Research Highlights

Planting Cover Crops Into Standing Soybeans

By Laura Temple

Although cover crops already provide many soil health benefits, ongoing research can help soybean farmers refine cropping systems to improve the effectiveness of the practice. In Pennsylvania, cover crops are commonly drilled into soybean fields after harvest. However, raising full-season soybeans with a longer maturity time or double-cropping soybeans limits the time available to plant and establish cover crops before low temperatures stop growth.

“Farmers I work with wanted to figure out if they could plant cover crops before soybean harvest,” says Heidi Reed, an agronomy educator with Penn State Extension. “In many of our cropping systems, farmers are limited to cereal rye and winter wheat as cover crops. They asked if planting earlier, into standing soybeans, would allow use of other species.”

For years, the Pennsylvania Soybean Board has funded a network of on-farm research with checkoff dollars to address practical questions like this. In the fall of 2020 and 2021, Reed led a study within this Pennsylvania Soybean On-Farm Network to help farmers test broadcast seeding cover crops before harvest. 

Trials at two Penn State research farms and in two cooperating farmers’ fields investigated broadcasting nine cover crop species into standing soybeans. At the research farms, Reed’s team planted small plots with cereal rye, winter wheat, annual ryegrass, crimson clover, red clover, Balansa clover, hairy vetch, canola and forage radish with a spinner spreader. 

Cooperating farmers chose one cover crop species or several from that group. They also chose planting methods, which included options like a high clearance commercial spreader and a drone. Cover crops were seeded at recommended population rates regardless of location or planting method.

“The goal was to time broadcast seeding as soybean leaves yellowed, just before leaf drop, which usually occurs in late September or early October in our region,” Reed explains. “However, actual planting timing ranged from the R6 stage of soybean growth to shortly after leaf drop.”

Initial Observations

Reed notes that overall cover crop stands were inconsistent. Cereal rye was the most productive species in a majority of trials, followed by winter wheat.

“We learned that even when planting sooner, winter cereal crops still produced the most ground cover and biomass,” she says. “Clovers still didn’t have time to get established, and likely need to be planted even earlier, so they don’t fit our systems as well.”

Brassicas also produced inconsistent results, though they averaged slightly better than clover species.

Reed adds that rainfall is paramount. The seeds need rain shortly after broadcasting to promote germination. However, a heavy rain can wash cover crop seeds away.

In most of the trials, dry matter production remained below optimal levels, according to the Natural Resources Conservation Service. However, even the lower levels of cereal rye biomass reduced spring soil nitrate content.

Ongoing Research and Refinement

The first two years of these trials raised many additional questions. Reed believes early cover crop planting is a tool that needs to be fine-tuned to work well in the region. 

“For our next step, we plan to directly compare the value of broadcast seeding cover crop grasses early with drilling them after harvest,” Reed says. “We want to understand the role of seed-to-soil contact in soil coverage and biomass development.”

As she focuses on quantifying yield and benefits from different cover crop practices, Reed hopes to narrow in on best practices for farmers in the region. 

“For example, broadcast seeding may make more sense in soybean fields planted with tramlines or tracks for equipment, to minimize impact on the standing crop,” she notes. 

As research continues, she also intends to look at the interaction between soybean planting dates in the spring and cover crop planting timing in the fall.

“This practical research will help farmers decide how best to incorporate cover crops into their systems and take full advantage of the benefits of this practice,” Reed says.

Published: Oct 3, 2022