By Lizabeth Stahl and M. Scott Wells

A key window to seed cover crops in soybean in Minnesota is around the time soybeans reach physiological maturity — roughly when 95% of pods on the plant reach a mature, brown color.

As soybean leaves drop off the plant, the canopy opens, allowing more sun to reach the soil surface to promote cover crop germination and growth.

Cover crops can also be interseeded into corn around the time corn reaches physiological maturity at kernel black layer, although results have been more mixed.

If corn plants stay green for a long time, the amount of light getting through the canopy can be limiting. Also, cover crop seeds may germinate but not survive in heavy corn stover. Adequate moisture and good seed-to-soil contact improve the odds for successful cover crop establishment.

Here are advantages to seeding around physiological maturity in corn and soybean.

• There is typically not enough time in Minnesota between grain harvest of corn and soybean and a killing freeze for a cover crop to successfully establish. Interseeding a cover crop into standing corn or soybean widens the window for cover crop establishment. As the cover crop has more time to grow, it can accumulate more biomass, increasing potential of the cover crop to protect soil from wind and water erosion. Greater biomass accumulation in cover crops like cereal rye has also been associated with increased uptake of nitrate-nitrogen, resulting in less potential for nitrates to leach through the soil profile.

• Interseeding a cover crop later in the season rather than earlier widens the window between preplant and preemergence herbicide applications and cover crop seeding. This results in more time for herbicides to break down, lowering the risk in many cases for herbicides to hinder cover crop germination and growth.

• Seeding a cover crop around physiological maturity in corn or soybean should not impact yield of the cash crop, assuming the crop is not driven over when interseeding. University of Minnesota research has shown that corn and soybean yield was not affected when cover crops were interseeded in corn or soybean around physiological maturity. It is important to note that interseeding earlier in corn, around the V7 stage of corn, has also been shown to not impact corn grain yield.

• Seeding later in the season offers more flexibility in postemergence herbicide applications compared with seeding the cover crop prior to application.

Cover crops that are less shade-tolerant should have a better chance of establishment when planted, as the crop canopy is opening up prior to harvest versus earlier in the season, prior to crop canopy closure.

No guarantees
Successful establishment of the cover crop, however, depends on adequate moisture for germination and growth of the cover crop. Inadequate seed-to-soil contact can lead to reduced stands and growth as well, while an early killing frost can put an end to cover crop growth.

Herbicide history of the field should be considered whenever interseeding a cover crop. How long the herbicide remains active in the soil, cover crop sensitivity to the herbicide, application timing and application rate are key factors to consider.

If you wish to graze the cover crop, be sure to read the pesticide label of any products used in the field. Rotational restrictions on the label MUST be followed if you wish to graze the cover crop. Note that cover crops often planted as cash crops — such as cereal rye, oats and wheat — will often have shorter rotational restrictions than cover crops.

For more information on herbicide and cover crop interactions, see Managing Risk When Using Herbicides and Cover Crops in Corn and Soybean.

Another good resource is the Midwest Cover Crop Council Decision Tool. This offers ideas on which cover crop species are best suited for planting later in the season.

And general cover crop information is available at the U-M crop production webpage under “Soil and Water.”

Stahl is a University of Minnesota Extension crops educator, and Wells is a forage and cropping systems specialist with U-M.