Lessons learned from field scale cover crop trials have transitioned to a watershed scale and continue to show the system’s benefits in controlling nitrogen loss.

Shalamar Armstrong, Purdue University assistant professor of soil conservation and management in the Department of Agronomy, is a co-investigator with Illinois State University faculty in the Lake Bloomington watershed trials.

He spoke of his findings from watershed project and other ongoing cover crop trials in a recent Illinois Nutrient Loss Reduction podcast.

“We focus on agronomics to economics to water quality to soil health,” Armstrong said.

Armstrong continues as the lead in field trials near Lexington that have been conducted for the past seven to eight years. He started that project while on staff at Illinois State University.

“We’re looking at 4R nitrogen practices, coupled with the inclusion of cover crops. We’re looking at the impact of those practices on the agronomics, on economics, on the environment as far as water quality, air quality. We’re looking at emissions of nitrogen into the atmosphere (greenhouse gas), and also we study the water quality implications and ecosystem services of those practices,” Armstrong explained.

Larger Scale

He is now involved in a collaboration that moves to a watershed scale using what was learned at the Lexington field trials.

“We located two large sub-watersheds at Towanda in the Lake Bloomington watershed. We put cover crops (500 acres) on half of the larger watershed and did not cover crop the other sub-watershed. We’re looking at the impacts of the mass adoption of cover crops,” he said.

“The trials look at the water quality because both of those watersheds drain into Muddy Creek, which then goes to Lake Bloomington.”

The watershed scale study is now in its fifth year.

“We see the cover crops are affecting not so much the concentration of nitrate in the tile drainage water, but it’s affecting the volume of water. So, when you affect the volume of water, volume multiplied by concentration gives you load, so we have a decreased load,” Armstrong said.

“We didn’t see that immediately. It took two or three years of cover cropping for that system to mature and for those differences to emerge.

“We’re also seeing the average growth of cover crop is maybe 1,000 pounds per acre of biomass and in that 1,000 pounds it averages 30 to 50 pounds of nitrogen that’s in that biomass.”

Water-Holding Capacity

With less water going into the tiles, Armstrong was asked if the cover crop itself is holding the water or is the rooting system allowing the soil to maintain a higher volume of water, or both?

“It could be both, but primarily we hypothesize that the cover crop is growing, so therefore there is transpiration occurring, so it’s using water for its growth. While doing that, it’s could possibly be drying out the soil,” he explained.

“So, you imagine if this field does not have cover crops it would take less rain to push water through the profile in order for it to be intercepted by a tile drain that then goes to the creek. But if you’re growing plants when you normally would not, now you have this plant growing during the fallow period, it’s using water and causing the soil to be a little drier compared to where there are no cover crops. So, it takes more rainfall in order to create drainage and there is less volume of water leaving.

“On top of that, you still have the cover crops taking up nutrients, taking up nitrogen and hold it in its biomass, which is also critical to nutrient loss reduction. Because if it’s in the biomass it’s not leaching and it’s also not denitrifying into the atmosphere, it’s captured in the biomass to be released at some point later.”

Armstrong said the same trends were found in both the field scale and watershed scale studies — cover crops reduce the amount of nitrogen leaving the field through tile drains.

“We can really be precise on the field scale and say there’s a 35% to 50% reduction in nitrogen loss. That’s what we’re seeing on a field scale,” he said.

“It’s more difficult on a watershed scale to come up with that precise number, but we think the reduction is somewhere around at least 30%. But it took time for that system to mature. After two years we started seeing those reductions.”


The research has also included looking at the agronomic side of a cover crops and cash crops system.

Cereal rye is the most commonly used cover crop in the Midwest Corn Belt because it does a good job keeping nitrogen from leaving the field. However, the challenge is growing cereal rye prior to corn.

“Those are two grasses and there are a lot of antagonist things that could occur. We recommend growing cereal rye before soybeans and some other crop species like radishes, oats or clover before corn,” Armstrong noted.

His research group is looking at ways that cereal rye before corn can be productive. This first-year study, in collaboration with Southern Illinois University, is being conducted in Illinois and Indiana.

“We’re looking at skip-row technology, so when we drill the cereal rye prior to the corn we drill in strips and leave a space for the corn to be planted where cereal rye was not planted. So, there are non-intersecting zones of growth of cereal rye and corn,” Armstrong said.

“We’re using precision agriculture that’s normally just used for corn and soybeans and using that same precision to plant cereal rye and leaving a space.

“We’re investigating that to determine if we are reducing some of this antagonistic relationship between cereal rye and corn by planting them in non-intersecting zones across the field. We want to see if we can get optimum yield doing that compared to the old way we planted cereal rye which was broadcast or drilled and not leaving a skip row.

“Do we have any negative tradeoffs with the environmental ecosystem services such as nitrogen loss reduction when we use this skip-row technology? Then, could we use less seed and get the same environmental ecosystem services and get equal or greater yield?”

Seed Selection

Armstrong advises those considering the use of cover crops to first know the needs of the field and pick the “tool” that is needed.

“Do you need erosion control because you have greater than 2% slope or do you have flatland 0% to 2% slope and you have tile drainage and you want to reduce the nitrogen that’s leaving? When you understand the problem that you’re trying to address, then you can select the proper cover crops,” Armstrong said.

“The other thought is plan ahead. We recommend a non-grass cover crop before corn until we figure things out. So, if you’re coming out of soybeans and going into corn, you’re going to want a cover crop that produces a low carbon-to-nitrogen ratio biomass that releases the nitrogen quicker and less antagonistic to your corn.

“If you’re going into soybeans there are no worries. You can use cereal rye.

“Think about your crop rotation and you might want to rotate your cover crops with your cash crops and think about the problem that you’re trying to solve.

“If you do that and plan and tap into your resources, or tap into your Extension services. You can go to the Midwest Cover Crop Council website where there are decision-making tools to allow you to select and know when to plant. Also if you want to do a cover crop mix and try some diversity, the tool is there on the website to help you determine the ratios to mix your cover crop species.”