As temperatures warm this spring, cover crop termination is on the to-do list for some Iowa fields. Killing cover crops with herbicides is the most common termination method. The effectiveness of herbicides at terminating a cover crop depends primarily on three things:  

Iowa State University Extension field agronomists Meaghan Anderson and Rebecca Vittetoe, along with ISU weed management specialist Bob Hartzler, provide the following observations, information and guidelines to help farmers successfully terminate cover crops in the spring. 

Choose right herbicide 

The cool and fluctuating temperatures encountered in spring often make terminating cover crops challenging. Depending on the cover crop species or mix you have in the field, you need to choose and use a herbicide treatment that will kill that cover. Chemical products and application rates needed to kill cover crops depend on cover crop species and growth stage. 

Farmers are limited to a few products like paraquat (Gramoxone; Group 22), glufosinate (Liberty; Group 10) or glyphosate (Roundup; Group 9) for cover crop termination. Glyphosate is the most consistent option for termination, especially as cover crops increase in size.

The Group 1 herbicides (clethodim, fluazifop, etc.) do not provide effective consistent control of cereal rye. If cereal rye or other grass species are seeded with a legume, inclusion of 2,4-D or dicamba with glyphosate will improve consistency of control. This addition can also be helpful if broadleaf winter annuals are present. 

Consistent control of cereal rye 

In a study encompassing eight site-years across five states, treatments containing glyphosate provided the most consistent cereal rye control. Cereal rye ranged from 5 to 54 inches tall at termination in the experiments. While control of cereal rye did not differ statistically between most paraquat and glyphosate treatments, paraquat-based treatments were much less consistent than glyphosate-based treatments. Glufosinate treatments were less effective and less consistent than glyphosate treatments.  

EARLY START: This cereal rye cover crop at the ISU McNay Farm in southern Iowa was planted in early September 2017. The photo was taken March 30, 2018. Photo by ISU 

While paraquat can provide acceptable control in some situations, neither glyphosate alternative (paraquat, glufosinate) provides as consistent control as glyphosate under cool and variable spring conditions. Dicamba combinations with the three burndown herbicides provided similar results to 2,4-D combinations. 

Always leave check strip 

Vegetative growth in rye requires temperatures of at least 38 degrees F. While air temperatures may be favorable some days, cool soil temperatures can slow growth. Herbicides are most effective on actively growing plants; thus, very early spring termination treatments may provide less than complete control. Leaving a small check strip is an easy way to see if the cover crop is dying following termination. 

ISU 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. Waiting to terminate until after your crop is planted, especially in non-GMO corn, can be risky. Termination options are more limited, and the cover crop can quickly become an uncontrollable weed in non-GMO crops. Additionally, it is important to check with your crop insurance agent for any specific cover crop requirements that they may have prior to planting corn or soybeans. 

Restrictions for next crop 

Always look at the herbicide labels for directions and any restrictions for the subsequent crop that will be planted in that field. A quick and easy place to look up herbicide labels is cdms.net or greenbook.net. 

For additional information on cover crop termination: 

Source: ISU, which is solely responsible for the information provided and is wholly owned by the source. Informa Business Media and all its subsidiaries are not responsible for any of the content in this information asset.