This season I received plenty of calls about perceived reductions in grass control after applying ACC-ase inhibiting herbicides, such as Select (clethodim) or Poast (sethoxydim). Some folks’ first reaction to this “lack” of control is that we suddenly have widespread ACC-ase resistance.
When its comes to the issue of resistance, I will never say never. But, before traveling down that bumpy road, I would like for you and your growers to consider the following 6 factors:
#1. Is resistance actually present?
Currently, only 2 grass species have been “officially” confirmed to have evolved ACC-ase resistance in Georgia, including large crabgrass and Italian ryegrass. Scientific confirmation of herbicide resistance takes lots of time, manpower, and greenhouse space.
#2. Was the grass too tall?
Labeled heights for optimum control of various common grasses with Select, including crabgrass, Texas panicum, crowfootgrass, and goosegrass, are 2 to 6 inches.
3. Right rate?
In one study, goosegrass control was reduced by 16% to23%, depending upon rate, when applied at the 4- to 6-tiller stage of growth.
4. Is Cadre to blame?
If Cadre (imazapic) was applied prior to the grass herbicide application, it is very likely that grass control will be reduced. Research has shown that Cadre can reduce the photosynthetic rate of goosegrass which then reduces the sensitivity of the ACC-ase enzyme to clethodim.
5. Are peanut plants shielding the target?
If peanut plants are kind of tall — 12 inches or more — any grass plants peaking out of the top of the peanut canopy are not likely to be adequately controlled due to size and coverage issues.
6. Double check your approach.
Before dropping the R-bomb, please double-check use rates, stages of growth, adjuvants, rain-free periods, and field history. The threat of herbicide resistance is definitely real but it does not happen in one night.