Glyphosate-Resistant Common Waterhemp Confirmed in Texas

Glyphosate failed to control isolated populations of common waterhemp (Amaranthus rudis) in 2006 and 2008 in Wharton County, Texas.  Therefore, biotypes of suspected glyphosate-resistant common waterhemp were collected and investigated to determine the level of glyphosate resistance exhibited under controlled conditions.  A known susceptible population was controlled at the 22 oz/A (1X) rate of glyphosate.  However, the resistant lines varied widely in the level of control by glyphosate.  Two lines, 15 and 19, were <5X more resistant than the susceptible population, while a known resistant line from Missouri (positive control) was approximately 12.5X more resistant.  Two lines, 1b and 6c, were > 25X more resistant than the known susceptible line. All resistant lines, however, were capable of producing seed set.  The resistant lines were also evaluated for control by alternative postemergence herbicides.  Under high humidity conditions, glufosinate and fomesafen at the recommended label rates provided the best control.  However, in low humidity, moderate temperature conditions, pyrithiobac at the recommended label rate provided the best control for most biotypes.  Management strategies for controlling and preventing the spread of glyphosate-tolerant common waterhemp will need to be tailored to the environmental conditions present at the time of herbicide application.