Cover crops
KILLING A COVER: Cover crops need warmer temperatures to grow before they can be terminated. Ideal conditions are daytime temps above 60 degrees F with sunny skies.

Cool spring thwarts cover crop termination

Sunny days with 60-degree temps needed for covers to grow before termination.

By Rebecca Vittetoe

Green cover crop fields are slowly starting to appear in Iowa. Despite the recent snow that fell across the state, the start of 2018 corn planting is right around the corner. The late-developing spring is posing challenges to farmers who are trying to find a window to terminate overwintering cover crops such as rye.

Farmers should have a game plan in place for how they plan to terminate their overwintering cover crop. Killing cover crops with a herbicide is the most common termination method. Keep in mind, however, the effectiveness of herbicides at terminating a cover crop depends primarily on three things:
cover crop species and growth stage
 herbicide and rate used

Tips to get a good kill
The cool temperatures so far may make terminating cover crops this spring more challenging. Using glyphosate alone is the safest bet to kill the cover crop in a timely and effective manner. It is recommended to use a 1-pound acid-equivalent rate of glyphosate per acre. This could be anywhere from 28 to 42 fluid ounces, depending on your formulation of choice.

Glyphosate formulations may contain from 3- to 4.5-pound acid equivalent per gallon; the concentration of a formulation is listed below the ingredient statement on the first page of the label. Check your product of choice to determine the appropriate application rate.

Since glyphosate is a translocated herbicide, the most effective herbicide applications will be made on a sunny day when temperatures are above 60 degrees F, plants are actively growing, and nighttime temperatures stay above 40 degrees.

Soil temperature
Cool spring temperatures mean cover crop plants aren’t actively growing. Vegetative growth in rye requires temperatures of at least 38 degrees. While air temperatures begin warming, soil temperatures can remain cooler and slow growth. Consequently, the plants may have trouble taking-up herbicides and termination may be compromised. Leaving a small check strip is an easy way to see if the cover crop is dying following termination.

GREENING UP: This cereal rye cover crop at the ISU McNay Farm in southern Iowa was planted early in September. At the end of March, the cover began greening up.

Iowa State University researchers generally recommend terminating the cover crop with herbicide 10 to 14 days prior to planting corn to protect yield. However, that time frame is less critical for soybeans. Check with your crop insurance agent for any specific cover crop requirements they may have prior to planting corn or soybeans.

Wait to terminate?
Waiting to terminate the cover crop until after your corn or soybean crop is planted, especially in non-GMO crops, can be very risky. Options become more limited, and the cover crop can quickly become an uncontrollable weed.

Always look at the herbicide labels for directions and any restrictions for the subsequent crop. Look up herbicide labels at or

Additional information on cover crop termination:

 ICM Blog: Terminating Cover Crops: What’s Your Plan?

• ICM Article: Terminating Cover Crops

• ICM Article: Research Shows Extra Cover Crop Growth Prior to Soybeans Provides Benefits

• ICM Article: Cereal Rye Cover Crops, Allelopathy and Corn

• Cover Crop Termination, IPM University of Illinois

• Cover Crop Termination, University of Wisconsin Extension

• Successful Cover Crop Termination With Herbicides, Purdue Extension

Vittetoe is an Iowa State University Extension field agronomist in south-central Iowa. Contact her at [email protected].

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