Weed diversity and weed management

The story of agriculture is the story of weed interference. After millennia of weed control we still have weeds. This situation has led many growers to observe that “the weeds always win.” One of the most important reasons weeds are so successful is their biodiversity. Biodiversity is an inevitable consequence of the struggle an individual weed species undergoes in the presence of neighbors, and by occupying a physical space in an agroecosystem. Weeds have evolved in response to cropping system practices by adapting and occupying niches left available in agroecosystems. Forces created by our cropping practices over evolutionary time have led to the weed diversity we observe today. Diversity underlies weed management in several important ways. A plant experiences diversity among its neighbors in at least five different ways. Weeds have adapted to selection in agroecosystems in several ways: (1) genetic variants within a species; (2) somatic polymorphism of plant parts; (3) success in diverse habitat microsites; (4) temporal adaptations within the community; and (5) floristic diversity of a community at higher levels than the species. Herein, weed diversity is discussed in this broader context, in terms of population behaviors that emerge as a consequence of the activities of individual components at lower levels of organization. Diversity is also discussed in terms of its implications for weed management. The potential exists to develop management strategies based on differences in weed and crop diversity. These strategies might be developed by characterization of weedy genetic and phenotypic diversity; enhancement of crop, cropping system, and agroecosystem diversity; and characterization of the spatial distribution of weed populations.