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Research Highlights

Research Highlights
Get More From Cover Crops: Plant Green

Soybeans seedlings grow taller when planted into green cover crops. Photo: University of Maryland

Part 2 of 2

By Laura Temple

Cover crops offer many agronomic 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 these benefits.

“Cover crops can do many things, but how they are managed determines what benefits they actually provide,” says Ray Weil, Professor of Soil Sciences at the University of Maryland. Since 2017, he has been researching ways to improve cover crop management in soybean–corn production systems with support from the Maryland Soybean Board. 

“Maryland has been a leader in cover crop adoption, in part because of state conservation cost-share programs,” he continues. “Our research directly influenced these programs, as farmers are now incentivized to lengthen the growing season of their cover crops.”

Weil’s work demonstrates that lengthening the growing season for cover crops significantly increases their value. That starts with seeding cover crops early, discussed in Part 1 of this series. Part 2 explores planting soybeans and corn into live cover crops or “planting green,” terminating them at or after.

Agronomic value of planting green

Figure 1. Conceptual diagram shows cover crop shoot growth and rooting depth over time when planted early and terminated late. Chart: Ray Weil

According to Weil, the work cover crops do in the spring is different than what they accomplish during fall and winter. After they are planted in the fall, cover crops take up nutrients to keep them in place and reduce leaching. Over the winter, they reduce erosion and runoff. But as temperatures warm up and growth resumes in spring, they help the soil prepare for the next crop.

“Cover crops actually help dry the soil profile out more quickly in the spring because they start using water as soon as they come out of dormancy,” Weil explains. “Farmers may not realize that’s what is happening in the total soil profile, because the growth covering the soil keeps it a bit cooler and wetter just on the top. This challenge increases the further north fields are. However, management practices like strip-till can mitigate it.”

He adds that spring is when cover crops produce the largest volume of organic matter to return to the soil. The biomass produced by cover crops can help maintain soil health and fertility. 

“The roots and shoots that cover crops grow in spring turn sunlight into carbon compounds that will become organic matter to feed soil biology and store nutrients for future crops,” he says. “In this area, farmers tend to terminate cover crops about a month before planting to make it easier to plant through. But skipping that month of growth means losing out on key benefits. Spring growth can produce two to five times the biomass being added to the soil.” 

Figure 2. Additional growing time resulted in three times more biomass and a greater diversity of cover crops. It also reduced weed growth. Chart: Ray Weil

Weil’s research also showed that additional spring cover crop growth changed the content of the biomass produced. In addition to replenishing soil organic matter, that biomass serves essentially as mulch, suppressing weeds and preventing water evaporation from the soil surface during the heat of summer.

Spring growth supports the cycling of nutrients the cover crops took up, eventually making them available to the next crop. The principle applies to both synthetic applied nutrients and to manure management. Weil recommends seeding a three-species cover crop mix that includes a grass, a legume and a brassica. However, specific species and seeding rates should be customized for the following cash crop.

“Each species provides different benefits,” he explains. “For example, brassicas like radish pull nutrients from much deeper in the soil profile than cash crops and bring them to the surface. Because they experience winter kill, those macro- and micronutrients are being released as the next crop grows.”

Ahead of soybeans, he recommends a higher rate of a grass species, because they hold and release nitrates slowly, and soybeans fix their own nitrogen. 

Legumes fix nitrogen so it is available for the next crop. Ahead of corn, Weil recommends a heavier rate of a legume, like a clover or vetch, and a much lighter rate of grass. Allowing the legume to grow longer in the spring allows the plants to produce more nitrogen.

“That additional month of spring legume growth increases the amount of nitrogen available for corn by 50 to 70 pounds per acre, according to our research,” he says. “They fix some nitrogen in early spring, but the additional growth can bring their total contribution toward 100 pounds of nitrogen, significantly reducing the amount of fertilizer needed.”

Figure 3. The charts show the pounds of nitrogen produced by cover crop biomass when planted green (top) compared to when terminated a month before planting (bottom). Planting green significantly increases nitrogen availability.
Chart/photo: University of Maryland

How does planting green work?

According to Weil, planting into green cover crops is much easier than many farmers expect. A wide variety of planter attachments, like row cleaners, are available to make it possible to plant into cover crops that are more than 3 or 4 feet tall. 

“The soil is in better condition to receive seed with the live cover crop growing,” he explains. “Planter coulters slice easily through the cover crop, and the furrows close well. Seed-to-soil contact is easier to achieve in these conditions than when cutting through cover crop biomass that has already been terminated and dried out. Farmers report better stands and fewer problems when planting green.”

He says that the burndown to terminate cover crops can be applied at or just after planting. The crop won’t emerge for several days. And herbicide-tolerant soybean or corn systems offer even more timing flexibility. 

Planting soybeans

Soybeans seedlings grow taller when planted into green cover crops. Photo: University of Maryland

Standing cover crops provide unique benefits to emerging soybeans, says Weil. The shade from the cover crops encourages soybean seedlings to reach for the sun, growing taller. Their lowest nodes will be farther above ground, making it easier to harvest them. Taller soybeans also make it easier to interseed cover crops early in the fall. The standing cover crop also protects soybeans from late frosts, as it traps heat at ground level. 

With many herbicide-resistance soybean seed programs, terminating the cover crop at or after planting is very manageable. Weil notes that heavy cover crops or high rates of a grass like cereal rye in the cover crop mix help suppress weeds in soybeans. Depending on growing conditions, farmers may be able to delay or reduce weed control passes in soybeans because the biomass effectively suppresses weeds. 

Planting corn

Planting corn into green cover crops requires more management because corn seedlings are much more sensitive to shade. Weil recommends rolling or crimping the cover crop while applying the termination herbicide as the corn is planted to ensure the seedlings will have plenty of direct sunlight. 

Rolling and termination can be done with the planter or as separate passes. However, the additional biomass and fertilizer savings from allowing the legumes in the cover crop mix to fix more nitrogen make the additional management worth the effort. 

To find research related to this Research Highlight, please visit the National Soybean Checkoff Research Database.