Cover Crops — More Than Just Weeds in the Field

If you walk through Andrew Reuschel’s fields at any time of the year, you will find some plant growing. To some, it may look like out-of-control weeds. To Reuschel, it’s the best return on investment he’s ever made.

Reuschel, a corn and soybean farmer from Golden, Illinois, has been experimenting with cover crops on his fields since the 2000s. His dad and grandpa have tried them off and on in their fields since the ’60s. He says despite renewed efforts about every decade, they were failing nearly every time.

Reuschel said the lack of information on cover crops back then contributed to their failures. However, thanks to those who didn’t quit trying in the ’70s, ’80s and ’90s, farmers like him have more data for successful cover crop implementation.

“Since the mid-2000s, we’ve been able to establish a baseline on our farm and then expand,” Reuschel said. “We had to go through all those growing pains and visit people’s farms, understand the rules and then do it firsthand.”

Now, Reuschel plants up to 30 different species of cover crops in his fields, and the species change constantly — much like his overall mindset on cover crops.

“In the beginning, my focus was on erosion control, and now it isn’t even in my top 10,” Reuschel said. “My mindset has changed because I know I am going to get erosion control just because I have roots in the ground.”

Along with erosion control, cover crops offer many benefits, such as a reduction in soil compaction, weed control benefits and improved soil structure.

Optimizing biomass produced from cover crops 

Reuschel grows corn and soybeans, and plants both of those into cover crops. If he’s planting soybeans into cover crops, then cover crop biomass is his No. 1 goal. He wants the biomass to help with both weed suppression and to create a thick armor that will provide his soil with temperature and moisture control throughout the summer.

Liz Stahl, an extension educator in crops at the University of Minnesota, has been experimenting with cover crops for the last 10 years. Initially, her interest was for soil health purposes, such as protecting the soil in Minnesota against erosion. However, as herbicide-resistant weeds continue to pose weed management challenges, she is particularly interested to see how the biomass generated by cover crops can help suppress hard-to-control weeds. 

Biomass production either comes from a living cover crop or a surface mulch created by a terminated cover crop.

Stahl says the more biomass a cover crop generates typically results in more benefits from planting a cover crop. Along with erosion control and weed suppression, a well-established cover crop can also help with nutrient retention, soil health quality, compaction reduction, increasing organic matter and soil water-holding capacity.

“Our challenge in Minnesota is to generate enough biomass from the cover crop in our traditional corn and soybean rotation,” Stahl said. “How much biomass you can produce is a driving factor in how much cover crops can contribute to weed control.”

Because fewer weeds manage to germinate and emerge through the thick biomass cover, Reuschel is able to easily manage weed populations with fewer herbicide applications than without the cover crop suppression. After several years of implementing this weed control practice, Reuschel believes cover crop biomass reduces the need for herbicide applications, which reduces the selective pressure that leads to herbicide-resistance issues.

Reuschel also sees biomass as temperature control to his soil in the summer during hot, dry years when his crops are most vulnerable. In years like 2019, with excess rain, cover crops and their biomass can also manage extra moisture. Some farmers have claimed planting into a cover crop went more smoothly than those without a cover crop. 

John Wallace, Ph.D., assistant professor of weed science at Penn State University, said both living cover crops and surface mulch can suppress weed emergence and growth. To optimize weed suppression potential, farmers need to think about two things: cover crop quantity and quality.

“A higher biomass production and higher ground-cover level generally results in higher levels of weed suppression,” said Wallace, whose expertise includes herbicide-based weed control and weed ecology. 

Farmers can achieve improved weed suppression in cover crops by increasing the seeding rate and allowing the cover crop to grow. Farmers can then terminate the cover crop with a pre-emergence herbicide application with a different site of action from what was used in last year’s crop.

Wallace says in the case of using surface mulches as cover crops, the quality influences the persistence of the mulch through the growing season and helps keep the weeds at bay.

Keeping up with the recommended rates 

Although biomass can help reduce weed pressure in a field, farmers should still maintain their regular herbicide programs. This includes rotating sites of action in a regular herbicide program from year to year and crop to crop to reduce the chance of resistance development in the weeds still breaking through the cover crops.

“One of our most problematic weeds is waterhemp. That’s a weed that emerges later on in the season and over a longer period, so it’s a real challenge,” Stahl said. “If you have a cover crop that overwinters, it may provide some weed control benefits with early-emerging weeds at the start of the season, but it may not have as much of an impact on weeds emerging later in the season.”

Stahl said using pre-emergence herbicides is critical as farmers fight weeds such as waterhemp with new herbicide trait technologies. 

“Using pre-emergence herbicides is still a key part of having effective control of a weed like waterhemp,” Stahl said.

Using the correct labeled rate of herbicide is also key for effective control. Farmers who are planting cover crops in the hopes of reducing weed pressure, but are also applying herbicides below the labeled rates, are only helping herbicide-resistant weeds build resistance.

“You have to be very careful in the selection of what cover crop you want to plant and look at what your herbicide program is. Try and get those as compatible as possible,” Stahl said.

To help fight herbicide resistance while planting cover crops, Wallace said there are a few goals farmers should keep in mind.

“One goal is employing cover crops that, due to their competitive interactions, reduce the size of emerged weed populations at the time of post-emergence herbicide applications,” Wallace said. “Spraying fewer and smaller weeds has the effect of reducing the intensity of selection pressure on our currently effective post-emergence herbicides.”

Another goal is to maintain weed control performance while reducing herbicide inputs. This could be through decreasing the number of passes or the number of herbicides within one season. He says this could help farmers more effectively diversify and rotate sites of action across crop rotations.

Farmers should already be scouting their fields throughout the growing season, and those efforts should continue when planting cover crops, Stahl said. Just because a cover crop was planted for weed suppression does not mean those weed seeds disappear. 

“You can’t just change one piece of the system and think everything can be the same,” Stahl said.

Choosing the right cover crops for your fields 

There are many cover crop options to help farmers meet their goals, whether that be weed suppression, nutrient management or soil compaction. Reuschel says he will typically have mixtures of cover crops each time he plants. If one species in the mix doesn’t survive, he can typically count on the other species to pull through.

“One of my favorite things to say is different roots produce different root exudates which feed different soil biology,” Reuschel said. 

Root exudates provide organic carbon and microorganisms in the soil, and each plant produces various quantities and compositions. 

“That’s what’s important about the mixes. The soil biology is taking the cover crops to the next step,” Reuschel said. “When you start to understand soil biology, you can make those soil health principles bend toward you.”

Stahl says some of the different cover crops they’ve experimented with in Minnesota are cereal rye, red clover, pennycress and oats. The university is also looking at cover crops that can act as a cash crop, such as pennycress and camelina. She recommends looking up different cover crop recipes based on goals and benefits.

Reuschel said no matter the mixture or time of year he plants his cover crops, annual ryegrass goes on every acre, every year. And, he plants it two to three times a year. If he’s planting soybeans into a cover crop, he always makes sure there is a cereal rye in the ground. Both cover crops have many beneficial attributes, such as reducing soil erosion and suppressing pests. Cereal rye also provides significant amounts of organic matter and fits in many different crop rotations. Annual ryegrass, another great option, helps build soil, improve water infiltration and catch leftover nitrogen. Annual ryegrass also works well as a companion crop. 

Farmers interested in planting cover crops or learning more about the cover crops planted in their fields can find many resources with information. For example, Midwest Cover Crops Council has developed cover crop recipes for farmers new to growing cover crops. Farmers can also work with the extension agents in their counties and their university researchers to help get the best results on their fields.

Reuschel suggests creating and bringing in a group of like-minded people.

“Build a community around you to optimize the success of your cover crops. You have to have that community to really be successful,” Reuschel said. “And that community doesn’t have to be in a couple-mile radius.”

Farmers should also be regularly connecting with their crop advisers to assure they’re managing their pesticide-resistance risk.

Reuschel is grateful for the trial and error of the farmers before him. And for the information he has access to today. “There’s not a day that goes by that I don’t talk to someone about cover crops or soil health,” Reuschel said.

For farmers interested in learning more about cover crops and weed suppression management, listen to the Take Action cover crop podcast featuring John Wallace.