Determining Glyphosate Susceptibility in Texas Populations of Common Waterhemp

Seeds from five populations of common waterhemp surviving two label-rate applications of glyphosate were collected from Fort Bend County, Texas in 2008.  Progeny were treated with glyphosate (Roundup PowerMax™) at 11, 22 (1X), 44, and 88 oz/A in a carrier volume of 10 GPA when plants were 6 in. or less in height. The treatments were applied using a stationary greenhouse sprayer equipped with TurboTee 110015 spray tips and pressure set at 28 PSI.  Plants surviving the 2X and 4X rates were allowed to cross-pollinate within each population.  Seeds were collected at maturity and maintained as separate lineages.  Seeds from each lineage were planted in 6 in. diameter pots and thinned to one plant per pot.  Confirmed glyphosate-susceptible and glyphosate-resistant biotypes were included as positive and negative controls.  A randomized block design was used to determine the tolerance threshold of all lineages treated when plants were 6 in. or less in height with 0X, 0.25X, 0.5X, 1X, 2X, 4X, 6X, 8X, 10X and 12X rates of glyphosate.  Each treated experimental unit (1 plant) was replicated three times.  Visual control was estimated 14 days after glyphosate application using a scale of 0 (no control) to 100 (complete control).  Plant height and fresh weight (shoot and leaf) were measured.  The resulting dose response curves were used to determine I₅₀ values for each suspect lineage, susceptible, and resistant biotypes.