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 many 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.

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.

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.

SOURCE: Ag Professional's "Dealer Update" newsletter.