Resistance to atrazine (a photosystem II [PSII] inhibitor) is prevalent in waterhemp [Amaranthus tuberculatus (Moq.) J. D. Sauer] across the U.S. Midwest. Previous research suggests that target-site mutation or rapid metabolism of atrazine mediated by glutathione S-transferase (GST) conjugation confers resistance in A. tuberculatus from Illinois. The distribution and mechanism of resistance to atrazine in A. tuberculatus populations from Nebraska (NE) are unknown. In this research we (1) evaluated the response and frequency of resistance in NE A. tuberculatus to soil-applied PSII (metribuzin and atrazine) and protoporphyrinogen oxidase (sulfentrazone) inhibitors, as well as POST-applied atrazine; and (2) determined the mechanism of atrazine resistance in NE A. tuberculatus. The chloroplastic psbA gene, coding for a D1 protein (the target site of atrazine) was sequenced in 85 plants representing 27 populations of A. tuberculatus. Furthermore, 24 plants selected randomly from four atrazine-resistant (AR) populations were used to determine the metabolism of atrazine via GST conjugation. Results from the soil-applied herbicide evaluation suggest that metribuzin (0.56 kg ai ha−1) and sulfentrazone (0.28 kg ai ha−1) were effective on A. tuberculatus management. PRE and POST screenings against atrazine in the greenhouse indicate that atrazine (1.345 kg ai ha−1) was not effective on 39% and 73% of the A. tuberculatus populations evaluated (total of 109 and 85 populations, respectively), suggesting the prevalence of atrazine resistance in A. tuberculatus in NE. Sequence analysis of the psbA gene found no known point mutations conferring atrazine resistance. However, the AR plants conjugated atrazine via GST activity faster than the known atrazine-susceptible A. tuberculatus. Overall, the outcome of this study demonstrates the predominance of metabolism-based resistance to atrazine in A. tuberculatus from NE, which may predispose this species to evolve resistance to other herbicides. The use of integrated management strategies for A. tuberculatus is crucial for the control of this troublesome species.

Glyphosate application to the rapid-response (RR) biotype of glyphosate-resistant (GR) giant ragweed ensues in loss of foliage via rapid tissue death, thereby reducing glyphosate translocation. Experiments were performed to determine if this GR response, in contrast to a non-rapid response (NRR) GR biotype, results in antagonism of the selective herbicides atrazine, cloransulam, dicamba, lactofen, and topramezone. Application of glyphosate at 1,680 g ae ha–1 in the greenhouse resulted in antagonism between all five selective herbicides for the RR biotype, whereas glyphosate applied at 420 g ha–1 was antagonistic only for cloransulam. Application of selective herbicides 2 d prior to glyphosate treatment avoided the antagonism observed in the RR biotype. In the field, glyphosate mixtures with dicamba and topramezone were antagonistic on the RR biotype across both 2015 and 2016 field seasons. Thus, the RR effectively reduces glyphosate efficacy but also has potential to diminish the activity of glyphosate mixtures with selective herbicides, and the degree of antagonism between these mixtures escalates at increasing glyphosate rates.

Johnsongrass is one of the most troublesome weeds of the world and is listed as a noxious weed in Arkansas. Reduced johnsongrass control with the recommended application rate of glyphosate (840 g ae ha−1) was reported in a continuous soybean field near West Memphis, AR, in the fall of 2007. A greenhouse study was conducted (1) to confirm and characterize glyphosate resistance in the johnsongrass biotype from West Memphis and (2) to determine whether resistant and susceptible biotypes have differential glyphosate absorption or translocation. Dose–response studies revealed that the resistant biotype was five- to seven-fold less sensitive to glyphosate than the susceptible biotype. Glyphosate absorption was similar in resistant and susceptible biotypes at 72 h after treatment (HAT). However, the treated leaf of the resistant biotype retained 28 percentage points more absorbed 14C glyphosate compared to the susceptible biotype at 72 HAT. Additionally, the resistant biotype had less 14C glyphosate translocated to the aboveground tissue below the treated leaf and to roots compared to the susceptible biotype at 24 and 72 HAT. Reduced translocation and increased retention of glyphosate in treated leaves is a probable mechanism of resistance in this glyphosate-resistant johnsongrass biotype.

Smallflower umbrella sedge is a problematic weed in direct-seeded rice in the midsouthern United States. It recently has evolved resistance to the acetolactate synthase (ALS) –inhibiting herbicide halosulfuron in Arkansas rice. Studies were conducted (1) to determine if the resistant biotype is cross resistant to other ALS-inhibiting herbicides, (2) to evaluate alternative herbicide control options, and (3) to determine the mechanism of resistance. Whole-plant bioassay revealed that halosulfuron-resistant plants were not controlled by bispyribac–sodium, imazamox, and penoxsulam at the labeled field rate of each herbicide. The level of resistance to these herbicides, based on the lethal dose needed to kill 50% of plants (LD50) was ≥ 15-fold compared to a susceptible biotype. Both biotypes were controlled >96% with bentazon and propanil and ≤ 23% with quinclorac, thiobencarb, and 2,4-D. Hence, effective control measures exist; albeit, the number of herbicide options appear limited. Based on in vitro ALS enzyme assays, altered target site is the mechanism of resistance to halosulfuron and imazamox. Massively parallel sequencing with the use of the Illumina HiSeq detected an amino acid substitution of Pro197-to-His in the resistant biotype that is consistent with ALS-inhibiting herbicide resistance in other weed species.

Glyphosate-resistant giant ragweed in Arkansas was reported in 2005. A study was conducted to (1) confirm and characterize the glyphosate resistance in giant ragweed, (2) determine if reduced absorption or translocation is the mechanism of glyphosate resistance in giant ragweed, and (3) evaluate the efficacy of nine POST-applied soybean herbicides to control glyphosate-resistant and -susceptible giant ragweed. Based on the rate required to kill 50% of plants (LD50 values), resistant giant ragweed biotypes from Greene and Jefferson counties were 2.3- to 7.2-fold less sensitive to glyphosate compared to susceptible biotypes. Glyphosate absorption and translocation for glyphosate-resistant and -susceptible biotypes was similar at 24 and 72 h after treatment. Thus, differential absorption or translocation is not a mechanism of glyphosate resistance in this resistant giant ragweed biotype. Control of resistant giant ragweed biotypes with glyphosate at a labeled field application rate of 840 g ha−1 was only 60% or less compared to complete control of a susceptible giant ragweed biotype. However, bentazon, carfentrazone, cloransulam, and fomesafen controlled both biotypes more than 95%.