Glyphosate-resistant Italian ryegrass management: Now is the time!

• By Daniel Stephenson and Josh Copes •

Mississippi State University weed scientists are leading the fight against glyphosate-resistant Italian ryegrass, such as this growing in a production corn field in Washington County  — photo by Jason Bond, Mississippi State University

Glyphosate-resistant Italian ryegrass has been an issue in the mid-South for the past 10 to 15 years. In Louisiana, growers have been battling this pest for five to 10 years, and the problem is spreading each year. Louisiana does not have ryegrass issues like Mississippi, but if control strategies are not implemented, Louisiana will.

Mississippi State University developed herbicide programs for glyphosate-resistant management, and the Louisiana State University AgCenter has adopted those programs for dissemination to Louisiana producers. The Mississippi program is divided into fall, winter and spring treatments for corn, cotton, soybean and rice.

The fall program — the best strategy for glyphosate-resistant Italian ryegrass management — begins with residual herbicides that are applied mid-October to mid-November. Residual herbicide choices are S-metolachlor at 1.27 to 1.6 lb/A of active ingredient, Boundary at 2 pt/A, Zidua WG at 2.5 oz/A, trifluralin at 1.5 lb/A of active ingredient and Command at 2 pint/A.

The crop to be planted dictates which residual herbicide should be used. S-metolachlor should be used for corn, cotton and soybeans; Boundary for corn and soybeans; Zidua for corn and soybeans; trifluralin for cotton and soybeans; and Command for rice. Regardless of which residual herbicide is applied, tankmixing with paraquat at 0.5 to 0.75 lb/A of active ingredient is a must to control any emerged ryegrass at application.

If a fall cover crop will be planted, many of the residual herbicides listed above should not be applied before planting. However, research has shown that applying S-metolachlor or Zidua one to three weeks after emergence of cereal rye, clover and Austrian winter peas will cause little to no injury of the cover crop and will provide residual control of many winter weeds.

Winter programs, meaning mid-January to mid-February, are limited. Research shows that clethodim at 0.094 to 0.125 lb/A of active ingredient is the only choice; however, ryegrass must be no more than 4 to 6 inches tall at application. Remember that clethodim must be applied at least 30 days before planting corn or rice.

Also, whether clethodim is applied alone or tankmixed with another herbicide, always apply clethodim with a high-quality crop oil concentrate rather than non-ionic surfactant or methylated seed oil. In addition, spray-grade ammonium sulfate should be added.

More than likely, the clethodim application in the winter will be tankmixed with other herbicides for burndown. If clethodim is tankmixed with any auxin herbicide like 2,4-D, then 0.125 lb/A of clethodim must be used. Auxin herbicides can antagonize the activity of clethodim on Italian ryegrass, thus reducing control, but applying 0.125 lb/A is enough clethodim to overcome the antagonism.

However, one major point is often overlooked. Italian ryegrass must be no more than 4 to 6 inches tall at this application if control is to be achieved. I have observed numerous claims of clethodim “failures” on ryegrass, then determined that the rate was less than 0.125 lb/A when tank-mixed with an auxin herbicide, was applied with an airplane so coverage was inadequate and/or ryegrass was much larger than 6 inches in height.

Incorrect herbicide rate, poor coverage and large weed size are consistent reasons for herbicide failures regardless of the weed targeted or time of the year. Yes, clethodim-resistant Italian ryegrass has been documented in Mississippi. Louisiana has some highly suspicious sites that are under evaluation, but I believe resistance begins when clethodim is applied incorrectly. Please take the time to do it right.

Imagine it is March 10, corn will be planted in a few days and 12- to 36-inch, multi-tillered glyphosate-resistant Italian ryegrass is present in the field. Based on data, paraquat is the only option available in the spring program.

Unfortunately, paraquat is not a great option. In this situation, applying paraquat at 0.75 to 1 lb/A plus atrazine at 1 qt/A followed by another paraquat application at similar rate 10 to 14 days later is required. An identical plan is required where cotton and soybeans will be planted, except tankmix diuron at 1.5 pt/A of active ingredient for cotton or metribuzin 75 DF at 4 oz/A for soybeans. Understand that these paraquat applications are not going to “melt” the ryegrass or make it disappear. It will die, hopefully, but its carcass could hamper planting and/or compete with seedling crops.

Earlier this year I wrote an article titled “Italian ryegrass is everywhere! Do not forget about it this fall” where I asked farmers, consultants and dealers to not forget about the ryegrass issue. Now is the time to take the plan discussed above and implement it. If we do not, then we cannot expect a different situation next spring.

Dr. Daniel Stephenson is an LSU AgCenter Extension weed specialists. Stephenson can be reached at Dr. Josh Copes is an LSU AgCenter agronomist. He may be reached at

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