Agronomy: Delay Terminating Covers Till Planting

Headline from Iowa State University: “Research Shows Extra Cover Crop Growth Prior to Soybeans Provides Benefits.”

Anyone who has planted cover crops, particularly winter cereal rye or annual ryegrass, will recognize the truth in this statement and has probably figured this out already.

This Iowa State University study, funded by the United Soybean Board, showed that letting cereal rye grow three more weeks prior to termination produced 300 to 400% more biomass and scavenged 100% more nitrogen prior to termination. And their research showed no impact on soybean yield if the cover was terminated 3 weeks or 1 day before planting.

I have planted rye as a cover for over a decade now, seeding it anywhere from mid September to mid December before corn and soybeans are planted. One thing I have realized is that spring growth is largely related to temperature and moisture, regardless of the spring growth window available. If it is a cold and/or dry spring, then rye won’t grow much until it warms up and the later the termination the greater the growth and benefit. Of course, in some springs it will warm early, the soil is moist and it will grow a lot and very early.

When covers started to become popular the recommendation was to terminate early, about 3 weeks before planting, to get them to start to decompose and break down. If you terminated say a week before planting, you would create a matt over the soil that would impede soil warming and drying, and both plantability and germination. However, terminating early never made agronomic sense because growers weren’t extracting the full benefit from their investment in a cover crop—biomass production, nutrient scavenging and stimulating soil biology.

So I quickly adopted the practice of planting directly into standing cover and terminating it immediately after planting with a burndown and residual mix. I realized three things: fields were easy to plant, I got more benefit from the cover and my stands were better than before. I made this discovery about seven years ago and have promoted it ever since. And now a university study confirms what I have been preaching.

This research in Iowa showed that covers before soybeans provided more benefit than cover planted before corn, because they have more time to grow before planting. This is true if soybeans are planted in the traditional timeframe. But today growers are planting both corn and soybeans early and often at the same time. Regardless of the crop, letting covers grow until the day you plant provides more benefit from the cover and the subsequent cash crop. I don’t know what to make of their statement “Cover crops must be killed at least 10 days before corn planting to avoid corn yield loss,” because that has never been my experience.

To read the full release click here

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

Dan Davidson
Soybean agronomist Daniel Davidson, Ph.D., posts blogs on topics related to soybean agronomy. Feel free to contact him at or ring him at 402-649-5919.



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