Tennessee: Glyphosate-Resistant Grass Species Management

Ryegrass in emerged corn. Photo: University of Tennessee

Ryegrass in emerged corn. Photo: University of Tennessee

A few years ago clethodim was rarely used in Tennessee but in the last couple of years that has really changed. Clethodim has become the tankmix of choice with glyphosate to control glyphosate-resistant (GR) ryegrass, johnsongrass, goosegrass and in some areas, barnyardgrass.

A common question is when Prefix is tankmixed with glyphosate and clethodim will the grass control suffer? From what we have seen so far, the answer is yes if the three way tankmix is used with a MSO surfactant. 

On the other hand, if NIS is the surfactant of choice, then grass control will be better. Grass control can also be improved if the clethodim rate is ran up to at least 12 ozs/A of a 1lb clethodim product.

There is also more Liberty Link soybeans being planted this spring and all the cotton will likely have Liberty applied POST so what about clethodim in those systems? Liberty is a very effective broadleaf herbicide and is also sold a little short on how effective it can be on grass weeds.

However, it is weak on some grass weed species, most notably goosegrass and crabgrass. But it can control these species if they are less than 3” tall or even with taller grass, with sequential Liberty applications applied about 7 days apart.

It is obviously more economical and convenient to control grass with the first Liberty application. So can clethodim be tankmixed with Liberty and still control goosegrass or crabgrass that is larger than 4” tall.

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Dan Reynolds at Mississippi State did some research several years ago on Liberty and clethodim tankmixes as well as sequential applications of those herbicides. His results clearly showed that some grass antagonism can be expected with a Liberty and clethodim tankmix.

However, this mostly can be overcome if the clethodim rate is bumped up to at least 12 ozs/A. Another method is to apply them sequentially with the clethodim application made about 7 days after the Liberty.

Finally, clethodim should not be the only plan for management of the GR grass species. The group 15 herbicides, Dual Magnum, Zidua, Outlook or Warrant should also be somewhere in the mix applied both PRE and POST. 

If not, these GR grass species in a short time will likely also be resistant to clethodim.