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.”

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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.”