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Don’t Sacrifice Weed Control for Later Cover Crop Stands

Residual herbicides can impact cover crop stands later in the season. It’s not a given, though.

Gregg Johnson is a fan of cover crops. 

“I am all for them,” says Johnson, who works with biomass cropping systems at the University of Minnesota’s Southern Research and Outreach Center (SROC) at Waseca. “But we can’t compromise on weed control to take advantage of the benefits of cover crops.” 

Preemergence and postemergence residual herbicides are great for managing weeds throughout the growing season. One downside, though, is they can stunt the growth of cover crops that are seeded in late summer/early fall. 

Johnson told those attending this week’s SROC summer tour that it’s hard to pin down the effect residual herbicides can have on cover crops planted later in the season. Results are mixed in university trials. 

In one year, use of Flexstar and Pursuit didn’t affect later-planted cover crops at a site in a University of Wisconsin trial. 

“The next year, they fried them,” says Johnson. 

Regional variability also exists. “There really is no consistency,” he says. “Predicting herbicide persistence is hard because there are so many factors going on,” he adds. “It’s related to soil factors like pH. The higher the pH, the longer the herbicide will be around.”

Ditto for the degree of soil microbes that are present to break a herbicide down. Tillage also plays a factor, he says.

A field’s weed spectrum also plays a role. “If your weeds are foxtail or lambsquarters, you have a good chance of finding a good herbicide that will give you the needed weed control without influencing the cover crop later that year,” he says. 

Not so with weeds with later-emergence patterns. “If you are dealing with weeds like waterhemp that germinate into the summer, you need residual chemistry that often isn’t good for the cover crop.” 

Given the variability of residual chemistry on cover crops, though, a sketchy cover crop stand isn’t always a given. Cover crops can help with soil health and erosion. The next spring, cover crops can also have positive weed-control benefits when it comes to early emergers like the ragweed complex, he says. 

Even if the cover crop stand is compromised, it can still benefit soil health and protect against erosion. 

“It might not put on as much biomass, but it is still there,” says Johnson. 

Cover crops vary in sensitivity to herbicides. For example, tillage radish is extremely sensitive to the lingering impacts of residual herbicides. Cereal rye? Not so much. 

“Cereal rye is a big old workhorse, as it can handle lots of different things,” he says. 

Above all, don’t cut herbicide rates hoping that it will help a cover crop planted later that year. “It is really, really tempting to cut rates, thinking it will help with the residual (impact) so you can plant a cover,” says Johnson. Do not do that. That just invites weed-control and herbicide-resistance issues.”

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