By Bob Hartzler
Slow planting progress this spring has led to all efforts focused on getting the crop in the ground. Often the first operation thrown to the wayside is application of pre-emergence herbicides in soybean. A decade ago this practice could be successful, but with widespread multiple herbicide resistance, it either sets up an imminent control failure or contributes to further evolution of herbicide resistance.
Applying pre-emergence herbicides ahead of planting provides the advantage of increasing the likelihood of the herbicide being activated by rainfall prior to weed emergence and reducing the risk of injury from certain pre-emergence herbicides, particularly the group 14 herbicides in soybean. However, application of residual herbicides after planting still provides major benefits in terms of protecting yield potential, controlling resistant biotypes and reducing selection pressure from post-emergence herbicides. Group 3 herbicides (Prowl, etc.) and group 14 herbicides (Authority, Sharpen, Valor, etc.) must be applied before soybean emergence. Group 15 herbicides (Dual II Magnum, Outlook, Warrant, Zidua, etc.) can be applied after soybeans have emerged, but these herbicides do not have activity on emerged weeds. If weeds are present at the time of application, include an appropriate post-emergence product to control the emerged weeds.
The limited time to complete spring field operations rarely allows things to fall in place exactly as one might have planned. However, the days when weeds could be treated as an afterthought are over. If planned pre-plant applications are bypassed due to delays in planting, adjust weed management strategies to include post-planting residual herbicides.