Research Highlights
Cover Crops Demonstrate Potential to Double as Forage

These cover crop plots in Knoxville were planted in mid-October and harvested for forage in April or left for residue prior to termination. Photo: University of Tennessee Institute of Agriculture

By Laura Temple

Interest in cover crops is growing. But some consideration ties to incentives like subsidies and cost-share programs, acknowledges Virginia Sykes, Assistant Professor with the Department of Plant Sciences, University of Tennessee Institute of Agriculture.

“As adoption increases and subsidies are reduced over time, we need to show the economic benefits of cover crops aside from current programs,” Sykes says. “One way to get value from cover crops beyond the ecological benefits over time is to harvest them as forage.”

Sykes is leading research that compares the economic and environmental benefits of cover crops when used both as cover and forage to traditional cover crops. This multi-year study started with national soy checkoff support, and it is now funded by the Tennessee Soybean Promotion Board. 

A small plot forage harvester allowed evaluation of cover crop species as both cover and forage. Photo: University of Tennessee Institute of Agriculture

Limited information is available about the suitability of specific cover crop species in Mid-South soybean and corn rotations, or about their performance as both cover and forage. In response, this research assesses the adaptation of 16 cover crop species to Tennessee regions under two cover crop season timings. Trials include dual-use and single-use plots in Knoxville and Spring Hill, and single-use plots in Milan. 

Regional production systems create two different season timings for cover crops planted after harvest and terminated just before planting. Cover crops following corn can be planted in September or early October and terminated shortly before planting soybeans in May. Cover crops following soybeans can be planted in mid- to late-October, but they are terminated in April, shortly before corn planting.

“In our trials, the cover crop plots following corn grew at least six weeks longer than the plots following soybeans,” Sykes reports. “Because of that, we found more options that work better as forage on corn ground rotating to soybeans. The cover crops on soybean ground rotating to corn had significantly less time to produce forage biomass.”

In two years of trials (2017-18 and 2018-19 cover crops), average forage yields from cover crops were adequate for the long season, when following corn and preceding soybeans (Figure 1A). Short-season yields following soybeans and preceding corn were much lower (Figure 1B). Green bars in the charts indicate species that had significantly higher biomass than the “no cover” control.

Figure 1A (left) and 1B (right). Cover crop biomass (tons/acre) when following corn (1A) and following soybeans (1B).

Regardless of system, all species provided adequate nutritive values for feed use. Ongoing economic analysis will determine more exact return-on-investment, according to Sykes. But she expects the option to harvest cover crops as forage offers will defray hard costs like seed.

Dual-Use Systems Maintain Ecological Benefits

This research focused on reducing erosion through off-season ground cover and increasing soil moisture, nutrient retention and weed suppression during the subsequent crop as environmental benefits of cover crops, according to Sykes. The team collected soil samples and evaluated ground coverage growth throughout the trials to determine if harvesting forage from the cover crops impacted those benefits. They also monitored weed and insect pressure to see if removing the growth affected pest management needs.

“We saw no reduction in ecological benefits to the soil under the dual-use system,” Sykes reports. “Perhaps over a longer period of time, organic matter would accumulate more slowly, but the root mass from dual-use cover crops provided the soil health and nutrient management benefits expected.”

Student Matthew Longmire evaluates insect populations in Spring Hill cover crop plots for his master’s thesis.
Photo: University of Tennessee Institute of Agriculture

Soybean yield and quality was consistent in both cover crop systems. In addition, they saw no change in weed suppression in the following crop, whether the cover species was harvested for forage or not. Nor did insect or slug populations vary. 

“Dual-use cover crop plots also produced high-quality forage,” she says. “The dual-use option appears to offer the best of both worlds, offering further economic incentive to cover cropping.”

Cover Crop Species Selection Key

Throughout these trials, adaptation, growth and ground coverage results varied significantly among crop species. Selecting species and varieties adapted to region and production system will help farmers optimize the benefits of both single- and dual-use cover crops.

“We recommend planting a mix of species in cover crops, but farmers need data to determine what mix will address their environmental and forage needs,” Sykes says. “In 2018-20 dual-use trials, we included 16 species of brassicas, cereals and legumes to evaluate both ecological and forage value factors.”

Cereals

In general, the five cereals species — barley, cereal rye, oat, triticale and wheat — provided higher percentages of ground cover when measuring fall growth. For example, the cereals averaged 32 percent ground cover just one month after planting. All species did better during the longer cover crop season following corn. However, only cereals provided significant fall and winter cover following soybeans.

As forages, cereals provided adequate yield. But they left the smallest nitrogen credit or a nitrogen deficit for the following crop. 

“We learned that all cereal species perform fairly well,” Sykes says. “Cereal rye is a common choice, but other cereals may be cheaper. Based on our data, farmers can choose a cereal species and variety for their cover crop mix that best fits their needs and budget.”

Brassicas

She notes that subsidized cover crop programs often require brassicas in a mix, such as the canola, forage radish and turnip that were included in these trials. Their roots address soil compaction and they offer nutritive value as forages.

“The varieties of brassicas we evaluated did not fit very well in Tennessee soybean and corn production systems,” Sykes explains. “These may not have been bred for this region. However, based on submissions to our cover crop variety trials, this seems to be an area where we are seeing a push to breed more adapted varieties.”

In both the long- and short-season trials, brassicas did not provide good full-season ground coverage. If planted early, forage radish was able to provide adequate ground cover for a short period in the fall, and canola and turnip had reasonable biomass at termination. 

Legumes

The research team included eight legume species in the trials: arrowleaf, berseem, crimson and red clovers; common, hairy and woolypod vetch; and winter pea. In general, legumes grow more in the spring and fix nitrogen that is then available for the following crop. In the fall, they averaged just 19 percent ground cover one month after planting, but by early March, some species provided more than 70 percent canopy cover. 

“Hairy vetch was the only legume to provide significant biomass at termination following soybeans,” Sykes reports. “In the long-season plots following corn, woolypod and hairy vetch stood out all season alongside crimson clover. Winter pea picked up in the winter, spring and at termination.”

Species Variety Selection

“While our study comparing dual-use and traditional systems focused on cover crop species, the selection of available cover crop varieties withing those species continues to expand,” she adds. “In 2019, we introduced a cover crop variety trial, evaluating 60 varieties, including brassicas, cereals and legumes, to provide head-to-head comparisons of available options.”

Detailed results from the 2019-20 cover crop variety trials share more specific data on biomass, forage quality and nitrogen release to support cover crop mix species and variety selection for either traditional or dual-use cover crop systems.

This project was funded by the soybean checkoff. To find research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.