Resistance to glyphosate herbicide by Palmer amaranth (aka pigweed or careless weed) is a serious matter and has increased its territory in the Cotton Belt, but other weeds are showing resistance too.
Dr. Paul Baumann, Texas A&M AgriLife Extension Service state weed specialist, College Station told attendees at the ongoing 2013 Beltwide Cotton Conferences in San Antonio that weed specialists had identified resistance to herbicide by common water hemp as early was 2005. But it was an isolated case and limited to Southeast Texas. But in recent years, such resistant water hemp has spread to Central Texas and the Blacklands. This is causing weed specialists like him greater concern.
In fact, especially when it's small, the common water hemp will look very much like a young pigweed.
Common water hemp became much more of a worry in 2010, Baumann noted, as "its resistance really took off."
He said the researchers grew out seed from the suspect water hemp in the greenhouse and exposed it to different rates of herbicide. A large majority of the collections from every site were resistant to herbicide at every rate. That was scary.
Meanwhile, some pigweed was identified as resistant to glyphosate herbicide by Baumann's research colleagues up on the High Plains, first in Terry County and then nearby counties in 2011. With all the vast acreage of cotton on the Texas High Plains, that has caused researchers to emphasize the urgent need to stop Palmer amaranth.
"We have to control every single Palmer amaranth weed," Baumann warned. "One resistant pigweed can shed seed for about a half-million offspring."
If a cotton grower spots a suspect pigweed, Baumann says he should get out of the truck or off the tractor or spray rig, and chop it up.
Look for signs, he told growers. If an uncontrolled weed species is surrounded by controlled weeds, that most definitely would be a likely suspect for herbicide resistance. Or an isolated patch of weeds unaffected by herbicide also would be a likely indicator of resistance.
Baumann suggested that growers return to including soil-active herbicide like their fathers once used such a pre-plant incorporated or so-called yellow herbicides to slow the development of resistant weeds. Using different modes of action can help deal with herbicide resistance, he said.
For what they achieve in weed control, Baumann assured, "Yellow herbicides are still a bargain."
He told the Beltwide crowd it behooves cotton growers to use many effective tools against resistance.