Palmer amaranth is the most economically damaging glyphosate-resistant (GR) weed in the southern United States. An understanding of the basic biology, including relative growth and competitiveness of GR and glyphosate-susceptible (GS) Palmer amaranth phenotypes from a segregating population collected from the same geographical location, may yield information helpful in the management of resistant populations. A segregating population of Palmer amaranth collected in North Carolina during 2010 was used as a plant source for both GR and GS traits. Research was conducted in the greenhouse to compare the following: level of resistance and shikimate accumulation in GR and GS phenotypes following glyphosate application; interference from GR and GS phenotypes on early-season vegetative growth of corn, cotton, and peanut; effect of various durations of imposed drought stress on GR and GS phenotypes; and response of GR and GS phenotypes to POST-applied herbicides. The GR50 (glyphosate rate providing 50% reduction in shoot dry biomass) was 17 times greater with the GR phenotype compared with the GS phenotype. Shikimate accumulated in both GR and GS phenotypes following glyphosate application, but greater concentrations were found in GS plants. The GR and GS phenotypes responded similarly when subjected to drought stress; grown with corn, cotton, and peanut; or treated with 2,4-D, atrazine, dicamba, fomesafen, glufosinate, paraquat, tembotrione, and thifensulfuron. These results indicate that in the absence of glyphosate selection pressure, resistance to glyphosate does not influence the growth and competitiveness of GR and GS Palmer amaranth phenotypes collected from the same geographical location.

Horseweed is an increasing problem in perennial crops and noncrop areas of the Central Valley of California. Similar to the situation in glyphosate-tolerant crops in other regions, glyphosate-based weed-management strategies in perennial crops and noncrop areas have resulted in selection of a glyphosate-resistant horseweed biotype in California. Research was conducted to determine the level of resistance to glyphosate in horseweed using an in vivo enzyme assay and to determine the distribution of the resistant horseweed biotype in central California. The resistant biotype was 4.8-fold more resistant to in vivo glyphosate exposure compared with the susceptible biotype, although enzyme function was inhibited in both biotypes at high glyphosate concentrations. An intermediate in vivo glyphosate dose was used to discriminate between glyphosate-resistant and glyphosate-susceptible individuals in a roadside survey conducted in 2006 to 2007. Overall, 62% of the individuals tested from the Central Valley were classified as resistant to glyphosate. Resistant individuals were found at most locations throughout the Central Valley, although the proportion of resistant individuals was slightly lower in the northern-most area. No correlation could be made between proportion of resistant or susceptible individuals and land use patterns likely because of long-distance seed dispersal or different selection pressure for resistant biotypes on field margins compared with that within fields. Horseweed with an economically significant level of resistance to glyphosate is already widely distributed in the Central Valley of California. Grower awareness of the problem and adoption of best management practices are needed to minimize the effects of horseweed in this highly productive and diverse agricultural region.