Glyphosate-resistant Amaranthus species are a recognized risk to U.S. agriculture. With affected cropland exceeding 1.2 million ha, this epidemic is particularly pertinent to agricultural regions that utilize an intensive glyphosate-based management program to control weedy pests. Before 2006, Texas had no identified glyphosate-resistant populations. Two independent common waterhemp populations exhibiting poor control by glyphosate were identified in Wharton County and Fort Bend County, TX in 2006 and 2008, respectively. The objective of the present research was to characterize the level of glyphosate resistance (50% lethal dose [LD50] and 50% reduction in growth rate [GR50]) in each population. Resistance levels in four putatively glyphosate-resistant common waterhemp biotypes selected from these two populations were compared with confirmed glyphosate-resistant and -susceptible common waterhemp populations under greenhouse conditions. The LD50 value for the susceptible population (736 g ae ha−1) was equivalent to the 0.9× labeled rate of glyphosate, whereas the putatively resistant lines exhibited a broad range of resistance with LD50 values ranging from 3.5 to 59.7× the labeled rate of glyphosate. The GR50 value for the most resistant line was 2.5-fold greater than the susceptible biotype (317 g ae ha−1 of glyphosate). These results confirm the first documented case of a glyphosate-resistant weed species in Texas.
Nomenclature: Glyphosate; common waterhemp, Amaranthus rudis Sauer AMATA.