Control failures with clethodim on glyphosate-resistant (GR) Italian ryegrass were common across the entire Delta, not just Mississippi, in 2016. With some exceptions, GR Italian ryegrass was larger during the early application window (January to early February) in 2016 than in previous years due to mild weather during late fall and early winter.
Also, GR Italian ryegrass plants were stressed at the time of application from a prolonged period of wet weather. Although it is wet in most areas right now, saturated conditions have not persisted as they did during the winter of 2016. However, this could change quickly.
Early burndown applications with clethodim generally work slowly (5 to 6 weeks for optimum effect), so making the application as early as possible has several benefits. Without fail, smaller weeds are easier to kill. Currently, most of the GR Italian ryegrass I have seen in the Mississippi Delta is still at a manageable size. However, the warm temperatures could stimulate rapid growth.
Herbicide Resistance Info
In addition to GR Italian ryegrass still being relatively small, the “spring flush” has not begun. Lastly, making an early burndown application in January or early-February for GR Italian ryegrass allows time to determine how well the first application worked and flexibility in deciding how to control escapes.
An early burndown targeting Italian ryegrass is more critical in fields to be planted to corn than those that will be planted to other crops. The window for burndown herbicide application is shortened in corn because of the early planting dates. Due to the competitive nature of Italian ryegrass, it is essential to control this weed prior to planting corn.
Clethodim must be applied at least 30 days prior to planting corn. If the clethodim application for GR Italian ryegrass was not timely, then corn planting may have to be delayed to avoid crop injury.
An early burndown application with GR Italian ryegrass as a target 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 of clethodim should be treated with paraquat (3 to 4 pints of Gramoxone SL or 2 to 2.67 pints of 3-lb paraquat).