It was not difficult to pick up on the topic du jour at the annual California Weed Science Society (CWSS) meeting in Monterey in late January. Growers and researchers continue to be stymied by the proliferation of herbicide resistance issues and the equally fast pace at which weeds themselves are spreading.

Palmer amaranth is a prolific weed impacting growers across the United States. It’s not the only one, but it is dominant.  In many areas researchers report it has developed resistance to glyphosate and other herbicides. According to Brad Hanson, weed specialist with the University of California, Davis, discovering glyphosate-resistant Palmer amaranth in California is a matter of time.

Even the chemical companies recognize the problem. At a recent company-sponsored meeting highlighting their new chemical portfolio, representatives with one company told growers they realize resistance issues will force them to use other company products with differing modes of action. That said, they assured growers and the professional crop advisers in the audience that their line of products are an effective part of a good herbicide rotation.

Want access to the very latest in agricultural news each day? Sign up for the Western Farm Press Daily e-mail newsletter.

Hanson said the proliferation of Round Up-ready technology has led to the bulk of herbicide resistant weed (HRW) issues. Lately, it seems the cases of glyphosate-resistant weeds seem to be growing almost exponentially. Resistance could expand to other modes of action, he said.

According to Hanson, the number of HRW species confirmed in California jumped from one in 1981 – Common groundsel in Asparagus was the first species confirmed resistant to a specific mode of action known as photosystem II inhibitor – to more than 20 by 2008.