In much of the upper Midwest, livestock and grain farms are in relatively close proximity. Due to herd expansion, rising fertilizer costs, and increasingly stringent nutrient management regulations, farmers are developing agreements in which manure is applied to grain farmers' fields. One concern of grain farmers, however, is increased weed pressure, via the entry of weed seed in the manure, or improved soil conditions that benefit germination. On-farm and on-station trials were established in 2004 (n=7) and 2005 (n=9) to ascertain the impact of dairy manure on weed pressure in corn. A simple factorial of with and without manure, and with and without herbicide resulted in four treatments. Weed communities were evaluated at approximately the V6 stage of corn to determine number of weeds (density) and biomass, to characterize species diversity, and to detect introduced species. Findings from 2004 indicated that there was little to no effect of manure on weed density, biomass, or diversity, in either weed control treatment. Although density and biomass increased significantly at some sites in the manured treatments without herbicide, farmers' weed management programs controlled weeds effectively in both manured and non-manured plots at most sites. Weed scouting indicated few examples of species present in the manured plots that were not present in the non-manured plots. In contrast, analyses from 2005, a record-dry season, showed significantly greater weed density and biomass, and greater species diversity and richness in the manured plots than the non-manured plots. However, although weed levels were statistically higher in the manured treatment, actual means were very low in both treatments where herbicide was used. The results of this research suggest that cash-grain farmers with successful weed control programs could incorporate manure into their systems with a low level of concern of increasing weed pressure.