Georgia Weeds: 6 Possible Reasons for Failed Grass Control

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Getting plenty of calls about perceived reductions in grass control after applications of ACC-ase inhibiting herbicides, such as Select (clethodim) or Poast (sethoxydim), have been applied.  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:

1. 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. Labeled heights for optimum control of various common grasses with Select, including crabgrass, Texas panicum crowfootgrass, and goosegrass, are 2″- 6″.  

3. The following is some data illustrating the effect of Select rate and timing on the control of goosegrass.  Please note that goosegrass control was reduced by 16-23%, depending upon rate, when applied at the 4-6 tiller stage of growth.

4. 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. At this time of the year, peanut plants are kinda tall (> 12″). Thus, 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. 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.

Figure 1. Goosegrass control with Select applied at different rates and timings. Click Image to Enlarge

Source URL: http://ugaweedscience.blogspot.com/2018/08/reductions-in-grass-control-in-2018.html