INFLUENCE OF TILLAGE SYSTEMS ON WEED POPULATION-DYNAMICS AND MANAGEMENT IN CORN AND SOYBEAN IN THE CENTRAL USA

Species composition and population densities of weed communities of arable land reflect agronomic practices. The trend toward reducing tillage in corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] production changes the environment where weeds are managed, survive, and reproduce. The shift from tillage systems that include extensive annual soil disturbance to systems that minimize soil disturbance will cause major changes in weed population dynamics. These changes often reduce the effectiveness of weed control practices. Reduced herbicide efficacy has slowed adoption of conservation tillage because many conservation tillage systems rely heavily on herbicides for weed management. Poor understanding of weed population dynamics and lack of suitable control alternatives often result in increased herbicide use in conservation tillage systems. While results have varied among experiments, some general trends in weed population dynamics have arisen as tillage is reduced. These include increased populations of perennial, summer annual grass, biennial, and winter annual species. Densities of large-seeded dicot species often decrease. The ecological and management aspects of these changes are varied and complex. Effective, economical, and environmentally sound weed management in conservation tillage systems will require integration of new information with established principles of weed management. New management systems and control technologies are needed to develop Integrated weed management systems for the altered ecosystems created by conservation tillage production systems.