Glyphosate Resistant Palmer Amaranth In Louisiana

Dose response experiments were conducted in 2010 at the Louisiana State University at Alexandria, LA greenhouse to determine the absence/presence of glyphosate-resistance and level of glyphosate resistance in a Palmer amaranth population collected from a cotton field in Louisiana.  The Palmer amaranth population was collected from a cotton field that had been previously farmed in cotton for three years.  Glyphosate was the primary herbicide applied by producer in all years of cotton production.  In the cotton field, the Palmer amaranth population survived an application of 0.75 lb ae/A of glyphosate followed by a 1.5 lb ae/A application of glyphosate.  Plants were collected, transferred to the greenhouse and allowed to produce seed.  Seed was collected and an absence/presence experiment was conducted using a 0.78 lb ae/A glyphosate rate plus 1% v/v non-ionic surfactant.  Data indicated that the Palmer amaranth population survived the application indicating the possible presence of glyphosate resistance.  Following the identification of possible glyphosate resistance, seed from the original Palmer amaranth population was screened against eight glyphosate rates ranging from 0.2 to 25 lb ae/A to identify the level of resistance.  A Palmer amaranth population identified as not resistant to glyphosate was used as a susceptible population and screened against eight glyphosate rates ranging from 0.02 to 1.6 lb ae/A.  Each dose response experiment utilized four replications and was conducted twice in the greenhouse.  After 21 days, plants were rated as alive or dead, and analyzed with PROC PROBIT in SAS.  Analysis indicated that the resistant population was 54 times less sensitive to glyphosate as compared to the susceptible Palmer amaranth population.  Based upon this data, producers in Louisiana are advised to implement resistance management strategies to mitigate glyphosate resistance.