Louisiana: Glyphosate-Resistant Italian Ryegrass Management Considerations

Italian ryegrass. Image from Mississippi State University

Italian ryegrass. Image from Mississippi State University

Glyphosate-resistant Italian ryegrass has become a significant issue for Louisiana farmers, especially in the northeastern part of the state.  I have been asked when ryegrass would begin to emerge.  That answer can be tricky, but a good rule of thumb is this:  when air temperature is less than 90 degrees for 7-10 consecutive days and soil moisture is not limited, then we should expect emergence.

I have observed that Italian ryegrass emergence pattern mimics henbit.  It can emerge in late September through March, but, historically, the majority of emergence occurs October through mid-December.  Therefore, based on the current weather conditions and the time of year, consider it ‘game-time’ for ryegrass emergence.

The front line of defense for glyphosate-resistant Italian ryegrass management is with a fall application of a residual herbicide.  Herbicides that contain S-metolachlor/metolachlor or Zidua are the better ones for residual control of ryegrass.  Also, both will provide some residual control of henbit. 

If the grower wishes to broaden the targeted weed spectrum, metribuzin plus S-metolachlor/metolachlor (aka Syngenta’s Boundary and other generic formulations) is a good option.  Whatever residual herbicide is chosen, if ryegrass has emerged at application, paraquat at 0.75 lb ai/A (Gramoxone at 3 pints/A) must be tank-mixed with the residual herbicide.

If the grower wishes to wait until spring burndown and will tank-mix clethodim with glyphosate and other herbicides, some issues arise. First off, tank-mixing clethodim with 2,4-D can reduce ryegrass control due to antagonism; therefore, the clethodim rate should be no less than 0.125 lb ai/A if mixed with 2,4-D or any growth regulator herbicide (Group 4). 

Also, do not tank-mix clethodim with any ALS-inhibiting herbicide (Group 2) such as LeadOff due to antagonism.  If the ryegrass is big, meaning multiple tillers, sequential applications of paraquat at 1 lb ai/A spaced 10 days apart is the plan.  Depending upon the crop to be planted, atrazine at 0.5 lb ai/A, metribuzin at 0.19 lb ai/A, or diuron at 0.5 lb ai/A should be tank-mixed with the first paraquat application.

Three more things that are quite important.

First off, clethodim-resistance is suspected in Louisiana. Overreliance, sublethal clethodim rates, and/or poor application procedures are possible reasons.  We must advise growers that glyphosate- and clethodim-resistant Italian ryegrass is a beast and, if present, fall-applied residuals and paraquat will be the only chemical treatments we have left. 

Second, regardless of what clethodim is tank-mixed with, it is important that a high-quality crop oil concentrate should be added or they may see decreased efficacy. 

Finally, if the grower has any concern that the water used for mixing is low quality (hard water, high pH, precipitants, etc.), then ammonium sulfate should be added.