Research Highlights

Research Highlights
Get More From Cover Crops: Seed Early

Photo: Joseph L. Murphy

Part 1 of 2

By Laura Temple

Cover crops offer many benefits, like improving nutrient cycling, reducing erosion, suppressing weeds and increasing organic matter. But common cover crops practices often limit or slow the realization of those benefits.

“The mindset about cover crops needs to shift from just qualifying for cost-share payments to managing for maximum benefits for soil health and profitability,” says Ray Weil, Professor of Soil Sciences at the University of Maryland. “We are investigating cover crop management in corn-soybean production systems to help Maryland and all U.S. farmers get more value from them. With management adjustments, cover crops more effectively capture nutrients, protect water quality, reduce inputs and boost yields.”

Most Maryland acres have cover crops because of state conversation cost-share programs, well above the national average. Weil has been researching ways to improve cover cropping systems with support from the Maryland Soybean Board since 2017. His work shows that the standard practice of planting a single cover crop after harvest greatly limits potential benefits.

According to Weil’s research, giving cover crops a longer growing season can significantly increase their value. Stretching the growing season both by planting it sooner and terminating it later gives the crop more time to accomplish the benefits they offer. Part 1 of this series focuses on seeding cover crops sooner. Part 2 will explore terminating the crop later in the spring.

Photo: Joseph L. Murphy

Value of early cover crop seeding

A primary goal of cover crops in Maryland and surrounding states is to capture unused nutrients, especially nitrates, to prevent leaching to groundwater and eventually to the Chesapeake Bay. Weil says those nutrients move during the winter, so to keep them in fields, they need to be captured in the fall.

“A standard regional practice is to drill wheat or rye into a field in October or November after the soybeans or corn have been harvested,” Weil says. “By the holidays, fields may show a bit of green, but soil coverage is very low, and crop roots haven’t grown deep enough to truly clean up nitrates in the soil profile.”

He adds that following a corn crop, 200 to 300 pounds of unused nitrogen can be found in the top 5 to 7 feet of the soil profile. The nutrients in the deeper soil layers are the first to be lost due to leaching.

Giving cover crops an additional four to six weeks to grow in the fall allows their roots to go deep, taking up those nutrients. They stay on the farm for future crop use, instead washing away.

“Establishing cover crops earlier makes a big difference in nitrogen leaching,” Weil says. “Our measurements show nitrate levels of 30 to 50 parts per million in leaching water from fields with standard cover crops, compared to just 5 to 10 parts per million when cover crops are seeded earlier.”

That additional growing time also allows cover crops to produce more vegetative ground cover that protects against surface soil erosion.

Cover crops also provide organic matter to improve soil composition. When the cover has more time to grow, the amount of organic matter put back into the soil increases. According to Weil, as the early-seeded cover crop system gets going, over time fields will need less fertilizer to meet yield goals. 

Photo: Joseph L. Murphy

How does early seeding work?

Several management practices support earlier cover crop establishment. Weil notes that choosing at least some earlier-maturing soybean varieties and corn hybrids can spread risk and allow earlier harvest. 

However, his work has focused on interseeding cover crops into standing cash crops. He has investigated cover crop mixes and technology advances that ease logistics of planting cover crops before harvest. 

For early plantings, Weil recommends seeding a three-way cover crop mix with a grass, a legume and a brassica. For example, annual ryegrass or rye as a grass cleans up unused nitrogen fairly deep in the soil and recompenses growth in sprain. In the fall, a brassica like radish or rapeseed captures nutrients from even deeper soil layers and helps fix soil compaction, as well. 

Specific species may vary when planting into soybeans ahead of corn compared to planting into corn ahead of soybeans. In Maryland, Weil has found red or crimson clover works well in soybeans ahead of corn. For planting into corn ahead of soybeans, he recommends planting heavier rates of grass and brassica species and lighter rates of the legume.

“We have observed that crimson clover grows more vigorously when it’s adjacent to a radish plant, and vice versa,” Weil adds. “Planting a mixture at the right time for maximum radish growth usually results in a clover-dominated cover crop in the spring, after the radishes die. Seeding the cover crop mixture a bit later gives the advantage to the grass species, usually resulting in a grass-dominated cover crop with very little clover the following spring. We don’t fully understand the interspecies interactions, but our observations have been fairly consistent across both plots and fields.”

Seeding into soybeans

According to Weil’s research, the ideal timing for interseeding cover crops into soybeans is when the crop starts to senesce. Before that, the soybean canopy is too dense to allow light to get to emerging cover crop seedlings.

Cover crop biomass measurements indicate the ideal interseeding planting window likely exists between the first two dates, when soybean leaves are all yellow, but before they begin dropping. The October cover crop was drilled after harvest and biomass was substantially lower than the interseeding trials.

“Our time-lapse cameras underneath soybean canopies in early-seeded trials show cover crops emerging and then wilting because the soybeans are more competitive for light and nutrients at that stage,” Weil explains. “As soybean leaves yellow and drop, increasing amounts of sunlight get through to the ground. The cover crop can get established and grow while the soybeans dry out.”

He described a few options for seeding cover crops into standing soybeans:

  • Aerial application via airplane.
  • Aerial application via drone, which is just starting commercially, but may become more accurate, cost-competitive and able to manage odd-shaped fields and those with obstacles.
  • Ground application with a highboy, modified with drop tubes that deposit seed below the soybean canopy.

The cover crop needs rainfall or a bit of irrigation to stimulate germination. The dropping soybean leaves can also protect seeds from drying out and encourage germination. 

One challenge for interseeding into soybeans is to choose cover crops that don’t grow very tall at first, so they don’t interfere with harvest. Choosing taller soybean varieties or planting soybeans into green cover crops (discussed in Part 2) so the emerging plants reach for the sun and their lowest pods are a couple of inches higher off the ground can help minimize harvest challenges. The goal is ensuring that the combine cuts soybeans below the lowest pod without getting plugged with the green cover crop.  

Seeding into corn

Weil says that planting cover crops into standing corn is much easier. The corn canopy allows more light to the soil, and corn ear height means cover crops won’t interfere with harvest.

A three-species cover crop mix seeded into standing corn in mid-July with a no-till drill. Photo: United Soybean Board

“Side-dressing knee-high corn with nitrogen is a standard practice in Maryland for improved nutrient use efficiency,” he explains. “And that practice may become more common throughout the country. New equipment has been developed to drill cover crops while side-dressing nitrogen.”

His trials show that a cover crop planted into corn will survive throughout the summer, even with limited rainfall. Then, as the corn dries down in the fall, the cover crop grows more quickly, scavenging leftover nutrients.

“We studied early-planted cover crop roots using a buried nitrogen tracer, and we saw that they grow deep fast enough to clean up the soil profile to 6-feet deep,” Weil adds.

According to Maryland Soybean Board Director Brian Johnson, who farms near Westover, grants for research like this directly support Maryland soybean farmers. “We select projects to receive grants based on research priority, cost-effectiveness and positive impact to farmers.”

Published: Jan 18, 2021

The materials on SRIN were funded with checkoff dollars from United Soybean Board and the North Central Soybean Research Program. To find checkoff funded research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.