Along with the planting delays we have experienced throughout the state, the wet conditions have also caused significant delays in the timing of our burndown herbicide applications. Many corn and soybean fields in the state are still covered with thick, green mats of winter annual weeds. Most of these weeds have begun to senesce and have already produced viable seed, but it is important to recognize that these weeds can still interfere with planting equipment and should be controlled as soon as possible.
In addition to these winter annual weeds, if there is any silver lining to the conditions we have experienced thus far it may be that many of our common summer annual weeds have already emerged in our corn and soybean fields as well. The most notable of these of course is waterhemp; our biggest problem weed throughout the state. I’ve checked our weed emergence models in Horizon Point for several areas around the state, talked to a number of people in different areas, and have personally been in fields in central and northeastern Missouri and in all instances the indication is that waterhemp is just starting to germinate. Of course there are a variety of other summer annual weeds that have already emerged as well; some of these include common and giant ragweed, smartweed, sunflower, giant foxtail, and others.
What all of this means is that we may have an opportunity here that we don’t always get in a “normal” year; the opportunity to eliminate a large portion of these summer annual weed populations with our burndown herbicide applications prior to planting. This could be a blessing in disguise, as we have waterhemp populations throughout the state that we aren’t controlling very well post-emergence in-crop due to their resistance to glyphosate and in many cases PPO-herbicides (Cobra, Flexstar, Phoenix, Blazer, etc.) as well.
So, I encourage you to take advantage of this situation and make sure you apply an effective combination of burndown herbicides for the control of the most problematic weeds that are present in your fields. I firmly believe that as a result of the prevalence of glyphosate-resistant horseweed, giant ragweed, and waterhemp throughout our state, we can no longer afford to apply glyphosate alone as a burndown and expect our fields to be clean at planting. And with the recent germination of waterhemp in many of our fields, it is perhaps even more important this year than most to make sure we add an effective tank-mix partner such as Sharpen, 2,4-D, dicamba, or metribuzin to our primary burndown herbicides which are usually either glyphosate or paraquat (Gramoxone).
For more specific information and recommendations pertaining to the efficacy of burndown herbicide applications, see our 2013 Missouri Pest Management Guide (University of Missouri Extension Publication M171).
REVISED: September 30, 2013