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.”

Unexpected connection

On an unexpected connection with resistant goosegrass across the globe…

“Interestingly, around 14 years ago, glyphosate resistant goosegrass was discovered in Malaysia. The resistance was due to a point mutation identical to what we have found in Mississippi. This doesn’t mean the Mississippi population came from Malaysia, but the same thing has happened in two locations though separated by several thousand miles.

“We don’t know how surprised we should be. It is interesting, though, that the same mutation was found. It might mean that there are simply many variations out there and we finally selected out the right one – or the wrong one, in this case.

“There could be other point mutations – some with no effect, some that are lethal to the plant, or some that might make a plant highly resistant to glyphosate. Such is the case in development of commercial varieties of Roundup Ready crops.  We found one mutation that allows the goosegrass to grow and also be glyphosate resistant after spraying with prescribed rates of glyphosate.”

On what this means for farmers…

“The resistance of this goosegrass is genetically-based. It isn’t because of something like ‘well, we had a dry weather pattern and the weed wasn’t controlled.’ Sometimes you get variable control with herbicides. That’s part of the deal. If you don’t use a herbicide under the best conditions some weeds may survive. This has nothing to do with that. These plants are resistant. A normal dose of glyphosate won’t kill them. The gene is here, is stable, and can be spread by cross pollination between populations.

“From a grower’s standpoint, if (they suspect) goosegrass resistance, a different mode of action should be included in their chemical weed control program. As far as we know, this weed remains sensitive to other common grass control products but further testing is needed to confirm this.

“Another concern is movement of goosegrass seed with irrigation water. Goosegrass seed is small and when it is shed it has a leaf-like pericarp around the seed. This enables the seed to float and can easily move in runoff from agricultural fields.”

 (Picture 4)

The good news…

“Fortunately, goosegrass is more of a nuisance weed. As it gets bigger, though, it becomes harder to control and produces much seed – next year’s weed infestation. Although it isn’t competitive like pigweeds or johnsongrass, it can still rob the soil and crops of nutrients and water.

“Moreover, goosegrass is unsightly and is one more thing that won’t be controlled with glyphosate. It’s increasingly rare that ‘glyphosate-only’ control programs will work. And goosegrass is just one more thing that can multiply and take over if left alone.

“The resistance in this goosegrass has already been found in a couple of locations in Washington County, Mississippi. We’re not sure how widespread it is. If there is suspected resistant goosegrass in Mississippi or Arkansas, we would like to know about it.

“Because this is a ‘point’ mutation, we’ve been able to develop an assay to detect it. That way we can ask ‘does a specific population contain the mutation?’ If it does, the population is resistant.”

On the federal government’s new Herbicide Resistance Group…

“We need more people involved in herbicide-resistant research because of the increasing number of weeds becoming resistant. In response to that, the USDA established the Herbicide Resistance Group, in the Crop Production Systems Research Unit at the Jamie Whitten Delta States Research Center in Stoneville.

“As part of this effort, we’re working on the important weeds we have in Mississippi and the Mid South. This is to complement the work being done by Mississippi State University.

“One of the areas we’re working on is pigweed (Palmer amaranth). Resistant populations of pigweed have appeared across the south. Research on pigweeds from Georgia showed that resistance in pigweeds occurred because the gene has been amplified. There have been many, many copies of the gene so the EPSP synthase enzyme remains functional despite glyphosate entry into the resistant plant. That’s why the plant survives.

“We’ve found the same thing is occurring here in the pigweed populations in Mississippi.  We’ve known resistance was in our pigweeds, but now we know why and are trying to understand how it happened.

“So how did resistant pigweeds get from the Georgia/South Carolina/North Carolina farm belt to the Mid-South so quickly? Weather systems? Seed? Pollen? Farm equipment? Birds? Cattle? There are many possibilities.”