Research Highlights
Do Cover Crops Impact Weed and Crop Germination?

The sample on the right treated with hairy vetch extract from Spring Hill, middle Tennessee, showed curled, wrinkled leaves that were smaller than the check on the left. Photo: Sam Purdom, University of Tennessee Institute of Agriculture

By Laura Temple

Cover crops have great potential to enhance complex soil ecosystems, and many research projects focus on their benefits. But as a team at the University of Tennessee Institute of Agriculture started looking at economic benefits of cover crops, new questions emerged.

“We know weed suppression is one benefit of cover crops,” says Avat Shekoofa, Assistant Professor with the Department of Plant Sciences, University of Tennessee Institute of Agriculture. “While weed suppression due to cover crops is well documented, the mechanics of that suppression are not completely understood. How do cover crops suppress weeds? How do specific cover crop species impact cash crops? There was very little information to address these questions, so we developed a process to find answers as part of a comprehensive cover crop research project.”

Shekoofa is studying the potential allelopathic effects of various cover crop species on both weeds and soybeans as part of a larger cover crop study. This multi-year study started with national soy checkoff support, and it is now funded by the Tennessee Soybean Promotion Board.

Allelopathy refers to a plant’s release of biochemicals, called allelochemicals, that interact with other plants. Through leaching, being exuded by roots, or other processes, allelochemicals can inhibit seed germination and plant development.

“We wanted to learn if cover crops have an allelopathic effect on weeds,” Shekoofa continues. “And if so, could they have a similar impact on row crops?” 

To find out, she started with controlled environmental studies in the laboratory in 2019 and 2020. Her team gathered above-ground biomass samples from seven cover crop species in two locations, Knoxville in eastern Tennessee, and Spring Hill in middle Tennessee. They processed these samples to create solutions that simulated plant leachate containing allelochemicals. They diluted those solutions with water to create 50 percent leachate for experiments on both weeds and soybeans. Trials expanded to the greenhouse to monitor early plant growth in 2020. And, she plans to conduct a similar trial in the field in 2021.

“So far, we have found that weed suppression and detrimental impacts on soybean germination and growth varied significantly by location,” she says. “Soil type, pH, environmental conditions, cover crop termination timing, and some other factors may influence allelopathic effects. Data from these trials will help farmers choose appropriate cover crop species for their operations.”

Weed Suppression

To begin investigating allelopathic impact on weeds, Shekoofa’s team considered goosegrass, a tough-to-manage weed in Tennessee. A variety of petri dishes containing 30 goosegrass seeds each were treated with percentages of leachate from canola, cereal rye, crimson clover, hairy vetch, wheat, winter pea and woolypod vetch, compared to just water. They measured various germination properties, like germination percentage, germination rate, seedling length and root and shoot length.

Most cover crop species tested in 2019 noticeably reduced goosegrass germination, but the results varied by location. In the samples from Knoxville, east Tennessee, only canola and woolypod vetch did not significantly impact goosegrass germination, as shown in chart 1A. In the samples from Spring Hill, middle Tennessee, only cereal rye and crimson clover did not suppress goosegrass germination, as the data shows in chart 1B.

Figures 1A-1B. Germination percentage of cover crop species in east Tennessee (1A, left) and middle Tennessee (1B, right).

Despite the differences in locations, these results indicate the potential for cover crops to be a tool for integrated weed management, to supplement herbicide control. The next target for allelopathic weed tests within this project will be pigweed germination, a weed with increasing herbicide resistance.

Soybean Germination

While allelopathic effects add value for weed control, a study in cotton showed that leachate from cover crops also negatively impacted cotton germination and seedling growth. Shekoofa also wanted to know if the allelopathic effects of cover crops species could negatively impact soybean germination. In the lab, soybean seeds were treated with cover crop leachates in petri dishes. In the greenhouse, soybeans were watered with leachates and compared to a control of pure water to monitor potential impacts for up to a month after planting.

Again, location was a major factor. With samples from Knoxville, east Tennessee, canola and cereal rye suppressed soybean germination the most, as shown in chart 2A. With leachate from Spring Hill, middle Tennessee, crimson clover had the most detrimental impact, shown in chart 2B.

Figures 2A-2B. Soybean germination percentage in various cover crops in east Tennessee (2A, left) and middle Tennessee (2B, right).

In the greenhouse studies, extract from hairy vetch from Spring Hill, middle Tennessee, impacted soybean germination and leaf development for the first 20 days of growth, as show in the photo. However, by 26 days after planting, the soybeans treated with this extract had more leaves than the untreated check.

Figure 3. Soybean root nodule count with various cover crops.

The most fascinating effect found in the greenhouse was that wheat extract from both locations had a positive impact on soybean root nodulation by 30 days after planting, shown in chart 3. More soybean root nodulation may improve biological nitrogen fixation in soybeans. 

While Shekoofa says additional data is needed, she can see how allelopathic studies can help farmers better manage their cover crops.

Cover Crop Management Implications

“Farmers need to learn what cover crop species suppress germination of their problem weeds, without inhibiting soybean germination,” she explains. “Species can be adjusted depending on the crop that will be planted the following spring, and allelopathic information could help farmers make management decisions like cover crop termination timing and soybean seeding rate.”

Regardless of the potential challenges for soybean production, she believes that supplement weed management tools will be critical as weed resistance to herbicides continues to evolve. And cover crops can be part of that solution.

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.