Recent rains may be too late for incorporation of soil-applied herbicides

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.  

Soil-applied herbicides remain an important part of weed control programs in corn and to a much lesser extent in soybeans. Because of the dry soil conditions that we experienced in April and the first couple of weeks of May, soil-applied herbicides were applied under conditions where there was little or no rainfall for incorporation leaving many of these herbicides on the soil surface. With the latest rainfall events the question remains, “How effective will these herbicides be that were applied in early to mid-April?” In order to answer this question, here is a quick review on how soil-applied herbicides work.

In order for a soil-applied herbicide to be effective, the herbicide needs to be available for uptake by the weed seedling. This usually means that the herbicide needs to be in solution before the seedling emerges, however there are a few herbicides that can control small emerged weeds under certain conditions (i.e., atrazine). Processes such as herbicide adsorption to soil colloids or organic matter can reduce the amount of herbicide available for weed absorption. Soil-applied herbicides do not prevent weed seed germination!! Soil-applied herbicides work by being absorbed by the root or shoot of the weed seedling as it emerges. Generally, this happens before the seedling emerges from the soil. For a herbicide to be absorbed by weed seedlings, the herbicide must be in the soil solution. Herbicides can be made available in the soil solution by mechanical incorporation (PPI applications) or incorporation by rainfall. Generally, with most of the soil-applied herbicide applications that we make in Michigan we rely on rainfall for herbicide incorporation. If we don’t receive adequate rainfall, the herbicide remains on the soil surface and is prone to dissipation processes, such as volatility and photolysis (depending on the herbicide). More importantly if the herbicide is not incorporated by rainfall or mechanically, the herbicide is not in the area of the soil where it would come into contact with germinating weed seedlings.

Many weed species, in particular small-seeded weeds, germinate from fairly shallow depths in the soil. The top one to two inches of soil is the primary zone from where these weed seeds will germinate, and therefore should be the target area for herbicide placement. In order for the herbicide to get into the weed seed emergence zone, 1/2 to 1-inch of rainfall is generally needed. To adequately incorporate the herbicides before weeds emerge, rainfall usually needs to happen within 7 to 10days after application. So what happens when rainfall occurs after that 10-day window? From this point forward the soil-applied herbicide will be effective on weeds that are just emerging, but if weed seedlings have already emerged through the soil surface, it will be important to implement other control strategies (i.e., POST herbicides). For example, if a soil-applied grass herbicide was applied on April 12 right after planting and there was no rain until May 10, that herbicide would have been on the soil surface for 4 weeks. Grasses that had emerged from the soil during this time would not be controlled and fields should be scouted to see if additional control measures are warranted.

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