Total postemergence (post) herbicide programs for corn and soybean weed control have increased dramatically during this past decade. There are several reasons for this shift including the introduction of effective grass and broadleaf post herbicides, restrictions on atrazine, the need to spread the work load, and transgenic crops that allow the use of nonselective post herbicides. However, the increased reliance on post programs can lead to two specific questions. The first question is when to spray the herbicide to maximize crop yield. If the herbicide is sprayed at the wrong time, crop yield could be reduced either from weed competition or from herbicide injury. The second question is when to spray to get the best weed control. This is a question because several post herbicides lack residual activity and weeds may emerge after the herbicide application. Weed competition and seed production from late emerging weeds must be considered.