Iowa State University
American farmers currently apply more than 200 million kilograms of herbicide-active ingredients each year (Aspelin and Grube, 1999). Although herbicides have reduced farm labor demands, heavy reliance on chemical weed control has had a number of undesirable consequences, including ground and surface water contamination (Barbash et al., 1999; Larson et al., 1999; U.S. Geological Survey, 1999) and shifts in weed community composition toward herbicide-tolerant species and herbicide-resistant genotypes (Heap, 1999; Radosevich et al., 1997). Weed management strategies that rely more on manipulations of ecological processes and less on herbicide technology are needed to address these and other problems.
Ecologically based weed management strategies begin with the premise that no single tactic will remain successful in the face of genetically heterogeneous weed populations, range expansions by dispersing weed species, variable weather conditions, and changes in crop management practices. Rather than relying on a single “large hammer,” such as herbicide technology, to suppress weeds, ecologically based strategies seek to integrate many “little hammers” that act in concert to stress and kill a wide range of weed species at many points in their life cycles (Liebman and Gallandt, 1997). By spreading the burden of crop protection over multiple, temporally variable tactics, ecologically based weed management strategies can reduce risks of crop loss and limit rates of weed adaptation to control tactics.
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PROFESSIONAL SOCIETIES and Ecologically Based Pest Management: Proceedings of a Workshop 5 Opportunities to Integrate Soil, Crop, and Weed Management in Low-External-Input Farming Systems MATT LIEBMAN Iowa State University American farmers currently apply more than 200 million kilograms of herbicide-active ingredients each year (Aspelin and Grube, 1999). Although herbicides have reduced farm labor demands, heavy reliance on chemical weed control has had a number of undesirable consequences, including ground and surface water contamination (Barbash et al., 1999; Larson et al., 1999; U.S. Geological Survey, 1999) and shifts in weed community composition toward herbicide-tolerant species and herbicide-resistant genotypes (Heap, 1999; Radosevich et al., 1997). Weed management strategies that rely more on manipulations of ecological processes and less on herbicide technology are needed to address these and other problems. Ecologically based weed management strategies begin with the premise that no single tactic will remain successful in the face of genetically heterogeneous weed populations, range expansions by dispersing weed species, variable weather conditions, and changes in crop management practices. Rather than relying on a single “large hammer,” such as herbicide technology, to suppress weeds, ecologically based strategies seek to integrate many “little hammers” that act in concert to stress and kill a wide range of weed species at many points in their life cycles (Liebman and Gallandt, 1997). By spreading the burden of crop protection over multiple, temporally variable tactics, ecologically based weed management strategies can reduce risks of crop loss and limit rates of weed adaptation to control tactics.
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PROFESSIONAL SOCIETIES and Ecologically Based Pest Management: Proceedings of a Workshop One approach for using ecological knowledge to increase the range of tactics available for weed management involves managing soil conditions to suppress weed emergence, growth, and competitive ability (Gallandt et al.,1999; Liebman and Davis, 2000). As a consequence of the fixed root habit, both weeds and crops are affected by soil conditions. Weeds and crops may differ in their responses to those conditions, however. Just as certain herbicides are toxic to weeds but do little or no harm to crops, certain edaphic factors may suppress weeds but have neutral or positive effects on crops. Legume residues, composts, and manures are widely used in organic and low-external-input farming systems to maintain soil productivity. Experiments conducted with several crop–weed combinations indicate that use of these soil amendments in place of synthetic fertilizer can also reduce weed density, biomass production, and competitive ability, at the same time maintaining or improving crop performance. This has been true for sweet corn and field corn grown in competition with Chenopodium album (Dyck and Liebman, 1995; Dyck et al.,1995); dry bean in competition with Brassica. kaber (Liebman and Ohno, 1998); and potato in competition with Chenopodium album and other weed species (Gallandt et al.,1998). Several factors appear to be responsible for the differential responses of crop and weed species to soil amendments. First, the substantial differences in seed size that exist between many weeds and crops (Mohler, 1996) may convey differential susceptibility to early-season stresses that limit photosynthesis and nutrient uptake. Because of greater seed reserves, seedlings of large-seeded crops, such as corn and soybean, are likely to be more tolerant of low-nutrient conditions and soilborne phytotoxins, whereas seedlings of small-seeded annual weeds are likely to be less tolerant of these and other stresses (Westoby et al., 1996). Second, many annual weed species are better adapted than crops for rapid nutrient uptake and biomass production early in the growing season (Alkämper, 1976; DiTomaso, 1995). Consequently, under conditions of high nutrient availability, many small-seeded weeds are highly competitive with large-seeded crops, despite their lack of seed reserves. Changes in the timing of nutrient availability have been investigated for their potential to alter weed growth and competitive ability. By delaying the application of synthetic fertilizer until several weeks after crop and weed germination, the growth of weeds such as B. kaber, Chenopodium album, and Veronica hederifolia can be reduced and the yield of crops such as corn and wheat can be increased (Alkämper et al., 1979; Angonin et al., 1996). Certain crop residues, composts, and manures may serve as slow-release nutrient sources that are better synchronized with the nutrient demands of crops rather than weeds (Dyck et al., 1995; Liebman and Davis, 2000). Specific rates of nutrient release from organic soil amendments will depend on substrate quality, soil temperature and moisture conditions, and other factors, but could be regulated advantageously. Third, crop residues, composts, and manures serve as sources of biochemicals that can affect crop and weed growth. Some of these compounds are growth inhibiting, whereas others are growth promoting. In addition to containing nitrogen, which generally stimulates plant growth, legume green manures can release a range of phytotoxic compounds (Liebman and Ohno,
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PROFESSIONAL SOCIETIES and Ecologically Based Pest Management: Proceedings of a Workshop 1998) that may be particularly damaging to small-seeded species (Liebman and Davis, 2000). Early in the decomposition process, composts and manures may release acetic acid, phenols, ammonia, and other organic compounds at concentrations high enough to be phytotoxic (Ozores-Hampton et al., 1999; Tiquia and Tam, 1998; Zucconi et al., 1985). Alternatively, mature composts and well-rotted manures can serve as sources of growth-stimulating substances, such as indole-3-acetic acid, and humic and fulvic acids (Chen and Aviad, 1990; Valdrighi et al., 1996). Beneficial effects of humic substances on plant growth are believed to result from increased membrane permeability, greater nutrient uptake, enhanced protein synthesis and photosynthesis, changes in enzyme activity, and effects similar to those resulting from application of plant growth regulators (Chen and Aviad, 1990). For reasons mentioned previously, small-seeded weeds may be more susceptible than large-seeded crops to compost-derived phytotoxins; this needs to be tested experimentally. It also remains to be learned whether growth-stimulating substances from compost differentially affect weeds and crops. The timing and balance of growth-inhibiting and growth-stimulating effects of soil amendments merit more research. Finally, applications of crop residues, composts, and manures may change soil microbial conditions affecting weeds and crops. Applications of fresh plant materials and manure can increase the activity of soilborne pathogens, such as Pythium, Phytophthora, and Rhizoctonia spp. (Cook and Baker, 1983; Dabney et al.,1996). Whether or not such pathogens can be managed to differentially attack weeds and crops needs to be investigated. Alternatively, organic soil amendments may reduce the incidence of pathogens attacking crops and improve their growth by increasing colonization of the root zone by beneficial microorganisms (Cook and Baker, 1983; Pankhurst and Lynch, 1995). More research is needed to determine whether such an effect could increase a crop's ability to compete with weeds. The aforementioned soil-related factors represent only a small subset of the many components of farming systems that can be manipulated to contribute to ecologically based weed management strategies (Liebman and Gallandt, 1997). Diversification of cropping systems through crop rotation, intercropping, and cover cropping offers a particularly powerful set of tactics for managing weeds that can effectively complement soil-based approaches (Liebman and Davis, 2000; Liebman and Dyck, 1993; Teasdale, 1998). Other soil- and crop-related approaches that may serve as components of ecologically based weed management strategies include: tillage practices that move weed seeds and vegetative propagules in ways that reduce their survival and disrupt the timing and success rate of seedling or shoot emergence (Buhler, 1995; Mohler, 1993); residue management practices that promote attack on weed seeds and seedlings by vertebrates, insects, and microbes (Boyetchko, 1996; Brust and House, 1988; Pitty et al., 1987); soil solarization techniques that use temporary plastic tarps to alter soil conditions and kill weed seeds before crops are sown (Stapleton and DeVay, 1995);
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PROFESSIONAL SOCIETIES and Ecologically Based Pest Management: Proceedings of a Workshop irrigation and fertilization practices that place water and nutrients where they are most available to crops and least available to weeds (Grattan et al., 1988; Rasmussen et al., 1996); use of crop densities, spatial arrangements, planting dates, and genotypes that increase crop competitiveness against weeds (Buhler and Gunsolus, 1996; Lemerle et al., 1996; Mohler, 1996; Teasdale, 1995; Teasdale and Frank, 1983; Wall and Townley-Smith, 1996). The successful integration of soil, crop, and weed management will require active collaboration among microbiologists, biochemists, agronomists, weed ecologists, agricultural engineers, and members of other disciplines. A better understanding of weed dynamics and soil-related processes on organic and low-external-input farms, which often use diversified cropping systems, crop residues, composts, and manures to maintain soil productivity, will also further the development of integrated weed management systems. Given that private industry has little incentive to aid farmers interested in reducing the use of herbicides and other purchased inputs, public funding to support research on ecologically based weed management is highly desirable. REFERENCES Alkämper, J. 1976. Influence of weed infestation on effect of fertilizer dressings. Pflanzenschutz-Nachrichten Bayer 29:191–235. Alkämper, J., E. Pessios, and D.V. Long. 1979. Einfluss der Düngung auf die Entwicklung und Nährstoffaufnahme verschiedener Unkräuter in Mais. Proceedings of the Third European Weed Research Society Symposium : 181–192. Angonin, C., J.P. Caussanel, and J.M. Meynard. 1996. Competition between winter wheat and Veronica hederifolia: Influence of weed density and the amount and timing of nitrogen application. Weed Research 36:175–187. Aspelin, A.L., and A.H. Grube. 1999. Pesticide Industry Sales and Usage—1996 and 1997 Market Estimates. Publication No. 733-R-99-001. Washington, DC: Office of Pesticide Programs, US Environmental Protection Agency. Barbash, J.E., G.P. Thelin, D.W. Kolpin, and R.J. Gilliom. 1999. Distribution of Major Herbicides in Ground Water of the United States . Water Resources Investigations Report 98-4245. Sacramento, CA: US Geological Survey. Boyetchko, S.M. 1996. Impact of soil microorganisms on weed biology and ecology. Phytoprotection 77:41–56. Brust, G.E., and G.J. House. 1988. Weed seed destruction by arthropods and rodents in low-input soybean agroecosystems. American Journal of Alternative Agriculture 3:19–25. Buhler, D.D. 1995. Influence of tillage systems on weed population dynamics and management in corn and soybean production in the central USA. Crop Science 35:1247–1257. Buhler, D.D., and J.L. Gunsolus. 1996. Effect of date of preplant tillage and planting on weed populations and mechanical weed control in soybean (Glycine max). Weed Science 44:373–379. Chen, Y., and T. Aviad. 1990. Effects of humic substances on plant growth. Pp. 161–186 in Humic Substances in Soil and Crop Sciences: Selected Readings, P. MacCarthy,
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