Clethodim gains popularity as glyphosate-resistant weeds increase

• By Larry Steckel •

glyphosate-resistant goosegrass
This resistant goosegrass survived two 22-ounces-per-acre applications of Roundup PowerMax — photo courtesy University of Tennessee

Clethodim is quickly replacing glyphosate as the “go-to” herbicide for grass control in West Tennessee. It began a few years ago as folks found that ryegrass and poa in many fields could no longer be controlled with glyphosate.

In more recent years, the pace has accelerated as the spread of glyphosate-resistant (GR) summer grasses like goosegrass, jungle rice and Johnsongrass have made it necessary to add clethodim to glyphosate to “prop up” the grass control.

As with glyphosate, there are many different brands of clethodim available. I counted over 30 in a brief search. Unlike glyphosate, the pounds of active herbicide in a typical clethodim product jug can vary by as much as 66 percent.

Another difference is that some of these clethodim formulations come “fully loaded” with surfactant while others have no surfactant included at all. This difference between clethodim herbicides can and does cause confusion.

It can also make it difficult for the average applicator to price shop as often the price quoted is not comparing the same percent active ingredient or surfactant package.

Valent’s Select 2EC was the first clethodim on the market, and most clethodim herbicide products today follow its lead and contain 2 pounds active clethodim per gallon. The standard rate of Select 2EC used for most annual grasses back in the day was 6 ounces per acre or 0.09 pounds active per acre.

The Select 2EC had no surfactant added in the jug and as such required 1 gallon of COC/100 gallons or (1% v/v) of water be added. In more recent years, Valent has moved to marketing Select Max which is a 1 pound per gallon active clethodim product with a robust surfactant included in the jug.

Many other clethodim herbicide products sold today range from 1 pound active clethodim per gallon to 3 pounds active per gallon. The most common clethodim products used in Tennessee would the 2 and 3 pounds active per gallonproducts. These typically do not have a surfactant included in the jug.

That said, it is easy to see why when you ask your consultant what rate of clethodim to use to control large GR ryegrass or GR poa for burndown, the answer can be complex. Of the two weeds, ryegrass is the most difficult to control.

The rate to control ryegrass would at least need to be 0.125 pound per acre. That would be equivalent to 8 ounces per acre of a 2 pounds-per-gallon formulation and 5.3 ounces per acre of a 3 pounds-per-gallon formulation. Most would round that 5.3 ounces up to 6 ounces.

One would also need to check the label of the clethodim product on what and how much surfactant to add. If the ryegrass is more than 12 inches tall and/or dicamba is in the mix, then consider using higher clethodim rates to obtain more consistent control. The maximum rate of clethodim that can be used is 0.25 pound per acre.

In the last year, there have been numerous reports of some glyphosate-resistant grass species, most notably jungle rice, that have escaped clethodim. We are investigating these reports, so please contact Extension if you have witnessed these type escapes.

Dr. Larry Steckel is a University of Tennessee Extension weed specialist. He may be reached at lsteckel@tennessee.edu.

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