Research was conducted to determine whether resistance to glyphosate among Palmer amaranth (Amaranthus palmeri S. Watson) populations within the U.S. state of Arkansas was due solely to increased EPSPS gene copy number and whether gene copy number is correlated with resistance level to glyphosate. One hundred and fifteen A. palmeri accessions were treated with 840 g ae ha−1 glyphosate. Twenty of these accessions, selected to represent a broad range of responses to glyphosate, underwent further testing. Seven of the accessions were controlled with this dose; the rest were resistant. The effective dose to cause 50% injury (ED50) for susceptible accessions ranged from 28 to 207 g ha−1. The glyphosate-resistant (GR) accessions had ED50 values ranging from 494 to 1,355 g ha−1, a 3- to 48-fold resistance level compared with the susceptible standard (SS). The 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene relative copy number was determined for 20 accessions, 4 plants accession−1. Resistant plants from five GR accessions (38% of resistant plants tested) did not have increased EPSPS gene copies. Resistant plants from the remaining eight GR accessions (62% of resistant plants tested) had 19 to 224 more EPSPS gene copies than the SS. Among the accessions tested, injury declined 4% with every additional EPSPS copy. ED50 values were directly correlated with EPSPS copy number. The highly resistant accession MIS11-B had an ED50 of 1,355 g ha−1 and 150 gene copies. Partial sequences of EPSPS from GR accessions without EPSPS amplification did not contain any of the known resistance-conferring mutations. Nearly 40% of GR accessions putatively harbor non–target site resistance mechanisms. Therefore, elevated EPSPS gene copy number is associated with glyphosate resistance among A. palmeri from Arkansas.

In 2007, populations of Italian ryegrass were observed surviving applications of glyphosate under field conditions in southeast Arkansas. At least 10 reports of Italian ryegrass escaping glyphosate applications followed in subsequent years in Arkansas. These were unconfirmed reports of resistance from county agents, consultants, and farmers. The objectives of this research were to confirm resistance to glyphosate in a suspected resistant population collected in 2007 (Desha 2007) and to determine the level of resistance of a putative glyphosate-resistant population collected in 2009, both from Desha County, AR. Other objectives were to determine the resistance frequency in these populations, to determine whether the 2009 population was also acetolactate synthase (ALS) or acetyl-CoA carboxylase (ACCase-resistant), and to determine the effect on plant size as it relates to dose–response to glyphosate. The Desha, AR, 2007 population exhibited a low level of resistance to glyphosate. The estimated glyphosate dose that would control this population 50% was 1,260 g ae ha−1, compared with 190 g ae ha−1 for the susceptible check. In 2009, a population of Italian ryegrass (Des03) was identified that survived a glyphosate application of 1,740 g ae ha−1 made in the field, which is twice the commercial use rate for glyphosate. Dose–response experiments determined that an estimated 3,890 g ae ha−1 glyphosate was required to obtain 50% biomass reduction of Des03; this was 23 times that of the susceptible standard. Neither growth stage nor glyphosate rate evaluated affected the level of resistance observed in the Des03 population. This population was determined to be more than 70% resistant at the levels reported. In addition to glyphosate, Des03 was also resistant to diclofop, a commonly used herbicide in wheat in Arkansas and other areas. As a result, alternative management strategies for Italian ryegrass are currently being explored.