Resistance in the Pollen
This photo shows Palmer pigweed in a Southern cotton field.
In an April 28, 2009 Southeast Farm Press article, Larry Steckel, University of Tennessee, and Stanley Culpepper, University of Georgia, demonstrated that the majority of movement of Palmer pigweed is through pollen. Steckel discussed these findings, "It's the reason it really snuck up on us in 2008. It was building up in the background, then all of a sudden we had glyphosate-resistant males out there shedding a lot of pollen."
While Palmer has been a significant problem for Georgia producers, it really exploded in Tennessee just this past season. Culpepper planted one male, glyphosate-resistant Palmer pigweed plant in a research field and planted glyphosate resistant females from 1 meter away to 300 meters away. After evaluating plants grown from the resulting seed, Culpepper found that 60 percent of the seed from the glyphosate-susceptible females within a meter of the glyphosate-resistant male proved resistant. At 300 meters, the susceptible females produced glyphosate-resistant plants from 20 percent of their seed.
Steckel says many growers who apply glyphosate alone this season "are going to be blindsided, particularly if they missed a few pigweed last year. We're going to need to use pre-emergence herbicides and overlap them if we have a really heavy infestation of Palmer pigweed." As Syngenta recommends, Steckel also suggests using a pre-emergence residual herbicide and to treat weeds when they are small, about the size of a pop can, to help reduce yield loss. "The days of catching a 2-foot-tall Palmer pigweed with glyphosate and melting it to the ground are over," he said.