As cases of resistance to herbicides escalate worldwide, there is increasing demand from growers to test for weed resistance and learn how to manage it. Scientists have developed resistance-testing protocols for numerous herbicides and weed species. Growers need immediate answers and scientists are faced with the daunting task of testing an increasingly large number of samples across a variety of species and herbicides. Quick tests have been, and continue to be, developed to address this need, although classical tests are still the norm. Newer methods involve molecular techniques. Whereas the classical whole-plant assay tests for resistance regardless of the mechanism, many quick tests are limited by specificity to an herbicide, mode of action, or mechanism of resistance. Advancing knowledge in weed biology and genomics allows for refinements in sampling and testing protocols. Thus, approaches in resistance testing continue to diversify, which can confound the less experienced. We aim to help weed science practitioners resolve questions pertaining to the testing of herbicide resistance, starting with field surveys and sampling methods, herbicide screening methods, data analysis, and, finally, interpretation. More specifically, this article discusses approaches for sampling plants for resistance confirmation assays, provides brief overviews on the biological and statistical basis for designing and analyzing dose–response tests, and discusses alternative procedures for rapid resistance confirmation, including molecular-based assays. Resistance confirmation procedures often need to be slightly modified to suit a specific situation; thus, the general requirements as well as pros and cons of quick assays and DNA-based assays are contrasted. Ultimately, weed resistance testing research, as well as resistance management decisions arising from research, needs to be practical, feasible, and grounded in science-based methods.

Safeners are chemical agents that reduce the phytotoxicity of herbicides to crop plants by a physiological or molecular mechanism, without compromising weed control efficacy. Commercialized safeners are used for the protection of large-seeded grass crops, such as corn, grain sorghum, and wet-sown rice, against preplant-incorporated or preemergence-applied herbicides of the thiocarbamate and chloroacetanilide families. Safeners also have been developed to protect winter cereal crops such as wheat against postemergence applications of aryloxyphenoxypropionate and sulfonylurea herbicides. The use of safeners for the protection of corn and rice against sulfonylurea, imidazolinone, cyclohexanedione, isoxazole, and triketone herbicides also is well established. A safener-induced enhancement of herbicide detoxification in safened plants is widely accepted as the major mechanism involved in safener action. Safeners induce cofactors such as glutathione and herbicide-detoxifying enzymes such as glutathione S-transferases, cytochrome P450 monooxygenases, and glucosyl transferases. In addition, safeners enhance the vacuolar transport of glutathione or glucose conjugates of selected herbicides. The safener-mediated induction of herbicide-detoxifying enzymes appears to be part of a general stress response.

BACKGROUND: The widespread occurrence of ALS inhibitor‐ and glyphosate‐resistant Amaranthus palmeri has led to increasing use of protoporphyrinogen oxidase (PPO)‐inhibiting herbicides in cotton and soybean. Studies were conducted to confirm resistance to fomesafen (a PPO inhibitor), determine the resistance frequency, examine the resistance profile to other foliar‐applied herbicides and investigate the resistance mechanism of resistant plants in a population collected in 2011 (AR11‐LAW B) and its progenies from two cycles of fomesafen selection (C1 and C2).RESULTS: The frequency of fomesafen‐resistant plants increased from 5% in the original AR11‐LAW‐B to 17% in the C2 population. The amounts of fomesafen that caused 50% growth reduction were 6‐, 13‐ and 21‐fold greater in AR11‐LAW‐B, C1 and C2 populations, respectively, than in the sensitive ecotype. The AR11‐LAW‐B population was sensitive to atrazine, dicamba, glufosinate, glyphosate and mesotrione but resistant to ALS‐inhibiting herbicides pyrithiobac and trifloxysulfuron. Fomesafen survivors from C1 and C2 populations tested positive for the PPO glycine 210 deletion previously reported in waterhemp (Amaranthus tuberculatus).CONCLUSION: These studies confirmed that Palmer amaranth in Arkansas has evolved resistance to foliar‐applied PPO‐inhibiting herbicide. © 2016 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Certified Crop Advisors of Arkansas and members of the Arkansas Crop Consultants Association were surveyed in fall 2006 through direct mail to assess the current situation of the red rice problem and early impact of imidazolinone-resistant (IMR) rice technology on red rice infestation. The information generated represented 40% (226,800 ha) of rice production areas in Arkansas. Barnyardgrass and red rice were the most problematic weeds, with 62% of fields infested with red rice. The estimated economic loss due to red rice averaged $274/ha. Red rice infestation was prevented mostly by crop rotation (96%) and use of certified seed (86%). Of the red rice–infested fields, 38% had light infestation and 26% had severe red rice problems before adopting IMR rice. Thirty-seven percent of infested fields had been planted with IMR rice once and 43% at least twice. Approximately 85% of the consultants reported > 90% red rice control when using IMR rice. The majority (92%) of IMR rice growers rotate to other crops, mostly soybean. Unsuitable field condition was the main reason for growing only rice. After 3 seasons, the consultants perceived that red rice infestation level declined by 77% on average. The herbicide-resistance gene had escaped to red rice in some fields, and 90% of growers are exerting effort to mitigate outcrossing.Nomenclature: Barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG, red rice, Oryza sativa L. ORYSA, rice, Oryza sativa L, soybean, Glycine max (L.) Merr