Herbicide Carryover Concerns in Soybean

JEFFERSON CITY, Mo. (DTN) -- It's been another drier-than-normal winter across much of the Corn Belt and Upper Midwest, creating potential for herbicide carryover and soybean injury this season that could be similar to what was experienced in many states in 2023.

"Last spring/early summer, I probably received more calls and complaints about herbicide carryover to soybean than I had received in my entire career up to that time," wrote Kevin Bradley, University of Missouri state extension weed scientist. "Most of these cases could be directly related to the persistent drought that we experienced in many areas of the state. Unfortunately, I haven't really seen a major shift in our weather patterns from last summer through this spring that alleviates these concerns as we head into this growing season."

According to the U.S. Drought Monitor map released the third week of March, nearly 67% of the Midwest region (including the states of Illinois, Indiana, Iowa, Kentucky, Michigan, Minnesota, Missouri, Ohio and Wisconsin) was experiencing at least abnormally dry conditions, compared to only 16.53% of the region one year ago. The change in condition was most pronounced in Wisconsin where the map showed an increase of 88.17% in abnormally dry or worse conditions this year as compared to last, followed closely by Missouri (81.53% increase).

Bradley said that herbicides will persist much longer in dryer soils due to decreased microbial and chemical degradation. Soil temperature also plays a role in herbicide degradation, slowing considerably in cool soils and completely ceasing when soils are frozen.

"While our soils haven't been all that cool this winter, they have been fairly dry," he said. "This is more than likely what led to the situation we experienced with herbicide carryover last year, and it is what I am concerned about this year."

Environmental conditions aside, the likelihood of herbicide carryover is dependent on the persistence (or half-life) of the herbicide in question. Bradley said that in Missouri in 2023, the herbicides that caused the most problems in soybean were the Group 27, HPPD-inhibiting herbicides such as mesotrione and topramezone.

"While these herbicides haven't historically been all that much of a concern for carryover to soybean from one year to the next, there are some conditions that are likely to increase their carryover potential," he said. "These include applications of these herbicides that were made much later than usual during the growing season than might typically occur, and/or 'double-up' applications of these herbicides in corn."

While he hasn't received many calls about clopyralid causing carryover injury, Bradley noted that this Group 4 herbicide has become a more common component in a lot of corn herbicide premixes in the past few years. Clopyralid depends exclusively on microbial populations for its degradation, so there is potential for carryover in dry soils.

Soil characteristics, such as pH and organic matter content, also factor in herbicide degradation and the likelihood of carryover in a particular field.

"For instance, degradation of atrazine (Group 5) and certain sulfonylurea (Group 2) herbicides is likely to be much slower in soils with a higher pH," Bradley said. "However, soil pH is likely not as important of an issue when it comes to the degradation of the Group 27, HPPD-inhibiting herbicides or clopyralid."

However, a soil's organic matter content can affect the breakdown of group 27, HPPD-inhibiting herbicides or clopyralid, he said. In general, the higher the organic matter content of the soil, the greater the microbial degradation.

Bradley said soybeans would be at greatest risk of injury from herbicide carryover if planted in a field with sandier and/or lower organic matter content in a drought area where herbicides last season were applied late or in a "double-up" fashion.

For those concerned about the possibility of herbicide carryover in their soybeans this season, Bradley recommended that farmers collect a soil sample from the field of concern and plant a few seeds in it. While this soil bioassay won't provide an exact measure of herbicide still present in the soil, it can show whether enough remains to harm the crop.

The University of Nebraska published a guide with step-by-step instructions on how to conduct a proper soil bioassay. It can be found here: https://extensionpubs.unl.edu/…

Should herbicide carryover be sufficient to cause damage to soybeans in a particular field, growers may consider planting another crop this season or at least avoid early soybean planting, as herbicides taken up by plants in periods of cold stress can often cause higher levels of injury. Tillage may also help dilute herbicide concentrations and encourage microbial activity.

Read more about herbicide carryover here: https://www.dtnpf.com/…

Jason Jenkins can be reached at jason.jenkins@dtn.com

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