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Strategic Farming: Let’s talk crops! focused on cover crops

Phyllis Bongard, Content development and communications specialist

Cover crop residue in corn (left) and soybean (right).
Timing can mean a lot when it comes to cover crops. Drs. Anna Cates, State soil health specialist, and Axel Garcia y Garcia, Sustainable cropping systems specialist, joined Extension Educator Liz Stahl for a wide-ranging discussion on cover crop termination timings in corn and soybean and planting green in the February 9 session of Strategic Farming: Let’s talk crops!

Benefits of cover crops

Cover crops provide many benefits, from reducing erosion to increasing water infiltration and retention. They reduce nitrate leaching risk by taking up nitrogen and can suppress weed growth, and these functions are greater when cover crops produce more biomass.

Why the focus on biomass?

Why is biomass production so important? Biomass covers the soil, retains nutrients, and feeds the soil food web. According to Cates, the living roots of cover crops feed the soil microbes that build soil structure. The more cover crop biomass you have, the more benefits your soil will gain.

Those gains in soil structure can translate into better water infiltration and retention for your crops. Cates has many anecdotal stories from farmers saying that infiltration had improved so much with cover crops, that they could get back in the field much faster after a big rain. She’s currently doing some research to try to quantify that benefit.

Recent research from Waseca looked at water retention through the soil profile with different tillage systems both with and without a cereal rye cover crop. Deeper than 15”, cover crops consistently reduced water, probably due to early spring water use. On the surface, the rye cover crop system had slightly more water, as cover crop residue probably decreased evapotranspiration. These changes in water content and as well as improvement in soil structure persisted months after the rye had been terminated.

Cover crops can even, over time, change the logistics of a farm. With established cover crops, for example, fall labor and tillage costs can be reduced, since those passes aren’t needed.

For more information, watch the short video Why soil structure is important in crop production.

Challenges in producing cover crop biomass

These benefits don’t come without challenges, however. In terms of Minnesota cover crop acreage, we are still quite behind other states, reports Garcia y Garcia. One challenge is that farmers don’t generally see an immediate economic benefit from incorporating cover crops.

A larger challenge, though, may simply be our short growing season. We don’t have much opportunity for cover crops to grow and produce significant biomass before the fall frost. How much biomass would Cates and Garcia y Garcia like to see? There is no simple answer to this, but they’d like to see biomass production of at least 3,500 lb/a.

In response to these challenges, UM research has focused on planting and termination date studies and methods to try to optimize biomass production of cover crops in a corn-soybean rotation.

Cover crop research

Planting date

Planting date matters, states Garcia y Garcia. When cover crops are seeded after harvest in a corn-soybean rotation, there’s very little time for those covers to get established and produce biomass. If that cover crop is not winter-hardy, it will be killed with the first frost and will have produced very little biomass. Cereal rye, on the other hand, is winter-hardy and will resume growth in the spring.

To optimize biomass production, Garcia y Garcia would ideally like to see cover crops seeded by August 20. This cannot be achieved in a traditional corn-soybean rotation, however. Seeding cereal rye by around September 20 should lead to substantially greater biomass production in the fall and spring than a later fall seeding.


Establishing cover crops is a little more challenging in corn. Garcia y Garcia has had most success with interseeding cover crops at V4-V6 (four to six leaf) stages. Results have shown that interseeding works well compared to seeding late in the season, since the amount of biomass is significantly higher. Seeding during earlier vegetative stages can affect corn yield and seeding into vegetative stages later than V8 can affect cover crop establishment due to competition with the corn. Cover crops can also be broadcast into maturing corn (R5) and after harvest.

Overall, good soil-to-seed contact and adequate moisture are critical for successful establishment.


Cover crops have been successfully established in soybean both before and after harvest in research conducted at Lamberton. When established before harvest, seeds were broadcast into R7 soybeans. This stage is close to maturity, when soybeans are yellowing and starting to drop their leaves.

Late fall

If there’s no time to seed cover crops earlier, cereal rye can be planted very late in the season. It may not emerge in the fall, but it will come up in the spring.

Termination timing

The other critical part of the equation is when the cover crop will be terminated. If cover crops are terminated 1-2 weeks before early planting corn and soybean (around April 20-25), biomass will be very low. By waiting another 3 weeks to around May 15-20, biomass production can double or even triple to 3,000 or more lb/a.

Waiting for mid-May to terminate the cover crop is ideal from a biomass perspective, but one must weigh the potential impacts on cash crop yield due to a later planting date.

Planting green

Planting green is an option that gives cover crops a little more time to grow in the spring. In this practice, a cash crop is planted directly into a living cover and then the cover is terminated soon after planting.

Early research at Lamberton showed that biomass of cover crops was very good with May termination dates. Yield of soybean planted May 12, 2020 was not affected by cover crop termination timing (5/2, 5/12, 5/21). Yield of corn planted the same date as soybean was reduced, however, when cover crop termination was delayed until planting or later. To see a summary of the first-year results, visit Planting Green in Minnesota.

For more information on benefits, establishment, species selection, and termination timing, see UM Extension’s Cover crops website.

Join the webinar series

University of Minnesota’s Strategic Farming: Let’s talk crops! webinar series, offered Wednesdays through March, features discussions with specialists to provide up-to-date, research-based information to help farmers and ag professionals optimize crop management strategies for 2022. Join us February 16 at 9:00 am when we discuss tar spot in corn. For more information and to register, visit

Thanks to the Minnesota Corn Growers Research and Promotion Council and the Minnesota Soybean Research and Promotion Council for their generous support of this program!

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