By now, the announcement of a newly-confirmed population of glyphosate-resistant weeds is hardly novel for Mid-South farmers. In May, growers in Tennessee were warned about goosegrass found to be resistant.

For more, see Glyphosate-resistant goosegrass in Tennessee.

It turns out goosegrass has also been the target of USDA-ARS research in Stoneville, Miss. Delta Farm Press recently spoke with Bill Molin, ARS-USDA plant physiologist, about that goosegrass, other studies and a new research group tackling weed resistance. Among his comments:

Is the goosegrass issue something you’ve been working on for a while?

“Several years ago, we noticed goosegrass along the edge of a field and a roadside. It had been sprayed a couple of times with glyphosate, but wasn’t dying. By the end of the season, we knew it had been sprayed repeatedly at recommended rates with glyphosate yet was still growing and producing seed.

“So, that alerted us to a potential problem. We harvested seed and established test plants in the greenhouse from these and other goosegrass populations. We performed side-by-side tests using plants which were 6- to 8-inches tall. These plants were sprayed with different concentrations of glyphosate. We initially found a two-fold increase in resistance.

“We repeated the test several times, collecting seed from multiple generations. We wanted to see if the resistance was being transmitted from generation-to-generation or was just an anomaly. We established sensitive and resistant lines. After two generations, we saw a four- to five-fold difference in tolerance to glyphosate. The resistant goosegrass survived rates up to 44 ounces of Roundup Weathermax.

“There generally is some leaf injury and growth reduction following glyphosate application (picture 1). Greenhouse tests with resistant goosegrass also showed substantial initial dieback of mature leaves but plants quickly became re-established (picture 2).  In the field, sensitive and resistant plants are easily distinguished (picture 3).

“In addition to the whole-plant greenhouse spray test bioassay, a biochemical test was done which measured the increase in a specific metabolite, shikimate, in response to the glyphosate treatment. Both tests indicated resistance.

“We also did molecular analysis and found, critically, that it is a ‘point mutation’ in the goosegrass that resulted in a change in the target site allowing the plant to be resistant.”

A “point” mutation?

“The herbicide glyphosate works by blocking the function of an enzyme called 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase. This enzyme is critical to the survival of plants. The sequence of the amino acids of EPSP synthase enzyme is very specific and is controlled by its gene in the plant’s DNA. We have determined that the gene for EPSP synthase has become altered at one point. This mutation at one point in the gene resulted in a change of one amino acid in the EPSP synthase protein.

“The bottom line: the plant has experienced a mutation in the gene coding for EPSP synthase which has resulted in a slightly altered form of the enzyme that does not bind glyphosate as well. This mutation is genetically stable and is being passed on to the next generation. It is in the population. This means resistant plants are being reproduced in the field.”