Extending cover crop growth until planting soybeans enhances the benefit.

Cover crops are popular for a variety of reasons including protecting the soil, suppressing weeds, scavenging nutrients and stimulating a robust microbial system that improves soil health.

There are a variety of species that can be planted into standing corn or seeded into cornstalks, in the summer after wheat harvest or in the early fall. Generally, you would plant a combination of species—some that winter kills and others that overwinter and continue to grow through the spring. 

Spring cover growth is important, as it adds more biomass to the soil and its actively growing roots feed the herd of microbes that contribute to soil health and nutrient recycling. However, the mistake growers often make is not recognizing the value of spring growth and how it contributes to soil health. Many growers want their covers to either die naturally during the winter so they don’t have to deal with termination or terminate early in the spring—after the winter covers green up—so it dies and decomposes before planting.

Well, there is evidence that shows that letting covers grow longer prior to planting soybeans is beneficial. The United Soybean Board funded a study to understand the potential for cover crops to perform in a corn and soybean rotation and how time of termination impacted that benefit.

Research conducted by Iowa State University shows that letting cereal rye grow an extra three weeks prior to planting soybeans produced about as much as 400% more biomass. Note the extra three weeks correspond with the recommendation to terminate rye three weeks before planting, so the residue begins to decompose and doesn’t have a negative influence on germination and emergence.

In addition, the study shows that soybean yield was not impacted—regardless if the cover crop was killed three weeks prior to soybean planting or one day before soybean planting. Another measured benefit was that letting covers grow an additional three weeks allowed the rye to take up twice as much nitrogen from the soil—from 40 lbs. per acre to 80 lbs. per acre. And their research shows that cover before soybeans provided more benefit than cover before corn because there is more time to grow in the spring, as long as they aren’t terminated.

But one of the benefits they did not measure is the impact of the extended growth on soil health. During this three week period, the roots leak organic molecules back into the soil and the microbes come to life early while feeding on these carbon substrates. This stimulates soil health and carbon and nutrient recycling three weeks earlier in the season and provides additional benefits to the crop.

Remember the green mantra, “roots in the ground all year all year round.” Living roots of cover crops contribute to soil health by feeding the herd of microbes in the soil.

Despite letting the cover crop grow an extra three weeks, the data shows no negative effect on soybean yields. By increasing biomass production and retaining nitrogen, the system will build soil health and have a positive impact on water quality challenges.

You can read the full study here.

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com.