For spot treatment, apply 41% Glyphosate at a rate of one quart in 3-10 gallons of water per acre by spot treatment to established rhizome Johnsongrass when the Johnsongrass reaches a height of 18 inches or more. Any alfalfa sprayed will also be killed and replanting areas killed will be necessary after the rhizome Johnsongrass is killed.
For general treatment in alfalfa that is infested with Johnsongrass throughout the field, an application of 41% Glyphosate through a wiper may be necessary. Mix one gallon 41% Glyphosate with two gallons water to prepare a 33% solution. Apply with ropewick or sponge bar when the Johnsongrass is at least six inches above the alfalfa. Apply the solution twice by wiping the Johnsongrass in both directions while maintaining a speed of five miles per hour or less to provide adequate coverage. Very good coverage is needed to kill rhizome Johnsongrass. Seedling Johnsongrass may be controlled with only one application.
Allow 14 days or more before cutting the alfalfa. Cutting could be toxic to livestock if the Johnsongrass formed hydrocyanic acid while under stress and if the Johnsongrass is a major portion of the cutting. Very heavy infestations of Johnsongrass should be controlled by rotating the cropland to another crop such as wheat for a few years to make it easier to get the rhizome Johnsongrass roots killed out with Glyphosate before rotating back to alfalfa. Crop rotation also helps control other pests.
Poast grass herbicide is also labeled for use in alfalfa, but established rhizome Johnsongrass may not be well controlled unless the field is well-disliked to chop the rhizomes into small pieces, the alfalfa replanted, and a Poast treatment and re-treatment plan is established and maintained at labeled rates. Read the Poast label for all rates and restrictions.