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Residual herbicide options as conditions remain dry

Liz Stahl, Extension Educator – Crops, Dave Nicolai, Extension Educator – Crops, Debalin Sarangi, Extension Weed Specialist

Fig. 1. Corn competing with weeds for water (right) when no
preemergence herbicide was applied, compared to where a
PRE herbicide was applied (left), Rosemount, MN on
 June 5, 2023. Photo: Debalin Sarangi
Although some areas of the state were inundated with too much rain earlier in May, the later part of May and early June has been very dry in parts of Minnesota. The portion of MN in the “abnormally dry” category according to the U.S. Drought Monitor more than doubled in size between May 25 (30%) and June 1 (66%). Even in dry soil conditions, weed growth often continues, resulting in competition for moisture with the crop.

Preemergence herbicides typically need at least 0.5 inches of rain within about five to seven days after application to “activate” or to mix with soil water and move into the zone where weed seeds are germinating. When conditions are excessively dry, up to 1.0 inch of rain may be needed for activation. Dry conditions after application can result in PRE herbicides moving from the soil water to be tightly bound to soil colloids, resulting in less availability of the herbicides to control germinating weed seedlings. Herbicides differ in their water solubility and how tightly they bind to the soil, but reduced availability in the soil water can result in inconsistent and reduced weed control.

Odds are that we will eventually get moisture to activate PRE herbicides at some point after application. If the herbicide stays on the soil surface for a longer period, photo- and microbial degradation can break down the herbicide into a compound that will not be lethal for the weeds. Preemergence herbicides help fields to stay clean for a few weeks earlier in the season before any postemergence herbicide is applied. Even if the first application of a PRE herbicide didn’t provide as much control as hoped, control will typically be better than if no PRE herbicide was applied. Moreover, layered residual herbicide programs, where a soil residual herbicide is tank-mixed with a foliar-active postemergence treatment, are very effective in controlling weeds like waterhemp that have an extended emergence pattern.

Dr. Amit Jhala, Extension Weed Management Specialist with the University of Nebraska-Lincoln, has pulled together some useful information listing crop stage and weed heights to help determine which residual herbicide products can be applied postemergence in corn and soybean. Always be sure to check the herbicide label for any updates and restrictions based on your field conditions and situation:

Dicamba cutoff looming

It is important to note that the cutoff date in MN for dicamba products labeled for use in dicamba-tolerant soybean (i.e. XtendiMax, Engenia, and Tavium) is June 12 south of I-94. North of I-94, the cutoff date is June 30. These products also cannot be applied in MN when the air temperature is above 85° F, or when the forecasted high is above 85° F. This temperature restriction is for the entire state. See a MN Department of Agriculture press release for more details:

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