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Cover crops can be a herbicide helper

Herbicides have been and always will be a major weed management tool. 

These days, though, they need help. Herbicide-resistant weeds are ravaging existing herbicide sites of action.

Enter cover crops. “They have potential to suppress weeds, provided there is enough biomass production,” says Liz Stahl, a University of Minnesota (U of M) Extension educator. 

Early seeding is key to establishing cover crops so they can snuff weeds. Unfortunately, weather can override this plan. Carryover and injury from cash crop herbicides can also decimate cover crop stands, which then gives weeds room to roam. 

Preliminary data from 2020 U of M tests at three sites with these cover crops – winter cereal rye (grass), red clover (legume), and camelina (brassica) – are encouraging. Cereal rye performed the best at suppressing early emerging weeds like giant ragweed, although red clover and camelina also worked well, says Ethan Ley, a U of M weed science graduate student. 

“If you have a really thick mat of cereal rye in the spring, there’s good potential to decrease early-emerging weeds like giant ragweed and grasses, too,” says Stahl. 

Control is more difficult with pigweeds like waterhemp, though. 

“Waterhemp has had a later-emerging pattern, while giant ragweed emerges much sooner,” says Ley. This enables waterhemp to dodge cover crops as a weed-fighting tool. 

Seeding Schedule 

Cover crop weed suppression requires timely seeding. Farmers who raise small grains, canning crops, or corn silage have an edge, as they may seed cover crops in late summer or early fall after harvest, says Stahl. 

The lack of a standing cash crop enables farmers to drill cover crops. “This helps ensure good seed-to-soil contact, which enhances establishment,” she says. A good stand also helps cover crops accumulate biomass that helps smother weeds. 

Liz Stahl in a soybean field
Photo credit: Gil Gullickson

Farmers who rotate soybeans with corn for grain do not have this extended window. For them, one of the best chances of cover crop seeding success occurs as crops near maturity, says Stahl. This works well in soybeans as they start to drop leaves.

“As soon as soybean leaves start to drop, the canopy opens and allows sun to hit the soil surface,” she says. “The fallen leaves also help retain moisture, which helps cover crop germination.” 

Planting options include aerial seeding or seeding with high-clearance ground equipment. A higher seeding rate is recommended compared with drilling cover crops, Stahl adds. 

Postharvest seeding is another option. This allows seed-to-soil contact through drilling; however, late seeding leaves less time for cover crop germination and establishment. 

A U of M trial at Lamberton, Minnesota, examined four 2019 cereal rye seeding rates – three in September and one on October 8. 

“The three September seedings had good establishment and biomass levels when they were terminated the next spring,” says Stahl. “The last seeding on October 8 just never took off. It’s just one year’s data, but it shows how the seeding date can affect biomass levels in the spring.” 

Some falls also don’t permit postharvest seeding. “In 2019, we had areas [in Minnesota] where the crops didn’t get harvested in the fall,” says Stahl. 

First Things First

Damage from lingering cash crop herbicides is also a concern with establishing cover crops. First things first, though. 

“We can’t compromise on weed control to take advantage of the benefits of cover crops,” says Gregg Johnson, who works with biomass cropping systems at the U of M Southern Research and Outreach Center at Waseca, Minnesota. If waterhemp is ravaging fields, controlling it comes first – before cover crops, he says.

In some cases, agronomic practices such as early soybean planting accompanied by early herbicide applications can fit with cover crops planted later that fall. 

“The earlier that herbicide applications occur in the year, there is a greater chance that fall-seeded cover crops will be outside the rotational restriction window for some products,” says Meaghan Anderson, Iowa State University Extension field agronomist. 

Iowa State University extension agronomist Meagan Anderson
Photo credit: Gil Gullickson

Don’t cut herbicide rates to avoid injury potential to a later-planted cover crop, adds Stahl. Not only does it crimp weed control, but reduced rates also select for resistant biotypes that survive a reduced herbicide rate, she adds.

Label Rotation Restrictions

Still, farmers need to think about the impact of cash crop herbicides if they are to make cover crops work, says Stahl. 

“Check the [herbicide] label if you are going to graze a cover crop; we don’t want illegal residues to end up in the consumer feed chain,” says Stahl. 

Herbicide labels don’t always reflect how carryover restrictions may affect cover crops, either. 

“We went through and looked at 240 herbicide labels in 2018,” says Daniel H. Smith, University of Wisconsin-Madison (U of W) southwest regional agronomist with the nutrient and pest management program. Labels they reviewed did contain rotational restrictions for crops grown for feed, food, forage, and livestock grazing. However, few labels specifically mentioned rotational restrictions for unharvested cover crops planted for purposes such as boosting soil health.

That can spawn confusion, since rotational restrictions may differ between the same species grown for forages or cover crop purposes. 

“Typically, forage crops will be established the season after [herbicide] application,” he says. “Cover crops are going to be established three or four months afterward. Even though we may get the legal OK to plant a cover crop for soil conservation purposes after applying these residual herbicides, we have to remember that significant injury may occur.” 

Carryover Damage

Herbicide carryover damage, though, hinges on factors including herbicide half-life, soil type, and soil pH. Farmers can’t do anything about half-life, as some herbicides inherently take longer to degrade than others, says Smith. Soils with a high pH and high organic matter content will also slow herbicide breakdown.

One factor within a farmer’s control is cover crop species. There’s a reason cereal rye is popular among farmers. U of W tests in 2013 and 2014 show winter cereal rye withstood a number of herbicide treatments on corn and soybeans. 

Other cover crops weren’t as fortunate. In 2013, crimson clover annual ryegrass, tillage radishes, and an oat/pea mix all suffered herbicide carryover damage in the U of W tests.

Damage doesn’t always occur, though. “We had absolutely no injury in any cover crop in the 2014 trials,” says Smith.

Wildly varying weather made the difference, says Smith. 

“In June 2013, we had lots of precipitation to activate the [preemergence] residual herbicides,” says Smith. “Then, hardly any rain fell until September. We hypothesize that the herbicide was still active in the soil at the time of cover crop planting in the fall.”

In 2014, warmer temperatures and more rainfall occurred during the growing season. This helped dissipate and degrade herbicides without injuring cover crops, Smith says. 

“There’s no cookie-cutter answer in managing all this,” says Stahl. Several universities, though, have conducted research and developed resources that can help guide farmers in how herbicide carryover can impact cover crop establishment. 

Even if a herbicide label doesn’t specifically address cover crops in rotational restrictions, it does address cover crops that double as cash or forage crops. This can indicate how long a herbicide will linger in the soil and if cover crops will be injured, says Smith.

Timely Termination

Cover crops have many benefits, but unfortunately, all things must come to an end. Overwintering cover crops need to be terminated in the spring to make way for emerging cash crops. 

“Make sure you have a plan ahead of time,” says Liz Stahl, University of Minnesota Extension educator. “You may need plan B or C as well, because things don’t always work out as planned.” 

Termination herbicides need to be applied to an actively growing cover crop. “You want daytime temperatures to be between 55°F. and 60°F., and avoid nighttime temperatures below 40°F.,” says Stahl. “That can be tough to do in states like Minnesota, so you really have to keep an eye on things.” 

Glyphosate works best for terminating cover crops such as cereal rye, says Stahl. If broadleaves are in the cover crop mix, farmers and applicators may add 2,4-D, dicamba, or Sharpen, says Stahl.

To ensure optimal coverage, use proper rate and gallonage for the herbicide, she adds.

Interseeding Options

Interseeding is also a cover crop establishment option. “In corn, the seeding window typically occurs between V5 and V7,” says Liz Stahl, a University of Minnesota Extension educator.

Unfortunately, interseeding is not an option in soybeans, as a good soybean canopy will shade out the cover crop, Stahl says. 

It’s also complicated in corn, as waterhemp can nix an interseeding strategy. 

“We typically need to use a residual herbicide to effectively control waterhemp,” says Stahl. “When you interseed, you have greater potential for a cover crop to be affected by herbicides, due to a smaller window between herbicide application and cover crop seeding.”

One potential option is to match a field’s weed problems and resulting herbicide with the cover crop, says Daniel H. Smith, University of Wisconsin-Madison southwest regional specialist, nutrient and pest management program.

“If you have a broadleaf weed like waterhemp or giant ragweed, use a broadleaf herbicide to control it and interseed a grass cover crop,” he says. “The opposite is true if you have grass weeds. In those cases, use a broadleaf cover crop. There are some good herbicides that provide 30 days of residual control that can work with interseeding.” 

Watch weather forecasts, too. “If rain is forecast and you don’t know when you’ll be able to return to the field to plant, apply the herbicide a little earlier than normal to make sure you get timely termination,” says Stahl.

The same strategy applies to dry weather. Although early termination can reduce biomass levels that snuff weeds, it prevents the cover crop from competing with the cash crop for water and nutrients, Stahl says.

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