Fifteen states now have acres claimed by glyphosate resistant weeds. Acres in Pennsylvania and Missouri have now confirmed glyphosate resistant marestail populations, and glyphosate resistant common ragweed has been confirmed in Arkansas.
William Curran, professor of weed science at Penn State University, documented resistance on as much as 500 Pennsylvania acres and Andy Kendig, associate professor for University of Missouri Extension, acknowledged more than 10,000 acres in the Missouri Delta. Chad Brewer, graduate student, and Bob Scott, extension weed specialist with the University of Arkansas, recorded as many as 50 acres with resistant common ragweed.
In Missouri, the first resistant population was confirmed in 2002. According to Kendig, in 2003 they were seeing a response similar to neighboring Tennessee populations in no-till fields. Everything in the field was controlled except horseweed. By 2004 and 2005, Kendig's calls on weed escapes multiplied dramatically.
"Southeast Missouri is similar to the flatlands of Arkansas and Mississippi in terms of tillage practices," Kendig says. "The horseweed has infested the majority of our cotton fields; it is essentially everywhere. Pigweed is also a big concern here. Residual herbicides are going to be key to controlling these weeds."
For Pennsylvania, the first resistant marestail population was confirmed in Chester County in 2003, followed by two farms in Lehigh County in 2004. In 2005, more sites are suspected to be confirmed. "Glyphosate resistant marestail first appeared in fields planted with continual glyphosate-tolerant soybeans for more than five years. We are seeing scattered pockets of glyphosate resistance throughout Pennsylvania," explained Curran. "Diverse cropping and tillage systems have prevented resistance from becoming as widespread here as other parts of the U.S."
Curran agrees with other experts that rotating modes of action is the best way to prevent resistance from developing. "We're recommending including 2,4-D, rotating herbicide sites of action and using effective tankmixes to control resistant marestail populations," Curran says.
"We are also watching other major weeds in Pennsylvania, such as lambsquarters, pigweed, eastern black nightshade and yellow nutsedge for emerging glyphosate control problems," Curran adds. "Lambsquarters, in particular, is our biggest concern because it is the number one weed in most crops and has historically driven our weed control programs."
Arkansas's first confirmed resistant common ragweed population was recorded in 2004, making it the second weed in the state with glyphosate resistance. The problem field is currently an isolated incident, originating in continuous glyphosate-tolerant soybeans. According to Scott, giant ragweed, Palmer pigweed and lambsquarters are also weeds to watch for glyphosate resistance in Arkansas.