Herbicide use may be reduced 30-80% without impacting crop yield with sitespecific weed management. Herbicide use is varied within a field to match the variation in the weed population. However, growers will not adopt this strategy until they are confident that the reduction in herbicide and other benefits of site-specific weed management will justify the cost of implementation and future weed control
will not be compromised. Predicting the outcome of site-specific weed management is difficult because benefits vary with the composition and spatial distribution of the weed population, possible herbicide treatments, and the resolution of variable management. WeedSite software was developed so growers could investigate the potential benefits of postemergence site-specific weed management in irrigated corn specifically for their weed populations and implementation of site-specific weed management. Users choose the resolution of patch spraying and may divide a field into management units. Georeferenced weed maps and GIS software are not needed. Net gain from site-specific weed management, area of the field not treated, herbicide use and cost, yield loss from weed competition, and weeds left in the field are all calculated from hand-drawn weed maps and results can be mapped. We think WeedSite can be useful for educating growers, agricultural consultants, and students about the potential benefits of postemergence site-specific weed management in irrigated corn in addition to published field experiments. Evaluations are consistent with what is known about variation in the benefits of site-specific weed management. Predictions from hand-drawn maps will be less accurate than predictions from field experiments but more germane. General features of spatial distributions may be more representative and users choose the weed species present, the resolution of management, and the candidate herbicides.