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    Avoiding losses from herbicide carryover

    sprayer in corn field

    Herbicides are a valuable tool protecting your crop from weed competition that steals moisture, nutrients and sunlight. However, residues of residual herbicides that protect one crop may inadvertently have a detrimental effect on the crop that follows — the occurrence known as herbicide carryover. 

    Poor stand establishment and reduced crop vigor are not only costly but also frustrating. And they can lead to additional management challenges in the future.

    “For example, if carryover results in a poor cover crop stand, you’re not getting the full benefits of this management practice, whether your goals are nitrogen retention, soil stabilization, moisture retention, weed suppression and, in some areas, protection from wind erosion,” said Eric Scherder, U.S. agronomy leader – herbicides and nitrogen stabilizers, Corteva Agriscience. 

    In the case of poor weed suppression, farmers may need to change herbicide programs to get ahead of the unexpected weed challenge, which are often small-seeded broadleaves like Palmer amaranth and waterhemp. Change adds up. 

    “You’ve already spent $25 to $30 or more per acre on the cover crop,” Scherder said. “Changing herbicide programs adds more costs and more time invested. When carryover impacts corn or soybeans, by the time you realize you have a problem, replanting or switching crops may not be an option.”

    Risk Management 101: Read the label 

    Understanding the factors that contribute to herbicide carryover and knowing how to minimize the risks are two important aspects of a successful crop management plan. Scherder said the herbicide itself as well as drought, soil pH, soil type and crop management practices all play a role. 

    “The first step to avoiding herbicide carryover is to understand the half-life of the active ingredients in each product and how long these ingredients take to break down to safe concentrations in the soil under normal conditions,” Scherder explained. “Certain actives have a longer half-life, so they persist in the soil longer and can carry over to the following year.” 

    This information is noted on herbicide labels as the “plant-back interval” and represents the normal time to allow between application and planting the next crop. Pay particular attention, because the interval may vary depending on the application rate and crop.

    In addition, when choosing herbicides to use preceding cover crops, such as oats, cereal rye or legumes, it’s also important to know if the chemistries you’re considering are really strong against those species. If the answer is yes, you may want to take a different approach. 

    Drought increases potential for carryover   

    During the growing season, microbial activity in the soil breaks down most herbicides, and this process is greatest when the soil is moist and warm. However, drought and cool temperatures slow the breakdown. This can extend the half-life into and beyond the growing season. 

    Late application and early planting add to risk

    “If for some reason you had a late application and it was a dry summer and fall, you can be left with a higher-than-anticipated concentration of a residual herbicide. That can create challenges in establishing a cover crop, not to mention the next year’s crop,” Scherder said. “In many cases, soybeans are planted earlier and earlier. If there’s carryover, by the time you realize it and replant, you’ve lost yield potential.”

    Management practices can help 

    Fortunately, herbicide carryover isn’t inevitable. Sound crop management practices also influence herbicide half-life. For example, for most herbicides, the odds of carryover are lower if soil pH is in the 6.5 to 7.0 range. Check the herbicide label for specific precautions related to pH and herbicide activity, particularly in no-till cropping systems.

    Tillage is another important management tool. When drought leaves herbicide sitting near the soil surface, or in bands over the row, running a tillage pass 4 to 6 inches deep can help reduce the chance of carryover in two ways: spreading out the concentrated product and introducing oxygen to stimulate microbial activity for herbicide breakdown.   

    Seek expert support 

    If you’re concerned the current growing season may have you set up for carryover next spring or if you’re interested in effective products that offer lower carryover risks, seek the help of your local crop protection retailer or Corteva Agriscience agronomist. 

    “These folks are boots on the ground in your area. They know your soil types, weather patterns and the weed challenges you’re facing,” Scherder said. “They can help you identify any potential issues and find a way to manage around them.”

    For more information, go to Corteva.us or contact your local Corteva Agriscience representative.