Clethodim has been the primary postemergence herbicide used for control of glyphosate-resistant (GR) Italian ryegrass in Mississippi for many years. Because that problem is widespread and because few other options were available, clethodim has often been used multiple times in a single year and in the same field in successive years.
So, it was only a matter of time before repeated use of the same herbicide selected for Italian ryegrass plants resistant to clethodim.
Control failures with clethodim on GR Italian ryegrass were common across the Mississippi Delta in 2016 and 2017. During both years, Italian ryegrass plants were collected from fields where clethodim failures were observed.
These plants were subjected to traditional herbicide response screening by Dr. Vijay Nandula with the USDA-ARS in Stoneville. Results of the screening confirmed that Italian ryegrass resistant to clethodim is present in Mississippi.
Counties known to contain populations of Italian ryegrass resistant to clethodim include several big ones in the Delta — Bolivar, Coahoma, Humphreys, Leflore, Sunflower and Washington.
When GR Palmer amaranth was spreading across Mississippi, the strategy was to assume populations were resistant if resistance had been identified in your county or an adjacent county.
A similar approach should be utilized with clethodim-resistant Italian ryegrass. For example, no clethodim-resistant Italian ryegrass populations were confirmed in Tallahatchie County. However, since clethodim resistance was identified in populations from adjoining Coahoma, Leflore, and Sunflower counties, Italian ryegrass from Tallahatchie County should be treated as resistant, as well.
While multiple-resistant Italian ryegrass is a driver weed for winter burndown applications, it does not exist in a monoculture, so burndown must go on even where Italian ryegrass is present. Because paraquat is the only alternative for postemergence control of Italian ryegrass and because not every Italian ryegrass plant in a field to be treated will be resistant to clethodim, an early burndown application should include the maximum rate of glyphosate plus 16 ounces of Select Max or 8 ounces of 2-lb clethodim formulation.
Auxin herbicides like 2,4-D and/or dicamba may be added to the herbicide mixture if the maximum rate of clethodim is utilized.
Italian ryegrass that escapes the early burndown application should be treated with paraquat (3 to 4 pints of Gramoxone SL or 2 to 2.67 pints of 3-lb paraquat). Large ryegrass (12 to 24 inches) will require two applications of paraquat (4 pints of Gramoxone SL or 2.67 pints of 3-lb paraquat) spaced 10 to 14 days apart. Atrazine, metribuzin, or diuron may be added to paraquat applications in fields to be planted to corn/grain sorghum, soybean, or cotton, respectively.