Earlier this month, we discussed advantages of and techniques for controlling emerged weed vegetation prior to planting. Several winter annual weed species have reached maturity while others, such as cressleaf groundsel (Figure 1),
began to flower within the past 10 days across areas of central Illinois. Control of these mature plants with herbicides or single passes of tillage equipment will continue to be challenging.
Preplant control is also needed for emerged summer annual grass and broadleaf weed species. It sometimes can be difficult to accurately identify recently-emerged broadleaf species (Figure 2),
but these plants will grow rapidly with the abundant soil moisture and warming air temperatures. One species that warrants special mention is waterhemp. The increasing prevalence of waterhemp plants and populations that demonstrate resistance to herbicides from more than one site-of-action family increases the importance of controlling any emerged waterhemp plants before planting. The potential for adverse, season-long ramifications from not controlling emerged waterhemp plants prior to planting might be more acute in soybean than corn, even though there are examples of waterhemp populations resistant to herbicides used primarily in corn (atrazine and HPPD-inhibiting herbicides, for example). This is simply related to having fewer viable herbicide options to control waterhemp postemergence in soybean than in corn, coupled with the increasing prevalence of waterhemp plants and populations resistant to both glyphosate and PPO-inhibiting herbicides. The waterhemp plants shown in Figure 3 emerged in a field where the population has been confirmed resistant to glyphosate. If these plants also demonstrate resistance to PPO-inhibiting herbicides and are not controlled prior to planting a glyphosate-resistant soybean variety, there would be no chemical options to control these plants for the remainder of the growing season.