The following are recommendations for using herbicides to terminate a cover crop, according to Dr. Mark Loux, OSU Extension.
Cereal rye: Generally easy to kill
Glyphosate up to 18 inches: Base rate: 0.75 lb ae (22 oz PowerMax). Increase the rate on taller rye, Antagonism with residual herbicides possible, increase rates or apply separately. Gramoxone can be effective: Use high rates on tall plants. Coverage is essential, 20 GPA, more effective with atrazine or 28%.
Winter wheat: Tougher to kill than cereal rye, more issues with antagonism, weather, and rate.
Glyphosate up to 18 inches: 1.1 to 1.5 lb ae (33 to 44 oz PowerMax). Increase rate on taller wheat, possible antagonism with residual herbicides, 28% a concern, most effective when applied alone in water. Most easily controlled when plants are small. Gramoxone is not consistently effective.
Annual ryegrass: Control is faster in warm weather, cold weather slows herbicide activity.
Ryegrass should be less than 6 inches tall. Glyphosate is most effective: 1.5 lbs ae/A minimum (44 oz PowerMax). Can use a higher rate if plants are large or in cold weather. Can add Select, Assure II. Gramoxone is variable, possibly high cost: Terminate small plants at high rates. More effective with atrazine, 20 GPA is preferable, aim for medium spray droplets.
Hairy vetch, winter pea: Fairly easy to kill, large vetch especially
Glyphosate plus 2,4-D or dicamba: Glyphosate: 0.75 to 1.1 lb ae (22 to 33 oz PowerMax). Gramoxone is effective on larger hairy vetch, Add 2,4-D and/or atrazine
Clover, alfalfa: Not necessarily easy to kill
Glyphosate plus 2,4-D or dicamba: Glyphosate: 1.1 to 1.5 lb ae (24 to 44 oz PowerMax). Clopyralid is very effective on these species: Surestart, TripleFlex, Hornet, Stinger. Gramoxone is generally not a good choice. Can kill larger crimson clover with 2,4-D.