MISSISSIPPI STATE -- Weed control is synonymous with glyphosate use to many row crop producers, but a resistant weed in the Delta is making producers change their management strategies.

John Byrd, weed scientist with the Mississippi State University Extension Service, said mare's tail or horseweed has become resistant to glyphosate applications in the Delta. Tennessee and Arkansas are fighting resistant strains of this weed, too, and Arkansas has just confirmed glyphosate-resistant populations of common ragweed.

"Producers who have resistant populations will have to use either an additional herbicide or some other form of weed control to remove these weeds prior to planting," Byrd said.

The use of glyphosate, a non-specific herbicide, became widespread when Roundup-Ready crops were introduced. Dan Poston, a Mississippi Agricultural and Forestry Experiment Station researcher, is working on the problem of herbicide resistance in weeds. He collaborates with Trey Koger, a U.S. Department of Agriculture, Agriculture Research Service scientist. Their work is being conducted at the Delta Research and Extension Center in Stoneville and the USDA-ARS Weed Science Laboratories.

"We've jointly documented the resistance problem and how widespread it is, primarily in the Delta," Poston said. "We're now working on control measures."

Poston said horseweed is not the first weed in the state to become resistant to a herbicide, but it is the first to gain resistance to glyphosate.

"All the interest is on glyphosate because it's so widely used and we're so heavily dependent on it," Poston said.

In typical Mississippi row crop production, glyphosate is used before planting to burn down weeds. Sometimes it is used again after harvest, and on glyphosate-resistant crops, it is sprayed once or more during the growing season to control weeds. Glyphosate leaves no residue in the soil to provide ongoing weed prevention, so new flushes of weeds grow up after each application.

Resistance is based on a genetic mutation that allows a small fraction of a plant's population to be tolerant or resistant to a particular herbicide. When one resistant plant manages to survive and produce seed, all or nearly all of the resulting seeds carry genetic resistance to that herbicide.

"With regular spraying, you keep killing the susceptible plants," Poston said. "What really got us into trouble is the constant use of one product with no other mixtures. When you add another herbicide such as 2, 4-D or dicamba, you change to a different mode of action and can kill the glyphosate-resistant horseweed."

Poston and Koger are continuing to look for ways to manage the resistant weeds and prevent others from becoming herbicide resistant.

Byrd said the practice of adding a second chemical to the mix to rid a field of glyphosate-resistant weeds is not a high expense.

"A herbicide like 2, 4-D is a very cheap material to add to glyphosate, and it will broaden the spectrum of weeds killed to include the horseweed that is resistant," Byrd said.

The main thing Byrd would like producers to do is break the cycle of resistance building in weeds. Using herbicides that act in different ways is the best way to prevent resistance from developing.