Glyphosate Did Not Control Grasses

Mark VanGessel, Extension Weed Specialist;

I have had a number of questions and conversations about grasses surviving an application of glyphosate plus Liberty. I have also visited a number of fields treated with glyphosate but not Liberty and the grasses survived. I think a few things are going on here.

First, the fields I have visited had fall panicum in them ( And I think a number of things might be occurring:

  • Fall panicum seems to be quite common in areas of drowned out crops and with the season we had last year, there is probably more fall panicum seed in the soil then in previous years and fall panicum densities are higher than expected
  • Dual/Medal is not very effective on fall panicum so there is not as much control from residual herbicides as we may expect.
  • Fall panicum is one of those species that is not as sensitive to glyphosate as other grasses. The Roundup PowerMax label says for effective control at 22 fl oz the maximum height of large crabgrass is 12 inches, giant foxtail is 20 inches, while fall panicum is 6 inches
  • It seems tankmixing some herbicides with glyphosate can reduce control (although I only have a suspicion that this is occurring). It seems glyphosate plus Liberty and glyphosate plus atrazine can reduce control

So, when more than one of these situations are occurring, fall panicum control can be reduced. Consider increasing your glyphosate rates or include a postemergence grass herbicide such as Select (clethodim) or Poast (sethoxydim) to improve control or to control those plants that escaped control.