Terminating Cover Crops

Failure to properly terminate cover crops can result in them acting as weeds and creating competition for your crop. For the most consistent control, make herbicide applications in the early spring while cover crops are small and actively growing. 

“Several factors influence the effectiveness of burndown treatments, including the cover crop species and growth stage, the herbicides and rates used, application parameters, and environment,” says Bob Hartzler, Iowa State University Extension weed specialist.

“As with controlling weeds, cover crops are easier to kill early in the spring while they are small,” says Hartzler. “They become progressively more difficult to kill as they approach reproductive stages.”

However, there are steps you can take to control a mature cover crop. One of these is to make sure you have proper spray coverage. 

“Increasing the herbicide rate and the spray volume to improve coverage will improve the consistency of control when dealing with large, mature cover crops,” says Hartzler. 

Another factor you need to consider when terminating cover crops is the temperature. Monitoring predicted temperatures during the day and night can help you plan applications. Avoid spraying in the early morning or in evening periods to avoid less than optimal temperatures. 

“Temperatures tend to fluctuate widely during the cover crop termination period and can lead to variable results,” says Hartzler. “Spraying when temperatures favor active plant growth minimizes problems; however, abnormally cold nights (<40°F.) may reduce activity even when favorable temperatures (>60°F.) occur during the day.”

“Glyphosate is the standard herbicide used for terminating cover crops,” says Hartzler. “Glyphosate is recommended at 0.63 lb a.e./A (18 oz Roundup PowerMax) for cereal rye 16 inches or less in height, and at higher rates for larger rye. Tank-mixing other herbicides with glyphosate may reduce the activity of glyphosate.”

“Control of cereal rye with glyphosate was reduced up to 50% when tank-mixed with atrazine or Canopy, whereas 2,4-D, dicamba, or Sharpen had little or no effect on rye control (K. Bradley, University of Missouri). The antagonism observed with tank-mixes was greater with late applications than applications made to small rye,” says Hartzler.

“The glyphosate label provides great flexibility in application rate. While the Roundup PowerMax label states an 18-ounce rate for cereal rye, this rate should only be used under ideal conditions. Increase the rate when tank-mixing with other products, with larger rye, or when applications are made during cool periods,” says Hartzler.