Follow the rules for killing a cover crop

Follow the rules for killing a cover crop

Cover crop termination guidelines are simple for Iowa farmers, says NRCS agronomist.

Rules for terminating winter hardy cover crops are clear and simple to follow for Iowa farmers and allow for maximizing conservation benefits, says Barb Stewart, state agronomist for USDA's Natural Resources Conservation Service.

With the increased popularity of cover crops, USDA's Risk Management Agency has simplified termination guidelines for winter surviving cover crops for farmers and crop insurance companies. Here are the guidelines Iowa NRCS announced on March 23 for farmers to follow in spring 2015:

•The western one-third of Iowa counties (referred to as Zone 3 by USDA) must terminate a cover crop at or before planting a cash crop.

PLAN AHEAD: Proper planning and execution of your plan will help lead to a successful termination of the cover crop. Improperly terminating it will result in a poor experience with cover crops.

•The central and eastern two-thirds of the state (referred to as Zone 4 by USDA) must terminate a cover crop prior to cash crop emergence.

"These are the same guidelines as last year, but we are still getting questions from farmers about the best time and the required time to terminate cover crops," says Stewart. "There is no set calendar date requirement for when to kill a cover crop – it all comes down to when the producer is ready to plant in the spring."

There is no rush to kill cover crops in spring
In fact, there is no rush to terminate cover crops. Keeping a living cover crop in the spring provides more erosion control and weed control on the field, as well as provides livestock feed, she points out.

Stewart says it is perfectly fine to plant soybeans into a living cover crop, and terminate the cover crop afterward. "For corn, I recommend terminating the cover crop two weeks prior to planting or before the cover crop reaches 8 inches in height," she says.

2015 Cover Crop Termination Zones

"Following these simple rules will provide the best opportunity to achieve the conservation benefits from cover crops, while minimizing the risk of reducing yields to the following crop."

Some cover crops winterkill, but others – like cereal rye – need to be terminated in the spring. Stewart recommends crimping the cover crop to kill it, or applying herbicide to kill the cover crop, and avoiding tillage. "Tillage negates most of the soil health benefits cover crops provide," she says, "and could lead to additional erosion issues."

For more information about cover crop termination, contact your local NRCS office or go online to and click on "Soil Health" or visit

Additional tips for terminating cover crops
Many farmers who try planting cover crops set their priorities around seeding methods and the potential benefits of the particular cover crop they choose to use. "But equal thought should also go into how you will terminate the cover crop," says Sarah Carlson, cover crop specialist with Practical Farmers of Iowa.

Generally, four or five methods are used to terminate cover crops: winterkilling, tillage, mowing and herbicides. Some farmers use crimping. Each method has advantages and limitations. For example, winterkilling is a highly effective termination method but only works with certain cover crops. Due to simplicity and effectiveness, many farmers prefer to terminate cover crops using herbicides. Iowa State University Extension weed management specialist Bob Hartzler has some suggestions when terminating cover crops with herbicides.

First, have a plan A and a plan B for the way you'll terminate the cover crop before you even plant it. You never know what kind of weather you'll have in spring. Make sure you select a herbicide that will kill the cover crop you plant. And know your planting restrictions. Think about the following cash crop and whether the herbicide you choose to use will negatively affect it. Check the herbicide labels for information on efficacy and plant-back restrictions.

Also important is to spray the cover crop with the proper herbicide rate. Consider the height and growth stage of the crop. Last but not least, watch the weather as Mother Nature will largely determine the success of the termination. Avoid spraying translocated herbicides on cloudy or cold days which slow or stop cover crop growth and uptake. Also, it's best to spray the cover crop before it begins reproductive growth. For glyphosate, the recommendation is you need three consecutive days of no cooler than 45 degree F temperature and daytime temperatures of 55 to 60 degrees F.
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