Clethodim and Managing Glyphosate-Resistant Grasses

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%.  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 lbs active clethodim/gallon.  The standard rate of Select 2EC used for most annual grasses back in the day was 6 ozs/A or 0.09 lbs active/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 lb/gal active clethodim product with a robust surfactant included in the jug.

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

All that being stated, 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 lbs/A.  That would be equivalent to 8 ozs/A of a 2 lb/gal formulation and 5.3 ozs/A of a 3 lb/gal formulation. Most would round that 5.3 ozs up to 6 ozs. 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” 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 lbs/A.

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.

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3 thoughts on “Clethodim and Managing Glyphosate-Resistant Grasses

  1. Hi Larry,
    Bit concerned there is no mention of the high likelihood of developing resistance to ACCASE herbicides with this practice.
    While use of ACCASE herbicide to control glyphosate resistant grasses occurs to some extent in Australia, we are careful to recommend a second control tactic or ‘Knock’ follows the herbicide application to control any survivors of the ACCASE herbicide.
    Any reason for using clethodim and not a ‘fop’ herbicide first??

    Andrew Storrie
    Australian Glyphosate Sustainability Working Group

    1. Terry Thanks

      I have had good luck with Capreno and atrazine managing Palmer amaranth in my tests. However, I agree with you that there is a resistance managment advantage with Armezon Pro or Acuron with two good residual modes of action on Palmer. So from a resistance managment perspective the Armezon Pro or Acuron are the better choices.


    2. Thanks Andrew for the question,

      You make a good point. The reason I mention clethodim is that is what the retailers carry. Most do not have any fop chemistry on hand and so my recommendation is something that can realistically be done this spring. I have visited with the basics and retailers about moving some fop chemistry in for 2020 and hopefully they will.

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