Though Palmer amaranth, commonly called Palmer pigweed, is the most pressing weed resistance problem for farmers from the Midwest to the Southeast, it is not the only weed showing resistance to glyphosate.
The first glyphosate resistant weed to create problems was horseweed, sometimes called marestail. Horseweed resistance was first found in the Carolinas in 2003, and it continues to be a problem. “Each year we find a little bit more — it is wide-spread all up and down the Coastal Plain of North Carolina,” says Alan York, long-time North Carolina State University weed specialist.
In 2006, glyphosate resistant common ragweed was reported in a handful of counties in North Carolina. “High rates of Weathermax uglied-up the terminal, but the ragweed didn’t die,” York says. He explains that to document resistance, there has to be proof that the resistant trait is heritable.
Giant ragweed is being investigated in Indiana, with distinct signs that it has developed resistance to glyphosate. Resistant giant ragweed would be a problem comparable to Palmer amaranth in some parts of the country.
Common ragweed is not likely to be a big problem for growers in the Carolinas and Virginia, according to York. The biggest problem may be in no-till or reduced-till systems which require a clean field to plant cotton. In these systems the problem is what to use for burn-down. If ragweed is resistant to glyphosate, the options are limited to dicamba, paraquat and 2,4-D.
Researchers in South Carolina have found glyphosate resistant cocklebur, and are in the process of documenting for certain that it is resistant. Clemson researchers are still conducting greenhouse tests, but the evidence is strong that at least one cotton field in South Carolina has glyphosate resistant cocklebur.
Virginia Tech researchers are at a similar place in time in documenting glyphosate resistant lambsquarters. More of a problem in the upper end of the Southeast, lambsquarters, prior to the introduction of Roundup Ready technology, was a constant problem in a number of row crops.