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Herbicide Loses Effectiveness When Weed is Water-stressed

LAS CRUCES - Controlling Russian knapweed becomes more difficult during times of drought when the weed is water-stressed, said a New Mexico State University weed scientist.


Russian knapweed is a problem in pastures and is toxic particularly to horses-, said Tracy Sterling, with NMSU's Agricultural Experiment Station. "Once established, the weed forms dense stands that out compete and ultimately exclude more desirable vegetation," she added.

In a recent study, only 11 percent of the herbicide picloram applied to Russian knapweed leaves was absorbed. The amount that translocated or moved throughout the plant was even less when the weed was water-stressed, reducing the herbicide's effectiveness, she said.

"When weeds are water-stressed, they have less than optimal amounts of water, so growth is decreasing and leaves are wilting," she said. "The weeds then shut down and their vascular systems don't move carbohydrates and other nutrients throughout the plant, and the herbicide doesn't move along either."

From this research and another project with snakeweed, Sterling said it seems like weeds are more sensitive to or respond better to herbicides when they've had ample amounts of water. "In New Mexico, this means weeds may be better controlled when the soil is moist," she said.

In the study, Sterling traced picloram's movement throughout the plants under varying concentrations and water conditions.

Because only 11 percent of the applied herbicide got absorbed, Sterling said trying to increase picloram uptake is a good approach to increasing herbicide-use efficiency. This means if more herbicide gets into the plant, ranchers may be able to apply less. She tried adding a surfactant, a chemical that encourages the leaf surface to absorb the herbicide. While the surfactant did increase herbicide absorption, it did not affect movement throughout the weed, which is necessary for the herbicide to be effective.

In a second study, Sterling treated two locoweed species, woolly loco and silky crazyweed (or white-point loco), with picloram and another herbicide called metsulfuron methyl. She found that silky crazyweed was 10 times more sensitive to increasing amounts of both herbicides. She thought the difference may be due to the two species, very different leaf surface characteristics, but her research showed that wasn't the case. She's planning further research in this area.

Results from the two studies were recently published in the journal Weed Science.