By Rebecca Vittetoe

Gearing up for the 2017 growing season, it’s important to successfully terminate cover crops. There’s a lot of talk about how to successfully establish cover crops; however, effectively terminating them is equally important to minimize any negative impacts they could have on the cash crop.

Iowa State University researchers generally recommend terminating grass cover crops when they are 6 to 8 inches tall and to terminate 10 to 14 days before planting corn to protect yield. That time frame is less critical for soybeans. Do check with your crop insurance agent for cover crop termination requirements prior to planting corn or soybeans.

Waiting to terminate until after your crop is planted, especially in non-GMO crops, can be risky. Termination options become more limited, and the cover crop can quickly become an uncontrollable weed. It’s better to be safe than sorry.

Winter-kill cover crops
The most common cover crop termination methods include choosing cover crops that winterkill, applying herbicides, using roller-crimping and using tillage.

PLANTER SETTINGS: This planter slot was not closed in a rye cover crop field going to corn. It’s important to have the planter set correctly to plant corn or soybeans into cover crop residue. (Photo by Rebecca Vittetoe)

With the mild winter, winter-kill cover crops such as oats and many brassicas may not have actually winterkilled. Scout fields to ensure that the cover crop did indeed winterkill. If it did not, you will want to have another plan in place.

Herbicides are popular option
This is probably the most common method used to terminate a cover crop. The effectiveness of a herbicide applied for terminating a cover crop depends mainly on three things:
• cover crop species and growth stage
• herbicide and application rate used
• environment (temperature)

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. Cover crops will need several days of this warm weather surrounding the herbicide application for optimal termination.

Contact herbicides, like paraquat, are significantly less consistent than translocated herbicides, like glyphosate, as a termination method and may require multiple passes for complete termination. Translocated herbicides move to the plant's growing points, so complete plant coverage is less of a concern than with contact herbicides. Due to the variability in environmental conditions during spring, a 1 pound acid equivalent rate of glyphosate per acre is recommended. This could be anywhere from 28 to 42 fluid ounces, depending on the glyphosate formulation you use. Glyphosate formulations may contain 3 to 4.5 pounds acid equivalent per gallon. You can find the concentration of a formulation listed below the ingredient statement on the first page of the herbicide label.

The addition of residual herbicides, liquid fertilizers, or other adjuvants may decrease effectiveness of the herbicide or slow termination. You may consider increasing the herbicide rate and spray volume if you are tank-mixing, dealing with larger and more mature cover crops, or making an application in cooler-than- ideal temperatures.  If you plan on grazing or mechanically harvesting your cover crop this spring, you’ll want to give that cover crop time to recover and get some regrowth on it before applying a herbicide to terminate it so there is enough leaf area present to absorb the herbicide.

Always read and understand the herbicide labels for directions, appropriate rate and spray additives, and any restrictions for the subsequent crop. A quick and easy place to look up herbicide labels is cdms.net.

Rolling, crimping can be effective Rolling or roller-crimping is an alternative termination method. Effective termination with this method depends upon proper timing of the crimping for the cover crop species present. For cereal rye, it’s recommended to wait until after the rye has shed pollen to get a consistent kill with a roller-crimper. A cover crop mix makes efficient control more complicated since the species can be at different growth stages at the same time. Examples of cover crops that can be controlled with rolling and crimping include hairy vetch (at full bloom), barley, triticale or cereal rye (all at milk or dough stage). Use tillage to terminate?

Tillage is an option but may fit better into some operations than others. Multiple tillage passes may be necessary to terminate the cover crop, which can negate the benefits the cover crop is providing to soil health and could result in erosion. Some species, like clovers, will be much more difficult to manage with tillage alone.

Following cover crop termination, be sure to check fields for regrowth or skipped areas that need further attention. This will allow for a successful cover crop termination, and, hopefully, a successful cropping season this summer.

In addition to successfully terminating cover crops, it’s important to think about other adjustments that may need to be made to your operation to get your cash crop off on the right foot. Start with your planter settings and check to ensure you are attaining optimal seed depth, getting the seed furrow closed, and not causing sidewall smearing or compaction from too much down pressure. For more information, go online to iowalearningfarms.wordpress.com/tag/cover-crop-residue.

From a nitrogen perspective, especially for cereal rye cover crop fields going to corn, Iowa State University data suggests using the same nitrogen rate for corn without a rye cover crop. ISU agronomist John Sawyer has conducted a study on this, and his results and recommendations are available online at agronext.iastate.edu/soilfertility/info/CoverCrop_CornYieldEnhancement_1-31-2016.pdf.

Finally, be sure to scout fields for potential insect pests like armyworm and black cutworm, or seedling diseases that create conditions such as damping-off.

Regardless of termination choice, it’s important to have a plan in place to minimize problems this spring. It’s smart to have not only a Plan A, but also a Plan B for terminating a cover crop, because Mother Nature can easily alter the best-made plans.

Vittetoe is an ISU Extension field agronomist covering southeast and south-central Iowa. Contact her at [email protected].