As the number of Iowa farmers using cover crops continues to grow, it’s important to help make sure they have a successful experience. With that goal in mind, the Iowa Department of Agriculture and Land Stewardship is offering some spring management tips for farmers using cover crops.

“We continue to see cover crop acreage expand as farmers see how they can reduce erosion, protect water quality and improve soil health,” says Mike Naig, Iowa secretary of agriculture. “Given the later planting date for many cover crops last fall and the cold weather this spring, cover crops have been slow to ‘green up.’ Management of cover crops over a range of weather conditions is critical to the success of using this important practice.”

While the lingering winter weather has created challenges this spring, it’s important to allow a cover crop to grow as long as possible to maximize the benefits. This is typically easier with soybeans that will be planted after winter cover crops because of the later planting date and less potential impact on the beans.

When to terminate a cover?
For corn following a winter cover crop, it’s important to fully terminate cover crops and provide enough nitrogen at planting, Naig says. If providing nitrogen at planting isn’t possible, then plan to terminate the cover crop 10 to 14 days ahead of planting. More experienced users may be more comfortable with closer termination windows if nitrogen is managed correctly.

The following information was put together with the help of the Iowa Cover Crop Working Group, which includes representatives from IDALS, Practical Farmers of Iowa, Iowa State University, Iowa Learning Farms and USDA’s Ag Research Service. For more information about using cover crops in your farming operation, visit these web pages from Clean Water Iowa, ISU Extension and PFI.

• Evaluate for winter kill. Snow cover after some winter cover crop growth had broken dormancy provided less-than-ideal conditions for cover crop growth and survival this spring. Scout your cover crop fields and check the crown of the plant for green plant tissue. Even if the leaves of the plant are brown, double-check whether the crown is brown or green. If the aboveground cover crop is brown and near the soil surface and no green plant material is present, then your cover crop winter-killed. 

Cover crops such as oilseed, radish and oats typically winter-kill and no additional spring management is needed. Other cover crops, such as winter cereal rye, winter wheat and winter triticale, consistently overwinter in Iowa.

• Termination options. Herbicides, tillage or a combination of the two can be used to effectively manage cover crops in spring.

Fir herbicide, the rule of thumb is: “Mow the yard once, and then get ready to kill your cereal rye. It needs to be growing.”

For successful herbicide termination, make sure the plant has greened up and has enough living surface area for the herbicide to work. Experienced farmers suggest spraying during the middle of the day and, if possible, spraying when daytime and nighttime temperatures add up to 100 degrees F. Unless you have experience, separate nitrogen application from a burndown herbicide application, or be sure not to dilute the herbicide effectiveness with too much nitrogen as the carrier.

Terminating cover crops with tillage can be effective but may take more than one tillage pass. Wet periods can delay tillage to terminate cover crops, and wet conditions following tillage can allow cover crop plants to survive anyway. Also, tilling a cover crop to terminate it eliminates the erosion prevention and potential weed control benefit the cover crop would usually provide in the early part of the growing season.

However, if spring tillage is a must, then make sure to fully bury the cover crop root balls that will have dislodged. Double-check your planter setup to make sure good seed depth is achieved for corn and soybeans.

• Consider N needs for corn. Winter cereal cover crops effectively scavenge nitrogen and reduce the amount of soil-available N in the period from late April through May. To protect yield, farmers growing corn after a cereal rye cover crop may want to apply 30 to 50 pounds of nitrogen at or near corn planting. This is not additional nitrogen, but within the farmer’s total fertilizer program. Options like starter in a 2-by-2 placement, nitrogen as the carrier for a weed and feed operation, or some form of available nitrogen over the top will be important to overcoming soil nitrogen that is tied up early in the season.

• Planter setup. A field planted after a winter cereal cover crop will be in a different condition than a tilled or no-till field with no cover crop. Evaluate planter setup and make sure to double-check that the seed trench is properly closed at planting. An open seed slot can be especially damaging to corn seed, while soybean seed seems to rebound better.

• Scout for insects. Although rarely an issue, sometimes true armyworm insects can emerge in cornfields following a winter cereal cover crop. These insects can only be treated once they have emerged. Plan to scout fields of corn where winter cereal cover crops biomass is thick.

• Know crop insurance requirements. Crop insurance rules state that a cover crop in Zone 3 (western third of Iowa) must be terminated by the day of cash crop planting. A cover crop in Zone 4 (eastern two thirds of Iowa) must be terminated within five days of cash crop planting. If using no-till, add seven days to either scenario. More information about insurance requirements can be found at this Risk Management Agency web page.

• Start planning now for fall cover crops. Determine what cover crops work with your current or planned crop protection program. Some residual herbicides have carryover restrictions for certain species of cover crops. Consult with your agronomist and/or cover crop seed representative to look at your specific management system with the integration of cover crops.

Consider assistance that may be available. Visit your local soil and water conservation district office to learn more about financial assistance that may be available. Also, Practical Farmers of Iowa invites farmers who sell soybeans into the ADM supply chain via its Des Moines plant or who sell corn to Cargill via the Eddyville plant to participate in a cover crop cost-share program. Farmers can reserve their place by signing up for a free, nonbinding, one-hour phone consultation with a cover crop expert at PFI.

“Cover crops are a brand-new practice to many farmers, and we want them to have a positive experience,” Naig says. “Fortunately, there is a wide range of resources available to help farmers successfully manage cover crops this spring.”

More information about cover crops and the Iowa Water Quality Initiative can be found at cleanwateriowa.org.