Integrated Weed Management: The Rationale and ...

Weed Science Society of America

Integrated Weed Management: The Rationale and Approach Author(s): Clarence J. Swanton and Stephan F. Weise Reviewed work(s): Source: Weed Technology, Vol. 5, No. 3 (Jul. - Sep., 1991), pp. 657-663 Published by: Weed Science Society of America and Allen Press Stable URL: http://www.jstor.org/stable/3987055 . Accessed: 21/12/2012 14:44 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp

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Integrated Weed Management:The Rationale and Approach' CLARENCE J. SWANTON and STEPHAN F. WEISE2 Abstract. A growing awareness of environmentalissues in Canadahas had a major influence on governmentpolicies. An initiative was launchedby the governmentof Ontarioto promote research toward the development of an integratedweed management(IWM) system. Research in IWM must take all aspects of the cropping system into considerationand evolve in a progressive manner.This approachmust encompass the role of conservationtillage, knowledge of the critical period of weed interference, alternativemethods of weed control, enhancementof crop competitiveness, modeling of crop-weed interference,influence of crop rotation and seed bank dynamics, and education and extension of the findings. The complexity involved in addressing these issues requires a multidisciplinary approach. Additional index words: Environmentalissues, sustainable system, research strategy, crop competitiveness, conservation tillage, multi-disciplinary. RATIONALEFOR AN INTEGRATEDWEED MANAGEMENT SYSTEM

and urbancommunities.The concept of sustainabilityis an attemptto move beyond short-termproductivityand In Canada, the 1980s were characterizedby a grow- efficiency as the dominantgoals of agriculturalresearch and towardgoals thatreflect food productionas a social ing awareness of environmental issues. One of the process (37). major elements that clearly distinguishedenvironmental The need for multi-disciplinaryteam collaborationis concerns expressed by Canadians from other social issues was the realization that alteration of individual recognized to insure that relevant problems are behavior could play a key role in protecting the envir- researched and appropriateevaluation criteria are used onment. Public attitudehas shifted from environmental (26). The issues involved in sustainable agriculture, such as concerns regardingfood safety and quality, will awareness to environmentalaction. Many of these concerns have focused on environmental, economic, and have a major effect on cropping systems research, quality control, and potential new markets for both onsocial impacts of conventional agriculture. farm produce and processed foods. In addition, these Aspects of currentproduction systems in agriculture issues will continue to have a major influence on have been under serious review. The emphasis of this government policies. review process has, at the very least, attempted to Federal government policies directed through Agrichange the focus of production practices away from culture Canada, have clearly stated that environmental yield maximization.The challenge is now to produce an will be a pillar of reform for our agri-food sustainability economic crop yield while preserving and enhancing industry (1). Part of this reform package is a comprelocal, regional, and global environmentalsustainability hensive review of the role of pesticides in food produc(44). This philosophical shift has resulted in the detion. In January 1988, the OntarioMinistry of Agriculvelopment of the concept most frequentlyreferredto as ture and Food announced a major research initiative sustainable agriculture. entitled Food Systems 2002, aimed at addressingpublic Attempts have been made by numerous authors to concerns in regard to sustainability,pesticide use, food provide a working defition of sustainable agriculture and food quality. Part of this initiative was (1, 26, 30, 36, 47). Within the various definitions, safety, designed to promote research toward achieving a 50% common themes include: environmentalquality, renewreduction in the total amount of pesticides applied into able resources, food safety, technology assessment, ecothe environment by the year 2002. This goal is to be nomic feasibility, and the enhancementof life in rural achieved while taining crop yields and providing effective pest control at moderate costs. In Ontario approximately71% of all pesticides sold 1Received for publication Jan. 18, 1991, and in revised form July 15, are herbicides (41). Most of these herbicides are used in 1991. the production of corn (Zea mays L.) and soybeans 2Assoc. Prof., Res. Assoc., Dep. Crop Sci., Univ. Guelph, Guelph, ON., Canada NlG 2W1. [Glycine max (L.) Merr.]. The challenge in Ontariois to 657


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develop a weed management system, particularly for these crops, in which weed control does not primarily depend upon herbicides. It is our opinion that the debate surroundingherbicide use is not in defense of their need, but rather the rationalization of their use with regard to environmental concerns. Such an approach requires the integrationof alternativeweed control measures. Integrated weed management (IWM)3 has been defined as the application of numerous alternative weed control measures, which include cultural, genetic, mechanical, biological, and chemical means of weed control (48, 50, 58). None of the individual control measures on their own can be expected to provide acceptable levels of weed control. However, if the various components of IWM are implemented in a systematic manner, significant advances in weed control technology can be achieved. Numerous authors have developed IWM practices for selected weed species (24, 33). There is a need to develop a logical sequence of studies that would incorporate the various components of IWM into a systems approach to weed management. This system must enhance the competitive ability of the crop and at the same time provide adequate weed control. The objective of this paper is not to provide a detailed literature review on these components, but rather to provide a perspective on a systems approachto the development of an IWM research strategy. AN APPROACHTO INTEGRATEDWEED MANAGEMENTRESEARCH The research approach to the development of an IWM system must take all aspects of the cropping system into consideration. Invariably, each cultural practice influences the competitive ability of both the crop and the weed community leading to a multitudeof complex interactions.However, efforts must be made to work within the existing productionpractice to ensure a greater likelihood of acceptance by the farning community. Thus, it is important to change the existing system in a progressive manner. This progression must be reflected in the research strategy. This would allow

3Abbreviations:IWM, integrated weed management. 4Letters following this symbol are a WSSA-approved computer code from Composite List of Weeds, Revised 1989. Available from WSSA, 309 W. Clark St., Champaign, IL 61820.

for the transferof specific components through education and extension, while research continues to refine and further develop the system. Such a strategy is outlined in Figure 1. Tillage system. One of the most important changes occunringin agriculturetoday is the increased awareness and acceptance by the agriculturalcommunity of conservation tillage practices. Conservation tillage, which advocates the maintenanceof crop residue cover on 30% of the soil surface, is soundly based witiin the framework of sustainable agriculture. Numerous authorshave criticized conservation tillage particularly in relation to the potential for lower yields, increased perennialweed problems, and most notably the perception that, as tillage is reduced, herbicide use increases (18, 31). However, extensive on-farm research in conservation tillage systems in Ontario has shown that average crop yields were comparable to conventional tillage yields (3). Under proper management,perennial weeds such as quackgrass[Elytrigiarepens (L.) Nevski. #4 AGRRE] have been readily controlled with proper herbicide selection and time of application (9). Research conducted in western Canada (15, 56) found that conservation tillage systems did not necessarily have weed communities that were more difficult to control than in conventional tillage systems. In addition, the presence of crop residue increased weed suppression and did not result in an increase in herbicide dosage or applications(21, 29). It is our hypothesis that the potential for a reduction in total herbicide use is greater in conservation tillage, particularly in no-till, than can be achieved in conventional tillage systems. This reduction in total herbicide use will likely be a result of changes in the population dynamics of seed banks (23, 54) and of rhizomes and tubers (9) caused by limited soil disturbance. Critical period of weed interference. To optimize herbicide use within a given tillage system and at the same time provide a logical frameworkfor the integration of alternativeweed control measures, information on the critical period of weed interferenceis essential. There are two recognized components to the critical period. The first component is the length of time weed control efforts must be maintainedto prevent crop yield loss; whereas the second component is the length of time weeds can remain in the crop before they interfere with crop growth and ultimately reduce yield (61, 64). This concept does not provide mechanistic descriptions of weed and crop interference but is primarily con-

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EDUCATION

TILLAGE SYSTEM CROP

CRITICAL

ROTATION ANDSEEDOFWE /

PERIO

/BANKDYNAMICSv

INTERFERENCE

PRODUCTION SYSTEM ALTERNATIVE MODELING OF CROP-WEED INTERFERENCE

METHODS OF WEED CONTROL

ENHANCEMENT OF CROP COMPETITIVENESS

EXTENSION Figure 1. Research strategy for the development of an integrated weed management system.

cerned with crop yield losses (64). Informationdefining the duration of the critical period can be used to potentiallyreduce the need for longterm residual control with herbicides, optimize the dose and timeliness of postemergence herbicides, influence the timing of cover crop seeding and mechanical cultivations. Studies conducted in Ontario have found the critical period to occur between the second trifoliate and first flower stage of growth in white beans (Phaseolus vulgaris L.) (63) and from the 4-leaf to 14-leaf stage of growth in corn (25). Although duration of the critical period can be affected by numerous factors such as weed species, planting pattern, and environmental conditions, an early assessment of this time-frame will provide a guide for future studies in alternative methods of weed control and crop-weed interference. Alternative methods of weed control. Cover crops. The growing of annual crops is characterizedby cropfree periods and, particularlyin row crops, periods of little ground-cover resulting in opportunitiesfor weed

5Personal communication. D. Falk, Assoc. Prof., Dep. Crop Sci., Univ. Guelph, Guelph, ON., Canada NlG 2W1. Volume 5, Issue 3 (July-September)

establishment and growth. The use of cover crops and their mulches duringthese periods is a way of suppressing weeds, particularlyin conservation tillage systems (20, 46). Cover crops can be sown into a standing crop or into a stubble after crop harvest. The timing of the sowing is critical in the first method, because establishment of a cover crop at too early a growth stage of the main crop can result in unacceptableyield losses. Eadie (19) used the end point of the critical period of weed interference as a guide for planting different cover crops into a stand of growing corn in a conservation tillage system. Successfully established cover crops may develop dense enough'canopies in the fall to interfere with the growth of perennial and winter annual weeds (49). In the spring, cover crops that have been killed by winter conditions (e.g., spring cereals) or winter-hardycover crops (e.g., fall cereals, perennial clovers) that have been mechanically or chemically killed prior to or soon after planting of the main crop form an organic mulch on the soil surface. In addition to the physical suppression of weeds by the mulch cover, certain plants used as mulches, e.g., rye (Secale cereale L.) and barley (Hordeumvulgare L.), contain allelochemicals that further suppress weed establishment and growth (45). Ideally, a mulch should adequately suppress the weeds duringthe critical period of weed interferenceto avoid yield loss in the main crop. Studies conducted in a no-till system with small grain mulches in soybeans on several soil types in Ontario have indicated the feasibility (except on very heavy soils) of using chemically-killed triticale (x Triticosecale Wittmack) and winter rye mulch to suppressweed growth (40, 57). The breeding of a winter-hardy cover crop that dies out naturallybefore the start of the critical period of weed interference of the main crop would further enhance this alternative method of weed control5. Cultivation.Inital flushes of anual weeds in row crops can be controlled nonchemically by shallow cultivation of the soil, often requiringa total of four to six passes (43). However, such intensive cultivation could potentially result in increased surface soil degradation and erosion. The banding of herbicides over the crop rows and the removal of weeds between the rows with one or two passes of an inter-row cultivator constitutes an alternativeweed control method. This method reduces the intensity of cultivation of the nonchemical system and contributesto the optimization of herbicide use in comparison with the conventional broadcastingof her-

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bicides. Furthenmore,inter-rowcultivation can be timed to keep the crop free of weed interference during the critical period. Studies in Ontario have shown that banding of preemergence herbicides combined with inter-rowcultivation can reduce the amount of herbicide required to obtain optimal weed control and crop yields by 50 to 70% in both conventional (63) and conservationtillage systems (19). The numberof cultivations and the herbicide dosage in the band depended on the level and type of weed infestation, soil type, and climatic conditions. Biological control. Biological control of weeds is the deliberate use of natural enemies, primarily insects or fungi, to suppress growth or reduce the population of weed species (60). The two most common approaches are the classical or inoculative method and the bioherbicide or inundative strategy (59, 60). The classical approach is generally used in perennial cropping situations like pastures, whereas bioherbicides are used in annual crops. The classical method is being used in Ontario to attempt control of leafy spurge (Euphorbiaesula L. # EPHES), cypress spurge (Euphorbia cyparissias L. # EPHCY), and common St. Johnswort (Hypericumperforatum L. # HYPPE) using different insects as control agents6. The advantage of this method is that once the exotic control agent has been tested successfully and released, it reproduces and disperses on its own to all suitable habitats containing the target weed, thus ensuring reasonably permanentmanagement and not resulting in recurrentweed control costs. Because restriction of control to a limited area is rarely possible and the measure generally not reversible, control of the target weed should not have any major ecological effect and the agent selected needs to be specific to the target weed population. In contrast, the bioherbicide method controls the weed only in the area of application by the periodic dispersal of an abundant supply of a control agent, often a fungal pathogen. Although presently only two bioherbicides are commercially available, Collegom for the control of northem jointvetch [Aeschynomenevirginica (L.) B.S.P. # AESVI] and DeVinem for stranglervine [Morrenia odorata (H. & A.) Lindl. # MONOD], many other bioherbicides are being deve-

6Personal communication.J. F. Alex, Prof., Dep. Environ. Biol., Univ. Guelph, Guelph, ON., Canada NIG 2W1.

loped, for example for the control of barnyardgrass [Echinochloa crus-galli (L.) Beauv. # ECHCG], field bindweed (Convolvulusarvensis L. # CONAR), roundleaved mallow (Malva pusilla Sm.), velvetleaf (Abutilon theophrasti Medic. # ABUTH) (60). Eradication of the target weed population is not possible with biological control (59, 60). However, rapid and complete weed control is not required, as long as the targetweeds are reduced in their vitality and their competitive ability and biological control is complemented by other measures within an IWM system (55, 60). Consequently,biological control could constitute an importantelement in the suppressionbelow the economic threshold level of a particularweed population escaping other IWM measures. Enhancement of crop competitiveness. Cultivar competitiveness. Crop and weeds compete for resources, particularlyfor light, water, and nutrients(5). An IWM programshould attemptto effectively exploit the competitive ability of crops in suppressing weed growth. Different crops and cultivars can reduce weed biomass from 4 to 83% during a full season of competition (39). Barrie (4) found that short early maturing varieties of soybean in Ontariowere less competitive than tall later maturing varieties. Similarly, Malik (38) reported that white bean cultivars varying in growth habit differed in their ability to compete with weeds under season-long weed pressure. Increased competitive ability of cultivars has been attributedto early emergence, seedling vigor, increased rate of leaf expansion, rapid creation of a dense canopy, increased plant height, early root growth, and increased root size (5, 22, 39). Futurebreeding and variety testing programsshould take factors of crop competitive ability into consideration. Informationon the degree of competitiveness of the recommendedvarieties must be collected. This informationcould contributeto optimizing herbicide use efficiency through factor-adjusteddose recommendations.In addition, the planting of competitive cultivars and more aggressive crops in rotations could complement other IWM measures in enhancing weed suppression. Planting patern. The establishment of a crop with a more uniform and dense plant distributionmay result in better use of light, water, and nutrients and lead to greater crop competitive ability (5, 39). Studies in Ontariowith soybeans and white beans have shown that weed growth was suppressed by planting in narrower rows (4, 38). Crops grown in narrower rows start

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competing with weeds at an earlier stage than those in wide rows because of more rapid canopy closure (35) and probably better root distribution. Although ongoing studies with corn indicate that increasing plant density can significantly reduce weed biomass accumulation7, higher densities of soybeans and white beans did not affect weed biomass within the ranges tested (4, 38). It appearsthat soybeans and white beans can compensate for lower plant densities with increased branching and possibly greater lateral root growth to rapidly occupy and exploit available resources. In addition, climatic and edaphic factors, such as soil moisture and fertility can have a significant effect on optimal plant density (39, 58). The manipulation of planting pattern for greater weed suppression should also allow for interrow cultivation where applicable, not increase the incidence of diseases and insects, ensure harvesting efficiency, and provide optimum crop yields. Nutrient placement. Crops and weeds generally compete for the same nutrientpool. Increasing the level of soil fertility can alter the competitive interactions between crops and weeds. Weed resource use often increases more rapidly with added nutrients,resulting in a greater ability of the weeds to compete for other resources (5, 39, 58). The influence of weed density (7) and time of nutrient application (51) on the relative crop response underscores the complexity of competition for nutrients. In addition, the application of nutrients to a soil, particularlynitrogen, can stimulate germination of dormant weed seeds (8). One way of potentially avoiding some of these problems would be to place the limiting nutrient source close to the crop (5, 51). Nitrogen, the major nutrient for which plants compete (2), could be banded close to the crop row, thus enhancing the crop's accessibility to the nutrient. Manipulating nitrogen nutrition in this manner would selectively enhance crop competitiveness over the weeds growing primarily between the crop rows. Modeling of crop-weed interference. The development of an integrated weed management system is a complex task and must be supported by a thorough understanding of crop-weed interference. Computer models can integrate available information and predict the outcome of crop-weed interference.

7Personal communication. A Dibo, Grad. Res. Asst, Dep. Crop Sci., Univ. Guelph, Guelph, ON., Canada NlG 2W1.

The hyperbolic weed density-yield model is an empirical regression model widely applied to depict crop yield loss associated with a particularweed density at a specific moment in the growing season (10). However, crop yield losses at a given density of a given weed species can vary between sites and over years. Much of this variation is attributedto differences in the time of weed seedling emergence relative to crop emergence (32, 53). In their critical period studies with white beans and corn, Woolley (63) and Hall (25), respectively, found that yield losses declined as seedling emergence of a natural population of weed species was delayed. The precision of weed density-yield models can be improved by incorporating relative time of emergence (12, 28) or relative leaf area as a factor (32, 53). Weed density-yield models are the basis for development of economic weed threshold models that predict the weed density at which specific post-emergence weed control measures are warranted (11). However, the prediction of an economic threshold level can be only as accurate as the underlying weed density-yield model and the cost and efficiency estimate of control. In addition, such models need to account for the longterm effect of allowing weeds at densities below the thresholdlevel to go to seed and potentially increasing the weed pressure in subsequent years. Populationdynamics models can be used for assessing the long-term iplications of control measures on weed populations (13, 17). Long-term economic weed threshold levels can thus be predicted by incorporating weed density-yield models into population dynamic models (13, 17). Despite these refinements, the general applicability of models based primarily on empirical regressions can be limited by potentially large fluctuations in model parametersdue to variability in edaphic, climatic, and cultural factors. The complexities of crop-weed interferenceappearto be predictable over a wider range of conditions by using a mechanistic approach to modeling (32, 52). This approach is based on physiological mechanisms and processes, and takes the effect of environmental factors on growth and development into account. Mechanistc models have been developed for most major crops, for example the corn growth model MAIS from the University of Guelph8. However, few mechanisticmodels exist for weed species (34, 42). The mechanistic modeling of crop-weed interactions has been used even more sparingly (32, 52). Furthermore, 661



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models have to be developed that are able to predict the time of weed seedling emergence and incorporatemore than one weed species, because crops in the field are generally exposed to varying weed populations. Crop rotation and seed bank dynamics. The composition and density of weed seed banks are frequently a reflection of long-term crop rotation and management systems (23). Several authorshave reportedchanges in the weed flora when monocropping has been replaced with crop rotation (14, 58, 62). However, in any given year the crop species, planting date, herbicide selection, and weather patterns may alter species composition of the weed community (14, 16, 23, 27). Thereforeknowledge of the role of crop rotation in seed bank dynamics would be useful in the refinementof an integratedweed management system. Particular research emphasis should be placed on assessing the impact of conservation tillage practices and crop rotation on seed bank dynamics. The long-term impact of less-than-perfect weed control must be examined in relation to harvestability, seed quality, and impact of weed seed return on seed bank dynamics. Research must be able to provide reliable evidence that the agronomic and ecological bases upon which the principles of integrated weed management are founded do not create unwarranted risks for the farner. Education and extension. The successful implementation of an integrated weed management system is highly dependentupon the efficient and thoroughtransfer of information and technology by education and extension (6). Efforts need to be focused within selected groups of progressive farmers. On-farm extension, workshops, tours, and seminars need to be carefully orchestratedto highlight recent changes. This core of innovative farmers can then act as the main vehicle for extending and implementing new information and technology within the farm community. To successfully implement an integratedweed management system, extension personnel must be much more than weed scouts. They must be competently trained in agronomy and ecology. Extension agronomistsmust be aware of all the importantbiological and physical components of their cropping systems and must integrate this knowledge at the community level if they are to meet the challenges of economic and environmental sustainability (44). 8Personal communication. M. Tollenar, Assoc. Prof., Dep. Crop Sci., Univ. Guelph, Guelph, ON., Canada NIG 2W1.

Integrated weed management systems will have to remain flexible to adjust to changing environmental, technological, economic, and social factors, while at the same time incorporatingthe long-term impact of specific measures. Consequently, IWM systems must be approachedand evaluated from different perspectives. The complexity involved in addressingthese issues will require a multi-disciplinary approach. ACKNOWLEDGMENTS The authors thank K. Chandl& and D. Derlsen for their constructive comments to the manuscript.The supportof the OntarioMinistryof Agriculture and Food (Food Systems 2002) is gratefully acknowledged.

LITERATURECITED 1. Agriculture Canada. 1989. Growing together. A vision for Canada's agri-food industry.Agiculture CanadaPubl. 52691E. Communications Branch, Agriculture Canada, Ottawa, ON. 74 p. 2. Anderson, W. P. 1983. Weed Science: Principles. 2nd ed. West Publ., St. Paul, MN. 655 p. 3. Aspinall, J. D., R G. Kachanoski, and H. C. Lang. 1989. Tillage 2000 Soil Conservation. Progress Report. Ontario Ministry of Agriculture and Food, Guelph, ON. 24 p. 4. Barrie,E. C. 1969. Influence of crop row spacing, populationand plant type on weed control in soybeans. M.S. Thesis, Univ. Guelph, Guelph, ON. 55 p. 5. Berkowitz, A. R 1988. Competition for resources in weed-crop mixtures. p. 89-119 in M. A. Altieri and M. Liebman, eds. Weed Management in Agroecosystems: Ecological Approaches. CRC Press, Boca Raton, FL. 6. Blair, B. D., and J. V. Parochetti. 1982. Extension implementationof integratedpest gement systems. Weed Sci. 30 (Suppl. 1):48-53. 7. Carlson,H. L., and J. E. Hill. 1986. Wild oat (Avenafatua) competition with springwheat: effects of nitrogenferdlization.Weed Sci. 34:29-33. 8. Cavers, P. B., and D. L. Benoit. 1989. Seed banks in arable land. p. 309-328 in h. A. Leck, V. T. Parker,and R. L. Simpson, eds. Ecology of Soil Seed Banks. Academic Press, San Diego. 9. Chandler,K., and C. J. Swanton. 1990. Effect of fillage on control of quackgrass [Agropyronrepens (L.) Beauv.]. Proc. QuwckgrassSymp. Oct. 24-25, London, ON. p. 135-150. 10. Cousens, R 1985. A simple model relating yield loss to weed density. Ann. Appl. Biol. 107:239-252. 11. Cousens, R D. 1987. Theory and reality of weed control thresholds. Plant Prot. Q. 2:13-20. 12. Cousens, R., P. Brain, J. T. O'Donovan, and P. A. O'Sullivan. 1987. The use of biologically realistic equations to describe the effects of weed density and relative time of emergence on crop yield. Weed Sci. 35:720-725. 13. Cousens, R., C. J. Doyle, B. J. Wilson, and G. W. Cussans. 1986. Modelling the economics of controlling Avena fatua in winter wheat Pestic. Sci. 17:1-12. 14. Dale, J. E., and J. M. Chandler. 1979. Herbicide-crop rotation for johnsongrass (Sorghum halepense) control. Weed Sci. 27:479-485. 15. Derksen, D. 1990. Weed control within crops in a conservation tillage system. p. 75-90 in G. P. Lafond and D. B. Powler, eds. Crop Managementfor Conservation.Proc. Soil Conserv. Symp. Feb. 22-23, Yorkton, SK. 16. Dowler, C. C., E. W. Hauser, and A. W. Johnson. 1974. Crop-herbicide sequences on a southeasterncoastal plain soil. Weed Sci. 22:500-505. 17. Doyle, C. L., R Cousens, and S. R. Moss. 1986. A model of the economics of controlling Alopecurus myosuroides Huds. in winter wheat. Crop Prot. 5:143-150. 18. Duffy, M., and K Hanthon. 1984. Retuns to com and soybean tllage

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WIED TECHNOLOGY practices. Agric. Econ. Rep. 508. U.S. Dep. Agric., Washington, DC. 14 p. 19. Eadie, A. G. 1991. The role of inter-rowcultivationand cover crops for weed management in conservation tillage systems. M.S. Thesis, Univ. Guelph, Guelph, ON. (in preparation). 20. Enache, A., and R. D. ILnickd.1988. Subterraneanclover: a new approach to weed control. Proc. Northeast. Weed Sci. Soc. 42:34. 21. Erbach, D. C., and W. G. Lovely. 1975. Effect of plant residue on herbicide performance in no-tillage cor. Weed Sci. 23:512-515. 22. Forcella, F. 1987. Characteristicsassociated with highly competitive soybeans. Agron. Abstr. 1987:111. 23. Froud-Williams, R. J. 1988. Changes in weed flora with different tillage and agronomic management systems. p. 213-236 in M. A. Altieri and M. Liebman, eds. Weed Management in Agroecosystems: Ecological Approaches. CRC Press, Boca Raton, FL. 24. Glaze, N. C. 1987. Culturaland mechanical manipulationof Cyperus spp. Weed Technol. 1:82-83. 25. Hall, M. R. 1990. The critical period of weed control in grain corn (Zea mays L.) and the impact of weed interferenceupon corn development. M.S. Thesis, Univ. Guelph, Guelph, ON. 99 p. 26. Hildebrand, P. E. 1990. Agronomy's role in sustainable agriculture: integrated farming systems. J. Prod. Agric. 3:285-288. 27. Holzner, W. 1982. Concepts, categories and characteristicsof weeds. p. 3-20 in W. Holzner and M. Numata, eds. Biology and Ecology of Weeds. Junk, The Hague. 28. Hume, L. 1989. Yield losses in wheat due to weed communities dominatedby green foxtail [Setaria virdis (L.) Beauv.]: a multispecies approach. Can. J. Plant Sci. 69:521-529. 29. Johnson, M. D., D. L. Wyse, and W. E. Lueschen. 1989. The influence of herbicide formulationon weed control in four tillage systems. Weed Sci. 37:239-249. 30. Keeny, D. 1990. Sustainable agriculture:definition and concepts. J. Prod. Agric. 3:281-285. 31. Koskinen, W. C., and C. G. McWhorter. 1986. Weed control in conservation ti11age.J. Soil Water Conserv. 41:365-370. 32. Kropff, M. J. 1988. Modelling the effects of weeds on crop production. Weed Res. 28:465-471. 33. Lee, G. A. 1986. Integratedcontrol of rush skeletonweed (Chondrilla juncea) in the western U.S. Weed Sci. 34 (Suppl. 1):2-6. 34. Ldgere, A., and M. M Schreiber. 1988. Simulationof redrootpigweed (Amaranthusretroflexus) growth, development and validation of the model AMSIM. VIIe Colloque Int. sur la Biol., l'Ecol. et la Syst. des Mauvaises Herbes 2:641-647. 35. Legere, A., and M. M. Schreiber. 1989. Competition and canopy architecture as affected by soybean (Glycine max) row width and density of redroot pigweed (Amaranthusretroflexus). Weed Sci. 37: 84-92. 36. Lockeretz, W. 1988. Open questions in sustainableagriculture.Am. J. Altem. Agric. 3:174-181. 37. MacRae, R. J., S. B. Hill, J. Henning, and G. R. Mehuys. 1988. Agricultural Science and Sustainable Agriculture: a review of the existing scientific barriersto sustainablefood productionand potential solutions. Res. Paper no. 6. Macdonald College of McGill University, Ste-Anne de Bellevue, PQ. 61 p. 38. Malik, V. S. 1990. Impact of white bean (Phaseolus vulgaris L.) cultivars, row-spacing and seeding density on annualweed interference. M.S. Thesis, Univ. Guelph, Guelph, ON. 93 p. 39. Minotti, P. L., and R. D. Sweet. 1981. Role of crop competition in limiting losses from weeds. p. 351-367 in D. Pimentel, ed. CRC Handbook of Pest Management in Agriculture, Vol. II. CRC Press, Boca Raton, FL. 40. Moore, M. J. 1991. The effect of cover crops on soybean [Glycine max (L.) Merr.] and weed development. M.S. Thesis, Univ. Guelph, Guelph, ON. (in preparation). 41. Moxley, J. 1989. Survey of pesticide use in Ontario, 1988. Econ. Info. Rep. no. 89-08. Economics and Policy CoordinationBranch, Ontario Mnistry of Agriculture and Food, Toronto, ON. 40 p.

42. Orwick, P. L., M. M. Schreiber, and D. A. Holt. 1978. Simulation of

foxtail (Setaria iridisvar. robusta-alba, Setariaviridisvar. robustapurpurea) growth: the development of SETSIME Weed Sci. 26:

691-699. 43. Parks, D. L. 1955. Successful Crop Production in Eastem Canada. McClelland and Stewart, Canada. 314 p. 44. Paul, E. A., and G. P. Robertson. 1989. Ecology and the agricultural sciences: a false dichotomy? Ecology 70:1594-1597. 45. Putnam,A. R. 1988. Allelopathy: problems and opportunitiesin weed management.p. 77-88 in M. A. Altieri and M. Liebman, eds. Weed Management in Agroecosystems: Ecological Approaches. CRC Press, Boca Raton, FL. 46. Putnam, A. R., J. DeFrank, and J. P. Banes. 1983. Exploitation of allelopathy for weed control in annual and perennial cropping systems. J. Chem. Ecol. 9:1001-1010. 47. Reganold, J. P., R. I. Papendick, and J. F. Parr. 1990. Sustainable agriculture. Sci. Am. 1990 (June):112-120. 48. Regnier, E. E., and R. R. Janke. 1990. Evolving strategiesfor managing weeds. p. 174-202 in C. A. Edwards,R. Lal, P. Madden, R. H. Miller, and G. House, eds. Sustainable Agricultural Systems. Soil Water Conser. Soc., Ankeny, IA. 49. Samson, R. A. 1989. On-farm evaluation of cultivation, cover crops and chemical banding for crop and weed management in integrated farming systems. MS. Thesis, MacdonaldCollege of McGill Univ., Ste Anne de Bellevue, PQ. 50. Shaw, W. C. 1982. Integratedweed management systems technology for pest management. Weed Sci. 30 (Suppl. 1):2-12. 51. Smith, R. J., Jr., and W. C. Shaw. 1966. Weeds and their control in rice production. U.S. Dep. Agric. Bull. 292. 64 p. 52. Spitters,CJ.T., and R. Aerts. 1983. Simulationof competitionfor light and water in crop-weed associations. Aspects Appl. Biol. 4:467-484. 53. Spitters, CJ.T., M. J. Kropff, and W. deGroot. 1989. Competition between maize and Echinochloa crus-galli analysed by a hyperbolic regression model. Anm. Appl. Biol. 115:541-551. 54. Swanton, C. J., and D. L. Benoit. 1989. Dynamique des populations de mauvaises herbes en pratiques culturales reduites. Joum6e d'information sur la malberbologie. Conseil des productions v6g6tales du Qu6bec. Agdex 640. p. 29-37. 55. Templeton, G. E., R. J. Smith Jr., and D. 0. TeBeest. 1986. Progress and potential of weed control with mycoherbicides.Rev. Weed Sci. 2: 1-14. 56. Thomas, A. G., and D. Derksen. 1990. Changes in weed spectrumwith changes in tillage practies. p. 59-74 in G. P. Lafond and D. B. Fowler, eds. Crop Management for Conservation.Proc. Soil Conserv. Symp. Feb. 22-23, York-ton,SK. 57. Voutsinos, Mi 1989. Effect of a rye (S. cereale L.) mulch on soybean (G. max L.) growth, yield, and microclimate, and on associated weed populations. MS. Thesis, Univ. Guelph, Guelph, ON. 87 p. 58. Walker, R. H., and G. A. Buchanan. 1982. Crop manipulation in integratedweed managementsystems. Weed Sci. 30 (Suppl. 1):17-24. 59. Wapshere, A. J., E. S. Delfosse, and J. AL Cullen. 1989. Recent developments in biological control of weeds. Crop Prot. 8:227-250. 60. Watson, A. K. 1989. Currentadvances in bioherbicide research. Proc. Br. Crop Prot Conf.-Weeds 3:987-96. 61. Weaver, S. E. 1984. Critical period of weed competition in three vegetable crops in relation to management practices. Weed Res. 24: 317-325. 62. Wilson, B. J., and P. A. Phipps. 1985. A long term experiment on tillage, rotationand herbicide use for the control of A. fatua in cereals. Proc. Br. Crop Prot. Conf.-Weeds 2:693-700. 63. Woolley, B. L. 1989. Integrated weed management in white beans

(PhaseolusvulgarisL.). MS. Thesis,Univ. Guelph,Guelph,ON. 144 P. 64. Zimdahl,R. L. 1988. The concept and applicationof the critical weedfree period. p. 145-155 in AL A. Altieri and M. Liebman, eds. Weed Management in Agroecosystems: Ecological Approaches. CRC Press, Boca Raton, FL.


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