The main study objective was to measure the effects of tillage (moldboard plow, chisel plow, and no-till) and weed management (intensive, moderate, and minimum) on weeds and crops in a spring barley monoculture compared with a spring barley–red clover rotation. The study was initiated in 1987 and conducted at two sites. Residual effects of treatments were measured in a wheat test crop at the loam site in 1994 and at the clay site in 1995–1996. Weed seed bank densities ranged from less than 300 to nearly 30,000 seeds m−2 and plant densities from 30 to 6,000 plants m−2. Seven species were recorded on average per plot aboveground and 10 species per plot in the seed bank. Species number in the seed bank varied little with treatments compared with species numbers aboveground. Crop rotation and tillage had little effect on weed species diversity but affected relative species dominance. The presence and abundance of species was also influenced by their degree of tolerance to the herbicides used in each system. Annual dicots largely dominated in minimum weed management treatments. Their relative importance in each rotation varied with their level of susceptibility to the different postemergence herbicides. Perennials were not exclusively found in reduced tillage systems. The relationship between perennials and tillage was dependent on the response of perennating structures to the type and frequency of soil disturbance. For example, quackgrass dominated in chisel and moldboard plow systems where rhizomes would be frequently fragmented. Field horsetail, also a rhizomatous species, dominated in the monoculture/direct-seeded no-till treatment under minimum weed management. Its absence from the rotation was explained by the regular removal of aboveground biomass during the forage production year. Overall, weed response was regulated by agronomic factors but was largely determined by specific biological attributes and environmental conditions.

A 6-yr project comparing four cash grain–farming systems relevant to the mid-Atlantic region of the United States was conducted from 1993 to 1999. A wide range of parameters was sampled including soil health, nutrient and agrichemical movement, economic viability, and insect and weed communities. The systems and their approaches to weed management were: continuous no-till corn without (System A1) or with (System A2) rye cover crop and preplanned herbicides based on expected weed infestations; System B was a 2-yr corn–soybean rotation with conventionally tilled corn and no-tillage soybean, with preplanned herbicides based on expected weed infestations; System C was a 2-yr rotation with no-till corn, conventionally tilled wheat, and no-till double-cropped soybean, using postemergence (POST) herbicides on the basis of field scouting; and System D was a 3-yr rotation of corn-soybean-winter wheat with rye and hairy vetch cover crops, using cultivation and reduced rates of POST herbicides based on field scouting. Spring weed assessment in 1999 was similar for species evenness (Shannon's E) and diversity (Shannon's H′) indices. Weed density was lowest in System C because wheat in this system received a spring herbicide application. In the final fall assessment, Shannon's H′ was greater in System D than System C. Common lambsquarters, eastern black nightshade, and jimsonweed were more abundant in System D than Systems A1, A2, and C. Fall 1999 assessment also indicated Canada thistle was more prevalent in Systems A1 and A2 than the other three systems. During the 6-yr period, densities of jimsonweed, eastern black nightshade, morningglory species, crabgrass, and fall panicum dramatically increased in a particular system for 1 to 2 yr, then declined to levels similar to other systems. Overall, weed communities were quite stable and effective weed management did not result in dramatic changes in the weed community, regardless of the approach to cropping systems or weed management.

The winter wheat production system of the Pacific Northwest is characterized by severe wind and water erosion and winter annual grass weeds requiring high herbicide input. Since 1985, numerous multi- and interdisciplinary, long-term, large-scale, integrated cropping systems studies have been or are currently being conducted. The primary focus of these studies was on weed biology, ecology, and management, whereas secondary evaluations were on alternative cropping systems, conservation tillage, and fertilizer or herbicide inputs. The 6-yr integrated pest management project, conducted in the high-rainfall zone (> 400 mm), showed for the first time that when weeds were adequately managed, conservation production systems were more profitable than conventional systems. In the intermediate rainfall zone (350 to 400 mm), a recently concluded 6-yr, three-state study integrated single-component research results into a multifaceted approach to managing jointed goatgrass. This project has been used as a model study for other western states and the National Jointed Goatgrass Research Initiative. At present (9 yr thus far), a study is being conducted in the low-rainfall zone (< 350 mm) to examine the feasibility of no-till spring cropping systems in lieu of the highly erosive, weed infested, wheat–fallow system. Because of these projects, the Washington Wheat Commission recognized the importance of long-term, interdisciplinary, cropping systems research and has therefore established an Endowed Chair at Washington State University for direct seed cropping systems research. Federal, national, and regional agencies have used information from these projects for farm plans and pesticide usage.