reasonably cheap tool

represented the `®nal solution' for the age-old problem of controlling weeds. This high

expectation has carried through to current marketing strategies where agrochemical companies

guarantee optimal weed control (Owen, 1997). Sagar (1968) indicated that `there is a widespread

impression that the weed-free environment is almost with us. This is an exaggeration, but the rate

of progress towards it is rapid and there is no a priori reason why it should not be technically

possible within, say, the next 10±20 years.' Thirty years on, the data show that no weed species

has been eradicated by the use of herbicides, whereas a number of new troublesome weeds have

increased in population size in the face of agricultural intensi®cation (Altieri, 1991).

Although weed management is still herbicide dominated in many important agricultural areas

of the world, there are strong indications that in the near future this will change. Some forces, of

which herbicide resistance is an important one, come from agricultural practitioners themselves.

The existence and risk of development of herbicide resistance makes herbicide-dependent

cropping systems increasingly vulnerable. Forces outside agriculture may be even stronger.

Widespread concern about environmental side-e�ects of herbicides combined with fear for public

health has resulted in the banning of several herbicides in some countries and increasing pressure

on farmers to reduce the use of herbicides (Matteson, 1995). This pressure will increase further,

particularly in those regions where the population is largely non-agrarian and food security is not

50 D A Mortensen et al.

Ó Blackwell Science Ltd Weed Research 2000 40, 49±62

an issue. The Dutch Ministry of Agriculture has recently formulated its policy towards the use of

chemical protection agents and has described the need to change the thinking of farmers away

from a `yes, provided that' to a `no, unless' attitude. Putting policy in such linguistic terms clearly

illustrates that crop protection, and particularly the use of biocides, are political as well as

agronomic issues.

For weed scientists this o�ers opportunities and also signi®cant challenges to meet

expectations and obligations. The only successful way to accomplish signi®cant reductions in

herbicide use is to work closely with farmers in the development, implementation and

maintenance of integrated weed management systems. Herbicides, apart from negative side-

e�ects, are generally unsurpassed as weed control tools: reliable, highly e�ective and relatively

inexpensive (when cost of labour is included and external costs are not). The introduction of

herbicides has strongly in¯uenced our concept of and attitude towards weed management. Weeds

have been regarded as a problem that can be controlled with herbicides, rather than managed

through cropping systems design. Because of this herbicide-centred control paradigm, farmer

actions and supporting research have mainly addressed the avoidance of yield loss in current

crops. Recent studies have largely advocated two directions for developing cropping systems

with a reduced reliance on herbicides (e.g. Sterrenberg & Brandt, 1996; Lotz et al., 1997). The

®rst, referred to as chemical re®nement, focuses on reduced use of herbicides through the

development of curative tactics. In this approach herbicides remain the central pillar of crop

protection, and consequently a change in perception of the weed problem is not really required.

Technological solutions, such as improved application technology, improved application timing,

factor-adjusted dosages and development of herbicides with low environmental impact have been

suggested as exponents of this direction. If reduced dependence on herbicides is the ultimate aim,

the weed problem should be envisaged in a di�erent perspective. Weeds can no longer be

regarded as a problem resolved by curative tactics; instead, integrated weed management should

be seen as a component of integrated cropping systems design. Prevention becomes the keyword,

and integrated crop management the new concept. In this cropping systems design approach, an

integrated system with numerous ®tness-reducing and mortality events is integrated to manage

weed populations where herbicides are used as a last resort, and in organic systems where no

herbicides are used at all (Lotz et al., 1997).

Attributes of ecological research thrusts that have affected on-farm weedmanagement practices

Although it is true that herbicide discovery and e�cacy have historically shaped our discipline,

research has turned to a more systems-oriented approach to problem solving and will need to

continue to do so for reasons stated earlier in this paper. Research directed at weed population

management has revealed important insights into population equilibria, density-dependent

mortality and life stages that are particularly important in regulating population size. These

insights have, on occasions, signi®cantly improved integrated weed management practices. We

have selected a number of such research studies to illustrate how research into weed ecology has

been used to address applied weed management problems. From these examples we will highlight

the characteristics of research programmes that are required in order to enhance the likelihood

that ecological principles will lead to improvements in the design of integrated cropping systems.

The paper takes a decidedly applied approach to this subject as we are interested in those


Ecology in weed management 51

characteristics of research approaches that are underpinned with sound ecological principles and

have a high likelihood of being adopted by farmers.

Curative case studies

The following three case studies focus on adjusting mortality intensity to enhance curative

strategies as components of integrated weed management practices. That is to say, these strategies

are deployed once a pest problem is present. All three cases feature long-term studies, a continuum

of basic to implementation tomaintenance research. Some involvemodelling, and all document the

importance of moving research ®ndings into practice in farmers' ®elds.

Population size and weed management outcomes

Annual weeds in Midwestern US maize (Zea mays L.) and soyabean production continue to be

the primary pest management target in these crops. Despite intensive weed management

programmes implemented each year in these crops, weed problems persist and are most acute

after poor weed control years. They are most di�cult to manage in areas of ®elds with

historically high weed infestations. Initial work to address the in¯uence of weed population size

on weed management outcomes relied on several research strategies. Hartzler & Roth (1993)

approached the question empirically by establishing weed populations at research station sites

representative of farm ®elds where weed control in the preceding year was poor. They found that

weed management e�cacy in the current year was strongly in¯uenced by weed management

e�cacy in the preceding year and that the outcome of weed management practices was directly

linked to weed population size. Mortensen et al. (1993) mapped weed management outcomes on

18 ®elds over a period of 6 years. At the same time, component patch studies (Dieleman et al.,

1999), whose characteristics were de®ned by weed patches observed in farmers' ®elds, were

established at the research station where controlled density by mortality studies were conducted.

The results of this work have given clear evidence that smaller populations require less intensive

mortality pressure to achieve desirable levels of control. This relationship appears to hold for a

variety of mortality sources including pre-emergence herbicides (Burrill & Appleby, 1978; Khedir

& Roeth, 1981; Winkle et al., 1981), post-emergence herbicides (Dieleman et al., 1999) and

mechanical control (Buhler et al., 1992).

Research continues in this important area, and research results have been successfully

implemented into extension programmes and adopted by farmers. Hartzler, following Hartzler

& Roth (1993), has incorporated results from this research into his farmer- and consultant-

oriented extension programme. One such extension publication discusses targeting high

infestation areas of ®elds with su�ciently e�ective weed management practices while reducing

the intensity of weed management across the remainder of the ®eld. Similarly, such an

approach was recently added to WeedSOFT, an integrated weed management decision aid

(Mortensen et al., 1999). These practices have arisen from research initiated in the late 1980s

and are being implemented today. This case underscores the importance of identifying an

important researchable question, the long-term commitment to addressing that question and

the need to shape future research by maintaining a connection between the researcher and the

farmer. Research in this area continues with mechanistic studies addressing the in¯uence of

population size on seedbank fate and seedling size hierarchies, plant sampling and spatial

population dynamics modelling.



Biologically e�ective doses

Herbicide labels detail the dose at which herbicides are to be applied. Such doses are generic in

that they are intended to provide acceptable levels of weed suppression under the worst-case

scenario; in practice, the herbicide dose is determined at a level that enables an agrochemical

company to guarantee product performance. Through extensive public-sector research, often

directly involving farmers, factors upon which herbicide performance depends have been studied.

Detailed knowledge on weed species, their growth, physiological state or developmental stage

and environmental conditions (Jensen & Kudsk, 1988; Kudsk, 1989; Ketel, 1996; Ketel et al.,

1996) can be used to reduce herbicide dose; such an approach is referred to as factor-adjusted

doses (Baandrup & Ballegaard, 1989). When these systems were conceptualized initially,

researchers recognized that many growers already used herbicide doses below those

recommended on the product labels (Baandrup & Ballegaard, 1989; Defelice et al., 1989).

Here is a case study that has grown out of farmer practice and by working with farmers and has

been re®ned through basic biological and ecological studies on both the short and long-term

consequences of reduced input weed management. The ultimate success of this approach should

not be measured in e�cacy alone, but rather the success of any system in which reduced doses are

used will be determined by the long-term fate of the weed population. To address these concerns,

research on the long-term consequences of reduced dose strategies should be initiated to

guarantee their long-term success.

In The Netherlands, a factor-adjusted dose approach has been developed at the Research

Institute for Agrobiology and Soil Fertility (AB-DLO). This methodology was based on

fundamental physiological research conducted during the early 1990s on factors a�ecting the

e�cacy of photosynthesis-inhibiting herbicides, such as application rate, weed species and weed

biomass (Ketel et al., 1996). In this research, measurement of chlorophyll ¯uorescence at 2 days

after herbicide application was found to be an early and reliable indicator of the e�cacy of

photosynthesis-inhibiting herbicides. The outcomes of this research, together with the research

methodology, were re®ned in the minimum lethal herbicide dose (MLHD) approach (Ketel &

Lotz, 1997). This consists of three steps: (1) determination of the MLHD based on a sample of

weeds collected in the ®eld; (2) application of the herbicide at the calculated MLHD; and (3)

determination of the e�cacy of the herbicide at 2 days after application by measuring chlorophyll

¯uorescence. This last step was included to reduce the associated risk of using a reduced dose of

the herbicides. Whenever the advised applications were insu�cient, farmers would be aware at an

early stage, allowing a second application in due course or other remedial practice. From 1994±95

onwards, the methodology was validated on experimental farms. Because of the promising

results, it was gradually introduced in practice through an on-farm demonstration project

conducted in close collaboration with the advisory service and farmers. Weed control resulting

from the MLHD approach was nearly always found to be equivalent to chemical control applied

at the full rate. In addition, a 5% yield increase was found in ®elds treated with a reduced dose, an

e�ect attributed to reduced herbicide-induced crop stress (Kempenaar & Lotz, 1999). Improved

crop safety was clearly evident after herbicide treatment by measuring chlorophyll ¯uorescence

of crop plants and contributed to the economic advantages of the MLHD approach. This

enhanced crop safety is believed to be one of the most important factors resulting in farmers

accepting the practice. This case study illustrates how results of basic research on plant

¯uorescence coupled with implementation and maintenance research can be of great value for the

development of practical management tools. The Research Institute now focuses on extending

the practice to include herbicides with a di�erent mode of action.



Selective spray topping

Annual grass weeds in wheat (Triticum aestivum L.) pose a major problem for wheat producers in

Australia. Initial work to address this important problem centred on enhancing wheat

competition through altered spatial arrangement and sowing rates (Medd et al., 1985). In that

work, wheat competitiveness was enhanced by increasing the seeding rate but was largely

unin¯uenced by spatial arrangement. Growers in southern regions of New South Wales have now

accepted the principle of enhancing weed control by increasing crop density. It is interesting that

the practice has been adopted only regionally, because in drier regions of the country there is

concern that an increased probability of moisture stress during grain ®ll will reduce wheat quality.

In spite of enhanced wheat competitiveness, a number of annual grasses are still a problem in

the cropping system. The research group then set out to identify the key factors that in¯uence

population dynamics and persistence of these species. This involved intensive studies of

population dynamics, simulation modelling (Medd & Ridings, 1990) and bioeconomic modelling

(Pandey & Medd, 1990, 1991; Pandey et al., 1993). From this knowledge of the wheat: Avena

fatua L. study system, it was determined by sensitivity analysis that seed rain by uncontrolled

A. fatua plants was the life stage that had the greatest in¯uence on weed population size and

persistence. As a consequence, the use of herbicides for killing weed seeds was pursued (Medd

et al., 1992, 1995). Very recently a modi®cation in the use pattern of ¯amprop-methyl was

approved (low doses during seed ®ll), which had to be supported by evidence through the

pesticide registration process.

The authors emphasize that a key to the success of this research relied on gathering

demographic data to enable sensible use of simulation or diagnostic modelling. The results of this

subsequently set directions for research into improving control. Another o�shoot of the work

arose through collaboration with an agricultural economist. This revealed that a `seed kill'

approach to weed management o�ered not only a way of reducing costs through better control

but could reduce farmer dependence on herbicides. This in turn could provide more management

options and retard the development of herbicide resistance in the targeted weeds. The work also

pointed to a need for change in the way farmers evaluated weed control. Greater pro®ts would be

realized if farmers adopted a long-term investment approach to weed control, and this hypothesis

has been pursued in recent work. These predictions have been validated in a long-term

population dynamics study of selective spray-topping and have been an important part of the

basis for commercialization of the technique, as outlined above.

Herbicide resistance management

The evolution of herbicide resistance in response to largely single-herbicide tactics is an example of

a failed approach to long-termweedmanagement. The systemswhere weed populations havemore

frequently developed resistance are those where diversity in space and time has been drastically

reduced and standardization of agricultural practices is high. Monoculture, intensive use of

herbicides with the same mode of action and reduced cultivation often characterize these systems.

Even a herbicide like glyphosate, which was thought unlikely to cause resistance if used repeatedly,

has recently been demonstrated to be able to select resistant populations of Lolium rigidumGaud.

in Australia (Powles et al., 1998). It has been noticed that populations ofAlisma plantago-aquatica

L. and Scirpus mucronatus L. resistant to ALS (acetolactate synthase) inhibitor herbicides, used in

northern Italian rice (Oryza sativa L.) crops, were much less frequently found in areas where weed

control strategies were, at least partially, shaped according to an integrated weed management

approach because of red rice (Oryza sativa var. sylvatica) infestations (Sattin et al., 1999).



The design of a scienti®cally and technically sound, economically acceptable resistance

management programme for one or more weeds appears to be a complex task. It cannot be done

without information on the biology/ecology of the resistant and susceptible populations and, in

general, on the speci®c agro-ecosystem. A positive example of how the knowledge of the biology/

ecology of a weed species can help to shape a sound integrated resistance management approach

is given by the resistant populations of Lolium rigidum in Australia (Powles & Mathews, 1996).

These cover large geographical areas and were the ®rst in the world to show a diversity of

resistance mechanisms and pattern of resistance in the same populations (Preston et al., 1996),

which makes their control very di�cult.

A long-term research e�ort (partly ®nanced by farmers' organizations) highlighted the major

biological and ecological features as well as genetics and dynamics of the populations involved,

the resistance mechanisms involved and the impact of several alternative control measures. It was

then concluded that the control of these resistant populations required a management

programme integrating chemical and non-chemical tactics. These were summarized by Powles

& Mathews (1996): delayed crop seeding because of the short residual life of L. rigidum seeds in

the seedbank; establishing vigorous crops (competition studies proved that barley (Hordeum

vulgare L.) and wheat crops are good competitors in relation to other crops); use of the low-

resistance-risk herbicide tri¯uralin; use of non-selective herbicides in the crop to control weed

seed production (spray-topping); catching L. rigidum seeds at crop harvest; use of physical

methods of control, such as burning crop residues, can destroy many of the viable L. rigidum

seeds; use of non-selective herbicides such as glyphosate and paraquat as pre-planting herbicides.

The success of an integrated weed management (IWM) strategy against a particular weed will

also require greater attention to other weeds (i.e. to their biology/ecology) that may invade the

biological space freed and/or develop resistance.

This approach is being successfully implemented in South and Western Australia, but it

involves a change in attitude by growers and advisors (Powles & Mathews, 1996). It should be

accepted that herbicides alone are not a suitable weed management strategy. Herbicides remain a

key component in this cropping system, but they must be used in conjunction with a variety of

non-herbicidal control methods, which suit the biology of the resistant weed in the context of a

speci®c environment and crop production system.

In Manitoba (Canada), where herbicide resistance is spreading fast (Heap, 1999), researchers

are moving along the same lines (Van Acker, pers. comm.). They are starting a medium- to long-

term research project on population genetics of Avena fatua because without this they believe

they can o�er little advice on resistance management or prevention.

From the above examples, it is clear that herbicide resistance management requires an

integrated systems approach that relies heavily on an extensive biology and ecology knowledge

base.

Preventative case studies

The following examples illustrate some of the methods that can be used to address weed

problems at an early stage as part of an integrated crop management strategy. In these

approaches, rather than focusing on weed control, crop management practices are adjusted such

that crop:weed interactions are altered to the bene®t of the crop. These cultural methods include

some of the oldest weed control practices, such as the transplanting of rice. Raising seedlings in a

seedbed gives the rice crop a competitive advantage over the weeds and this has long been one of



the main components of weed management in traditional rice cropping systems like those in

South-east Asia. During the last decade, however, high labour costs coupled with the shortage of

on-farm labour have caused a rapid shift to direct seeding practices (De Datta, 1986; Erguiza

et al., 1990). This illustrates that integrated crop management is mainly an optimization

approach. Several often partly con¯icting objectives have to be realized at the same time,

resulting in a situation where some can only be achieved at the cost of others. All three cases

clearly have an optimization character. Some rely heavily on fundamental knowledge about basic

ecological processes, whereas others demonstrate that long-term aspects become increasingly

important. Those cases that deal with cropping systems design are characterized by a strong

interaction between research and practice, both to ensure that the proper questions are being

addressed and to increase the likelihood of adoption by farmers.

Weed suppressive ability

Direct seeding practices have caused weeds to become a rapidly increasing problem in rice

cultivation, and therefore other means to reduce relative ®tness of weeds are being investigated.

Options for using the inherited weed-suppressing abilities of rice, such as competitive ability and

allelopathy, are being explored and should ultimately result in new lines that are better suited for

direct-seeding. Using the intrinsic weed-suppressing ability of crops has previously been

investigated for several other crops such as sugarbeet (Beta vulgaris L.) (Lotz et al., 1991),

Maize (Lindquist et al., 1998), soyabean (Callaway & Forcella, 1993) and wheat (Lemerle et al.,

1996). Procedures for selecting genotypes with an improved competitive ability are often labour-

intensive and expensive if they are based on direct selection of genotypes grown in the presence of

weeds (Wall, 1983). Furthermore, direct selection with real competition is only possible in the later

stages of a breeding programme when su�cient seed is available. Useful selection criteria in this

approach are absolute grain yield under weed-free conditions (yielding ability), relative grain yield

in the presence of weeds (competitive ability) and weed biomass (weed-suppressing ability). At the

West Africa Rice Development Association (WARDA) in the Ivory Coast, this methodology was

elaborated further by having promising lines compete with a range of crops, each of which

represented a functional group of weeds (D Johnson, pers. comm.). The advantage of using crops

as `model' weeds is that it is relatively simple to obtain and reproduce a uniform stand.

Indirect selection is an alternative screening procedure in which selection is aimed at

attributes, such as plant height, that are associated with competitive ability (Lemerle et al., 1996).

Selection can thus begin early in the breeding programme and can be carried out in the absence

of weeds. In this procedure, traits contributing to competitive ability need to be identi®ed before

actual selection, which requires a thorough quantitative understanding of crop:weed interactions

in various environments. Mechanistic models for crop:weed competition have been shown to be

useful tools for identi®cation of key traits of competitive ability. Bastiaans et al. (1997)

introduced the so-called isoline approach, in which the implications of experimentally

determined di�erences in phenology, physiology and morphology between genetic lines of rice

for competitive ability were quanti®ed by constructing hypothetical isolines. The weakest

competing genetic line was used as a starting point and the various attributes of this line were

then represented by a set of experimentally determined parameter values that were used as input

to the model. Various isolines were constructed by successively replacing original parameter

values with experimentally determined parameter values on more competitive cultivars. The

model analysis combined the e�ect of each single attribute with the experimentally determined

genetic variability to identify those attributes with the strongest potential for improving



competitive ability. The model analysis further enabled a quantitative estimation of the trade-o�

between competitive and yielding ability. For instance, a canopy with droopy leaves minimizes

light penetration, so reducing the amount of light available for emerging weeds. At the same

time, this characteristic brings about a poor light distribution within the canopy, resulting in

suboptimal utilization of the intercepted radiation for crop production. The analysis revealed

that under well-fertilized irrigated conditions the relative leaf area growth rate and plant height

were the most important determinants for competitive ability. For plant height, a trade-o�

between competitive and yielding ability was found, related to an increased risk of lodging.

Modelling in this case was found to be a good means of communication between scientists with

various backgrounds, like agronomists, plant physiologists, plant breeders and weed scientists.

Development of weed management strategies within an integrated crop management setting

needs such interaction, because by nature this approach deals with various disciplines.

Allelopathy is another option through which interactions between crop and weeds might be

altered to bene®t the crop (e.g. Putnam & Weston, 1986). At the International Rice Research

Institute, the opportunities of exploiting allelopathy as a new component of an integrated weed

management strategy in rice are under study (Olofsdotter et al., 1995). The research programme

covers a wide range of topics from the fundamental to the more applied. Elementary scienti®c

issues that are being dealt with are, for example, separation of allelopathy and competition,

identi®cation of allelochemicals, identi®cation of the genetic background of allelopathy and

investigation of the mechanism of selectivity of allelopathic rice. More applied aspects deal with

the relevance of allelopathy under ®eld conditions and the possible correlation between

allelopathic potential and physiological, morphological or phenological traits that are important

from an agronomic point of view. This last topic is also relevant from a breeding point of view,

because it might result in the identi®cation of easy-to-determine traits indicative of allelopathic

potential. Facilitation of future breeding e�orts was established through the development of a

laboratory screening technique for identifying allelopathic potential and the development of a

pot experiment technique for evaluation of promising allelopathic germplasm (Navarez &

Olofsdotter, 1996). The research group included fundamental and applied research items in their

work. This ensured and facilitated the use of allelopathic potential in breeding programmes,

whenever the results of their work would direct attention towards this. Identi®cation of

allelochemicals is currently considered to be the main bottleneck for further study, since more

mechanistic studies, for instance on selectivity and mechanisms of allelopathy, are seriously

hampered as long as the causal agent is unresolved (M Olofsdotter, pers. comm.).

Cash crops as cover crop

At the Swiss Federal Research Station for Fruit-Growing, Viticulture and Horticulture in

Wadenswil, a research programme is under way using intercropping to overcome weed problems

in vegetable ®eld crops with a weak competitive ability. Crops that are sown, and those with a

slow juvenile development, such as carrots (Daucus carota L.), onion (Allium cepa L.) and leek

(Allium porrum L.), are very sensitive to weed competition. Environmental concerns and the

growing market for high-value ecologically produced vegetables have stimulated farmers to

convert their production to integrated or organic farming. This has required a reduction in or

elimination of herbicide use. The introduction of clover and grass species as a cover crop in row-

seeded ®eld vegetables resulted in reasonable weed suppression, but, if planted too early, also

produced a clear reduction in crop yield because of resource competition between the main and

the cover crop (MuÈ ller-SchaÈ rer & Baumann, 1993). As a result, attempts were made to reduce



competition in these interplanting systems through mild suppression of cover plant growth,

through screening for less competitive cover crops and through a delay in the sowing of cover

crops. This last option gave good results in leek, when a delayed interseeding of the cover crop

was combined with mechanical weed control during the ®rst 5±6 weeks after transplanting of the

crop. In addition, there was a signi®cant reduction in attack by Thrips tabaci Lind. and a

strongly reduced nitrogen loss compared with the control plots that had bare soil after harvest

(Baumann & Imhof, 1996). However, the growers rejected the implementation of this system,

mainly because its management was too complicated and very laborious. The same research

group then started to work on the introduction of a secondary cash crop instead of a cover crop.

Intercropping of leek by celery (Apium graveolens L.) in a row-by-row replacement design

increased canopy light interception, resulting in a shorter critical period for the intercrop than for

the a pure stand of leek. Senecio vulgaris L. in¯orescence and seed number were considerably

reduced in the crop mixture, indicating a strong e�ect of light competition on the reproductive

potential of weeds (Baumann & Bastiaans, 1999). The relative yield total of the leek:celery

intercrop exceeded that of the pure stands by 10%, but the yield and the quality of leek were

negatively a�ected by interplanted celery (Baumann & Krop�, 1999). Further research is focused

on a better understanding of crop:crop competition through the development of a mechanistic

model for plant:plant interaction. This should result in a further optimization of this

intercropping system, with special emphasis on the quality of leek, which is closely related to

individual plant size. This case study illustrates once more that integrated crop management

involves optimization of various objectives and it emphasizes the importance of farmers'

involvement right from the start of research projects to ensure adoption later on. It is promising

that farmers have already started to adopt this intercropping system.

Focus on crop rotations

In The Netherlands, an innovative project for ecological arable and vegetable farming was

conducted as a joint activity of the Research Institute for Agrobiology and Soil Fertility in

Wageningen and 10 organic farms in the central clay area in The Netherlands (Vereijken et al.,

1994). A multifunctional 6-year crop rotation model was designed as a basis for achieving

objectives related to various matters such as soil fertility and the environment, product quality,

economics and energy e�ciency. The multifunctional crop rotation involved developing a well-

balanced `team' of crops so that inputs necessary to maintain soil fertility and crop health could

be minimized. For weed management, an alternation of highly competitive mown crops (e.g.

cereals) with less competitive lifted row crops (e.g. carrot, onion) was expected to have a positive

e�ect on weed management in row crops. Because of the alternation of crops, the cultivation

method would change with each crop, increasing pressure on the weeds. This, strengthened with

the intrinsic competitive ability of the mown crops, was expected to suppress the reproductive

output of weeds resulting in the maintenance of weed populations at low equilibrium densities.

Evaluation of this rotation model on ecological farms showed that weeds remained one of the

major problems. For adequate management of the weeds, the registered number of hours of

hand-weeding varied between 490 and 3100 hours per farm, mainly spent in the carrot and onion

crops, whereas 500 hours per farm was set as the target (Schotveld & Kloen, 1996). A detailed

survey conducted by Mertens (1998) showed that most of the need for hand-weeding in crops like

onion and carrot was caused by seed production of poorly competing, hardly detrimental, weeds

that developed in the canopy of the preceding cereal crop. A shift in weed ¯ora composition was

also observed on ecological farms, where weeds like Stellaria media (L.) Vill, and to a lesser



extent Poa annua L. and Capsella bursa-pastoris (L.) Med., had become major constituents. This

example clearly illustrates that in cropping systems that do not rely on herbicides long-term

aspects and particularly the population dynamics of weeds become increasingly important.

Consequently, in crop:weed interaction research, more attention should be given to the e�ect of

the crop on growth and development of the weed (Bastiaans & Drenth, 1999). It is clear from the

results of the survey of Mertens (1998), for example, that the weeds in cereals should be

considered for their seed-producing potential rather than for their immediate e�ect on grain

production. This example demonstrates again that integrated crop management involves

optimization of sometimes partly con¯icting objectives. Evaluation of the multifunctional crop

rotation model demonstrated that, although a suitable concept for a variety of objectives, weed

management remained the `Achilles heel' because of its reliance on hand-weeding. Current e�orts

for the interactive improvement and adjustment of the rotational model are therefore to a large

extent directed at improving weed management.

Characteristics of research strategies that aid in the design of sustainableweed management practices

The examples used as case studies in this review are not meant to be exhaustive. They were

selected as examples of areas in our discipline where knowledge of basic biology and ecology

underpins, and has helped to enhance, practices that have been, or soon will be, implemented in

the ®eld. Future research should be developed so that there is a reasonable chance that its results

will help to improve the design of cropping systems and ensure the ecological and economic

integrity of such systems.

To this end, we have identi®ed common attributes of research programmes that run through

the case studies highlighted in this paper. We argue that these attributes should be carefully

considered before research is undertaken. As was argued at the 11th EWRS Symposium in Basle,

Switzerland, and borne out in surveys by Moss (1994) and Norris (1992; 1997), we must avoid

`copycat' research about favourite or in-vogue subjects, and we must see the research questions

that we ask in the context of a cropping system rather than as an isolated piece of a larger system.

Common attributes

To ensure that the results of weed ecological research will bene®t cropping system design and

performance, we must begin by understanding the precise nature of the problem being addressed.

In addition, the research must be seen in the context of an integrated crop management system.

The literature is replete with examples of studies where a particular statistical method or

ecological approach is applied out of context of the study system. The result is a high level of

understanding about a particular process without an understanding of how that process is

in¯uenced by a multifactor interdependent system. If we want to see the results of our work have

an impact upon farming practices, then we must spend a considerable amount of time with

farmers in order to understand the true dimensions of our study systems. Such an approach

suggests that teams of individuals with complimentary skills will be needed to address

increasingly complex systems problems. All of the case studies highlighted in this review relied on

having a broad team to adequately de®ne the problem and with a great deal of expertise in order

to conduct the basic implementation and maintenance research needed to put basic research into

practice. Although this inclusive team-oriented approach is natural for some, it often goes



against the behaviour of hierarchical institutions in which researchers are assumed to have a

complete knowledge of all integrated cropping systems questions.

In all of the examples presented, transformational rather than transactional knowledge

resulted from the work. The former is what growers are increasingly seeking, and this requires a

full understanding of the relevant questions. The role of the applied researcher will increasingly

include building teams with non-traditional players, and learning with farmers rather than

instructing them on how a practice is to be performed (e.g. Vereijken, 1994; Vereijken & Krop�,

1995).

Successful research programmes will address the short- and long-term implications of their

®ndings. In all of the examples, signi®cant commitments in time were made. In fact, it could be

argued from these examples that systems studies take considerably longer (a factor of four or

greater) than the average life of grant-funded research projects (2±3 years). The duration of

such systems projects must re¯ect the time required to adequately address the problems. In

addition, research-funding bodies must apportion equal value to basic implementation and

maintenance research if important basic ®ndings are to result in changes to farmer practices.

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the role of ecology in the development of weed management systems: an outlook

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