By Bob Hartzler, Department of Agronomy
Slow planting progress this spring has led to all efforts being focused on getting the crop in the ground. Often the first operation thrown to the wayside is application of preemergence 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 preemergence 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 preemergence 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 postemergence 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 soybean have emerged, but these herbicides do not have activity on emerged weeds. If weeds are present at the time of application, include an appropriate postemergence 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 preplant applications are bypassed due to delays in planting, adjust weed management strategies to include post-planting residual herbicides.
Bob Hartzler is a professor of agronomy and weed science extension specialist with responsibilities in weed management and herbicide use. He can be reached at hartzler@iastate.edu or 515-294-1923.
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