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Cover crops battle resistant weedsCover crops battle resistant weeds

Get control of tough to handle herbicide resistant weeds with cover crops.

Jim Ruen

October 4, 2016

4 Min Read
<p>Where the cover crops ended, the weeds began in this wheat field on Ralph Upton, Jr.&#39;s south central Illinois farm. He has seen similar benefits in his corn and soybean fields, reducing and in some cases, eliminating the need for post herbicide applications and controlling glyphosate and PPO resistant marestail. </p>

Think Different

  • Plan cover crop mix for weed suppression as well as soil health.

  • Keep costs down with lower cost cereal rye and ryegrass.

  • Integrate herbicide program planning with cover crop planning for maximum return.


Ralph Upton has cut his herbicide costs, controlled herbicide resistant marestail and waterhemp, improved soil health and reducd erosion on his south central Illinois fields. And while the weather impacted his herbicide program, weed suppression by cover crops kept working.

"Where I had cover crops, I seldom saw weeds," says Upton. "Waterhemp is hitting us real hard. We've had issues in the past, but nothing compared to this year."

Wet, cold weather in April delayed planting until May 10, with additional rain delays keeping him out of the field until June 1. As a result, he planted only 200 acres of corn compared to his normal 800 to 900 acres. That same weather also led to poor herbicide burndown. After years of relying on glyphosate he tried alternatives, only to discover apparent PPO as well as glyphosate resistance in his waterhemp.

Fields with cover crops aren't immune to waterhemp. "I did get a few coming through and where I did, I sprayed the field, but I should have spot sprayed," he says. "We had corn fields with a hairy vetch cover crop that we didn't put any herbicide on. Where I had to replant I did use herbicide, but even there it was on only eight to ten acres out of 80."

Stifled winter annuals, marestail

Mark Anson also reports significant weed suppression with delayed weed seed germination using cover crops. Weed control combined with other benefits justifies Anson Farms continuing to invest in cover crops while looking for ways to reduce other costs. The 20,000-acre operation in southwest Indiana and southeast Illinois has about 65 percent of its acres in cover crops.

"We find that cover crops control winter annuals really well and my brother, who does a lot of the spraying, has pointed out fields that used to have heavy marestail pressure have very few out there," says Anson. "We seeded cover crops in a river bottom field, leaving a 100 ft. perimeter bare. The cover cropped area was clean, but the 100 ft. perimeter was heavy with weeds."

"We are clearly seeing weed suppression with cover crops," says Larry Steckel, row crop weed specialist, University of Tennessee.  "If you have a good cover crop stand, you don't have to worry about marestail and winter annuals. Sometimes you avoid that extra burndown needed for marestail and it's a big help with summer annuals like Palmer pigweed. A decent cover crop stand can delay pigweed 20 to 30 days which buys time and puts less stress on resistance management."

Steckel notes seeing the most consistent weed control with wheat/vetch, cereal rye/vetch or cereal rye/crimson clover combinations. He does admit that they can require a two-pass program to terminate, such as dicamba, a few weeks before planting and gramoxone behind the planter.

He also has found that cover crops can complicate pre-emerge herbicide programs. "Atrazine in corn or metribuzin in soybeans work better than others," says Steckel. "Prowl or Dual seems to hang up on top (the soil) while encapsulated acetoclor works well."

Green planting helps

Mike Plumer has three years of research (eight replications each year) to back up claims of marestail suppression with cover crops. "We saw 95 to 98 percent control," reports Plumer, consultant, Conservation Agriculture and a former University of Illinois extension specialist. "The best control was with 60 to 80 pounds of cereal rye seeded the second or third week of October  - You can go lighter and still get weed control if planted early enough to tiller. If you're shooting for full season weed control of a broad spectrum of weeds, let cereal rye grow until planting. If planting cereal rye into corn stubble, 25 to 30 pounds of nitrogen in fall can increase stand, improve growth going into winter and increase growth in the spring improving weed control."

Upton tries to get vetch planted on soybean stubble by mid-October even if that means planting a shorter season soybean variety as he did this year. He aims to plant 35 to 40 pounds of cereal rye and 10 to 12 pounds of ryegrass on corn stubble by Thanksgiving.

He feels he is getting physical suppression from the rye combination, as well as some allelopathic affect. As he has focused more and more on weed suppression, he waits longer to terminate which adds more complications. "To get more biomass, we need to let it grow longer," he says. "Increasingly, I will be terminating right at planting with glyphosate on the rye and 2, 4-D on the hairy vetch."

Convince landlords of value

This year Upton had too much biomass. Delayed planting allowed up to 25 percent more biomass than usual. "The planter didn't get set up right to handle it and that affected the stand which in turn allowed some water hemp to slip through," he says.

Even with the complications, Upton, like Anson, has no plans to back off on his cover crop program. He is even considering expanding it to rental ground. "It has been hard to justify the cost on land you may not farm the following year," he says. "However, I'm seeing enough weed control and other benefits that I may be able to convince landowners to work with me on it."

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