There is an increased interest in use of cover crops due to the many agronomic and environmental benefits they offer. An important consideration when incorporating cover crops into your cropping system is their termination. Failure to completely control cover crops at planting results in them acting as a weed and competing with the crop, notes Bob Hartzler, an Iowa State University Extension weed management specialist.
Several factors influence the effectiveness of burndown herbicide treatments, including the cover crop species and growth stage, the herbicides used and application rates used, application parameters and the environment. Hartzler offers the following considerations and management recommendations for timing of application and getting a good kill on a cover crop.
Some cover crops are easier to kill than other covers
Research in Missouri has shown that cereal rye and hairy vetch are usually consistently controlled with appropriate treatments, notes Hartzler, whereas winter wheat, annual ryegrass and red clover can be more difficult to kill.
As with controlling weeds, cover crops are easier to kill early in the spring while they are small. They become progressively more difficult to kill as they approach reproductive stages, he explains. The Roundup PowerMax label states better performance is achieved when applied before the boot stage of cereal rye. Increasing the herbicide rate and the spray volume to improve coverage will improve the consistency of control when dealing with large, mature cover crops.
Temperatures tend to fluctuate widely during the cover crop termination period and can lead to variable results. Spraying when temperatures favor active plant growth minimizes problems; however, abnormally cold nights (lower than 40° F) may reduce activity even when favorable temperatures (higher than 60° F) occur during the day.
Glyphosate is used most often to kill cover crops
Glyphosate is the standard herbicide used for terminating cover crops. Glyphosate is recommended at 0.63 pounds a.e. per acre (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.
"That's according to research by Kevin Bradley at University of Missouri," notes Hartzler. "The antagonism observed with tank-mixes was greater with late applications than applications made to small rye."
Best control is obtained when applied in early spring
Unfortunately, "we have a limited ability to assess the susceptibility of cover crops to burndown herbicide treatments," adds Hartzler. "The most consistent control will be achieved with applications made in early spring while cover crops are small and actively growing. You should avoid spraying in the early morning or evening during periods with less than optimum temperatures. Also, note that the glyphosate label provides great flexibility in application rate."
While the Roundup PowerMax label states an 18 ounce per acre application rate for control of cereal rye, this rate should only be used under ideal conditions, says Hartzler. He recommends you increase the application rate when tank-mixing Roundup PowerMax with other products, or when applying it to larger rye, or when applications are made during cool periods.