We recently rated our Italian ryegrass screen in the greenhouse for glyphosate and clethodim resistance. The first two runs were conducted on 13 populations across 11 counties in West Tennessee.

A quart of Roundup PowerMax completely controlled only 2 of the 13 ryegrass populations. Increasing the rate to a half gallon of Roundup PowerMax provided complete control of 7 of the 13 populations. The lack of control with the high rate of glyphosate points to why most growers have moved to clethodim to manage ryegrass.

The results from the clethodim screen were better than those of glyphosate, but still concerning. A pint of a 2-lb clethodim completely controlled 9 of the 13 populations. Of particular note, 2 populations from Gibson County not only survived a pint of clethodim but a quart as well.

These results are consistent with those of growers who reported poor ryegrass control with glyphosate across the state, as well as, less frequently , sketchy ryegrass control with clethodim. Our greenhouse screen results, using both herbicides, confirm those field observations.

Keep in mind the greenhouse screening was conducted on ryegrass that was about 4” tall in an ideal environment for control.  One could expect less control in the field, particularly next spring, when ryegrass is more well-established. These data point to a change in management tactics aimed at controlling this weed.  Fall-applied residual herbicides that limit emergence would be one recommended approach that, in our research, has provided better results than relying entirely on POST applications.