Post Now, Continue Scouting Later

May 19, 2000
The rapid corn and soybean planting progress across Illinois this season will allow for completion of many postemergence herbicide applications much earlier than usual. Thus far, most areas of the state have not had to deal with postemergence herbicide application delays caused by excessive precipitation, and many postemergence corn herbicides have already been applied in a timely manner. Concerns with large weeds during postemergence applications have not been extremely widespread (at least so far).

Issue no. 4 of the Bulletin contained information on weed emergence sequences ("Knowing When to Look for Weeds"). While many weed species have already emerged, there are several species that have not started to make significant emergence. With a considerable amount of the growing season remaining, fields that have already received a postemergence herbicide application should continue to be monitored for emergence of additional weed species, especially those species that typically do not emerge until about midseason. In particular, waterhemp emergence has begun across about the southern third of Illinois but has not yet made significance emergence across the northern one-half of the state. Keep in mind that this species can emerge well into late June and early July. Other species that may emerge in a few weeks include crabgrass and nightshade, and, while the competitive effects of later-emerging weeds may not be as large as those that emerge with the crop, these species can often cause other types of problems later in the season (for example, harvesting soybean fields infested with eastern black nightshade). If significant weed emergence occurs after the initial postemergence herbicide application, additional management strategies may be needed to reduce the probability of problems later during the season.--Aaron Hager and Christy Sprague

Author: Aaron Hager Christy Sprague