Johnsongrass in soybeans may be controlled by wiping. Light infestations of Johnsongrass can be controlled by wick application of 41% Glyphosate at a rate of one gallon of 41% Glyphosate to two gallons of water in the wick applicator. Apply by wiping twice, wiping in both directions when the Johnsongrass is at least six inches above the soybeans. The more Johnsongrass plant above the soybeans the better will be your results. Keep wiper speeds below five miles per hour.
Roundup-ready soybeans are also now available. This new technology allows the producer to spray Glyphosate directly over the Johnsongrass and the soybeans to control all grasses and broadleaf weeds in the soybeans and is very good combination to get established Johnsongrass under control in the field.
Poast grass herbicide is also labeled for use in soybeans. Poast herbicide works well on rhizome Johnsongrass only when the rhizomes are well-chopped before planting soybeans and the Johnsongrass is sprayed with one pint of Poast with two pints of crop crop oil concentrate or Dash spray adjuvant in 5-10 gallons of solution per acre when the Johnsongrass is 15-25 inches tall. A repeat application of Poast when the Johnsongrass regrows to a height of 6-12 inches will be necessary with the same rates and adjuvants. Labels must be read to avoid damage and provide optimum control. Poast does not control sedges or broadleaf weeds. The Poast label must be read to find out all restrictions before using Poast.
Best results on established rhizome Johnsongrass in Roundup-ready soybeans can be had by applying to Johnsongrass when the Johnsongrass is at or over 18 inches tall. Use the 32-ounce rate of 41% Glyphosate and the addition of ammonium sulphate or an ammonium sulphate replacement.