When terminating cover crops ahead of corn, manage them with your future crop in mind: eliminate them early to plant clean and stay clean.
Common, research-based wisdom says to spray a herbicide 10 to 14 days before corn planting. This herbicide spray timing is the best way to reduce risk, aside from using a low-risk cover crop that doesn’t overwinter. While a cool, wet spring that slows both fieldwork and cover crop growth may not optimize biomass to deliver greater early weed suppression, the long-run soil health benefits outweigh the challenges.
An excellent resource by the Sustainable Agriculture Research and Education (SARE) program, “Managing Cover Crops Profitably,” lists numerous early termination benefits: faster residue decomposition to improve planting, increased soil warming and soil water availability, reduced disease incidence and phytotoxic effects of residue, increased nitrogen mineralization from lower C:N ratio cover crops and easier control of shorter cover crops and weeds.
Farmers with multiple years of cover crop experience have experienced the weed reduction results. “My colleagues and I hear stories from farmers who say, ‘Hey, my weed population is less where I had cover crops versus where I didn’t have them,’” says Lizabeth Stahl, University of Minnesota agronomist and crops extension educator.
“Granted, what your key weed species are depends on where you farm," she adds. "A key headache in Minnesota is waterhemp, which emerges later and longer into the season. That's why residuals are an important part of a herbicide program. They can be combined with a burndown when controlling the cover crop."
A good termination plan requires timely scouting to determine the right herbicide combination to control both the cover crop and existing weed species, given their growth stage and environment. A multi-state research project on cover crop termination examined control of nine species with common herbicide treatments. The lead researchers, Eric Oseland and Kevin Bradley with the University of Missouri, showed that cereal rye can provide good control of winter annual weeds and other troublesome species like horseweed (marestail) and early season suppression of summer annuals like waterhemp.
Oseland and Bradley's research offers details on herbicide program efficacy given grass and legume cover crops. It also cites Iowa State University research on the effective use of residual herbicides when used 2 to 3 weeks before planting on 12- to 18-inch tall cover crops. The report mentions that delayed termination of taller cover crops may not allow enough residual herbicide to reach the soil for uptake by germinating weed seeds.
Stahl encourages growers to examine the residual longevity of preemergence herbicides included in a cover crop burndown program. “Some herbicides offer longer residual, which can be important for waterhemp control. When you seed a cover crop in the fall versus interseeding early in the spring, the wider window between herbicide application and seeding can increase the chance of successful cover crop establishment. Selecting a cover crop species with good tolerance to your herbicide program will also increase the potential for successful establishment.”
Suppose spring weather doesn’t cooperate for cover crop termination two weeks before planting corn. In that case, all is not lost if you have plans B, C and D. According to cover crop termination recommendations from 12 Midwest university specialists, you can spray a few days before or right after planting corn, depending on your USDA NRCS termination zone. And they recommend using starter nitrogen or early sidedress nitrogen to aid early corn growth. Corn yield was consistently reduced in a wet spring when cereal rye was terminated after planting, according to a 2019 University of Wisconsin research. The report cites that early-season nitrogen availability was the suspected limiting factor.
There are disadvantages with these late-termination plans, along with planter/drill performance concerns, so check with local cover crop specialists and experienced farmers for the best advice. “Planting green into a cover crop works better with soybeans than corn because you can create a green bridge where insects like true armyworm or black cutworm can transition from dying cover crops to feeding on young corn,” Stahl says. “Terminating earlier reduces overall risk.”
Another tip from Stahl regards hybrid selection. “If you’re considering planting green, a Bt hybrid that offers control of true armyworm and black cutworm, for example, would reduce risk compared to a conventional hybrid, but you want to scout for pests regardless.”
Finally, be sure to consult with your crop insurance provider to understand cover crop termination rules for your area.
Check out more termination details by viewing the University of Minnesota Cover Crop Virtual Field Day. And if you’re a beginner cover cropper, view these cover crop recipes by state to help you get started.
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