Ryegrass control: We’re losing the battle

• By Daniel Stephenson •

glyphosate-resistant ryegrass
In 2008 and 2009, samples of suspected glyphosate-resistant ryegrass were collected from a Franklin Parish, Louisiana, field — photo courtesy LSU AgCenter

The major issue I have discussed with Louisiana crop producers, Louisiana State University AgCenter parish agents, agricultural consultants and agricultural industry this year is glyphosate-resistant Italian ryegrass. It is not a new problem for Louisiana producers as we have been dealing with it for five or six years, and it is a problem in most parishes where row crops are produced.

In 2021, Italian ryegrass has caused significant issues. Even with the educational endeavors by the LSU AgCenter and others, Italian ryegrass is “stomping a mudhole in us.” This is not a new pest for the Mid-South. Mississippi has been dealing with glyphosate-resistant Italian ryegrass for many years. Mississippi State University weed scientists developed a great program to manage this pest and the LSU AgCenter has adopted that approach.

This program is divided into three options. The first option is a residual herbicide application or tillage in the fall. Second is a clethodim application in January. Third option is sequential applications of paraquat with the first application having either atrazine, diuron, or metribuzin tank-mixed. Implementation of the fall option is best, followed by the second and third options.

First and best option

The first and best option, residual herbicide or tillage in the fall, is not popular in Louisiana. The primary complaint is bed erosion due to fall and winter rainfall. Use of a residual herbicide has the potential to control most winter annual weeds, thus no weeds are present to help prevent soil erosion. This is a very valid concern.

However, Italian ryegrass emerges predominately in the fall, so by not implementing a management plan in the fall, we are effectively inviting an infestation.

Another option: fall cover crop

An option to consider is seeding a cover crop in the fall. LSU AgCenter research data has shown that a residual herbicide like S-metolachor or Zidua can be applied over the top two weeks after emergence of cereal rye, wheat and many broadleaf cover crops without injury to the cover crop.

Therefore, the cover crop will maintain bed integrity and physically compete with the Italian ryegrass slowing its growth. Plus, the addition of a residual herbicide will provide a barrier to help manage the ryegrass. Yes, this step requires money and effort that may be in short supply in the fall. However, compare that to what you have tried to do this spring to manage Italian ryegrass, and it is easy to see that implementing a fall management program will pay dividends.

Italian ryegrass is “stomping a mudhole in us.”

Honestly, the second and third options are failing Louisiana producers. Tankmixing clethodim with other burndown herbicides (which is not part of Mississippi State’s plan) has consistently provided poor Italian ryegrass control because many burndown applications have had sublethal clethodim rates. Tankmixing clethodim with 2,4-D or dicamba in a burndown treatment oftentimes leads to poor ryegrass control because 2,4-D and dicamba antagonize clethodim, which decreases clethodim efficacy.

To overcome antagonism, at least 0.125 lb ai/A of clethodim must be applied. Another issue is the ryegrass is too big at clethodim application. For maximum clethodim efficacy, ryegrass should be less than 4-inches tall. Antagonism, low use rate, and large ryegrass at application has led to potential clethodim-resistant Italian ryegrass.

The LSU AgCenter is investigating multiple population for resistance. Clethodim-resistant has been documented in Mississippi.

The third option

The lack of a fall program and the poor control provided by clethodim leads to the third option….paraquat plus atrazine/diuron/metribuzin followed by another paraquat application 10 to 14 days later. Many times, the first paraquat treatment may get applied, but the second application does not. Why?

Because the corn has already emerged before the second application can be applied. This leads to a terrible situation in which no herbicide options are available. Dr. Jason Bond and others with Mississippi State, the team that developed the management program, applied almost all known herbicides to Italian ryegrass in the spring with little to no success. The lack of in-crop herbicides to manage ryegrass intensifies the need to implement a management program the fall.

I have no intention to be a fear monger. I have always tried to tell the truth. The truth is that we are losing the battle with glyphosate-resistant Italian ryegrass in Louisiana. Unless we implement a fall management program, Louisiana will continue to struggle with this pest.

Please feel free to contact your local parish agent or me. My email is dstephenson@agcenter.lsu.edu and my mobile number is 318-308-7225. Good luck.

Dr. Daniel Stephenson is an LSU AgCenter Extension weed scientist. He may be reached at dstephenson@agcenter.lsu.edu.

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