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Background: Early detection of herbicide resistance in weeds is crucial for successful implementation of integrated weed management. We conducted a herbicide resistance survey of the winter annual grasses feral rye (Secale cereale), downy brome (Bromus tectorum), and jointed goatgrass (Aegilops cylindrica) from Colorado winter wheat production areas for resistance to imazamox and quizalofop. Results: All samples were susceptible to quizalofop. All downy brome and jointed goatgrass samples were susceptible to imazamox. Out of 314 field collected samples, we identified three feral rye populations (named A, B, and C) that were imazamox resistant. Populations B and C had a target‐site mechanism with mutations in the Ser653 residue of the acetolactate synthase (ALS) gene to Asn in B and to Thr in C. Both populations B and C had greatly reduced ALS in vitro enzyme inhibition by imazamox. ALS feral rye protein modeling showed that steric interactions induced by the amino acid substitutions at Ser653 impaired imazamox binding. Individuals from population A had no mutations in the ALS gene. The ALS enzyme from population A was equally sensitive to imazamox as to known susceptible feral rye populations. Imazamox was degraded two times faster in population A compared with a susceptible control. An oxidized imazamox metabolite formed faster in population A and this detoxification reaction was inhibited by malathion. Conclusion: Population A has a nontarget‐site mechanism of enhanced imazamox metabolism that may be conferred by cytochrome P450 enzymes. This is the first report of both target‐site and metabolism‐based imazamox resistance in feral rye. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry. [ABSTRACT FROM AUTHOR]

Field studies using glyphosate-resistant corn (Zea mays L.), sugarbeet (Beta vulgaris L.), and spring wheat (Triticum aestivum L.) were conducted at Scottsbluff, NE, Torrington, WY, and Fort Collins, CO, over 6 yr to evaluate weed population dynamics under glyphosate- and conventional herbicide-based weed control strategies. We report the response of common lambsquarters (Chenepodium album L.), wild buckwheat (Polygonum convolvulus L.), and all grasses combined to four management strategies imbedded in two crop rotations. There were no consistent or highly significant benefits from a diverse crop rotation versus continuous corn when weed control was considered. Rotating herbicide mode of action for general weed control provided no benefit above that provided by glyphosate at 0.8 kg acid equivalent (ae) [ha.sup.-1] applied twice each year. There was no evidence that any weed species developed resistance to glyphosate. The most striking finding of these studies was the buildup of common lambs-quarters and wild buckwheat that occurs when glyphosate is applied at 0.4 kg ae [ha.sup.-1] twice each year. Such a low-use-rate approach is to be completely discouraged since it appears to enrich the gene pool for individuals that survive low rates of glyphosate. The continuing decline in the real cost of glyphosate should eliminate any legitimate reason for growers to consider using less than the full labeled rate of glyphosate. All treatments that included glyphosate at all three locations over 6 yr provided significantly better grass control than the nonglyphosate conventional treatments.

The potential introduction of wheat (Triticum aestivum L.) cultivars with transgenic traits has generated increased interest in pollen-mediated gene flow (PMGF). The objectives of this study were to estimate wheat PMGF between commercial fields across multiple years and locations, and to compare estimates from large fields to those from smaller experimental plots, The study was conducted in a total of 56 commercial field locations in eastern Colorado in 2003 2004, and 2005. We measured PMGF by tracking the movement of an imidazolinone herbicide resistance gene from resistant to susceptible cultivars, sampled at distances of 0.23 to 61 m. At least one sample from all 56 fields and from all 18 evaluated cultivars had detectable PMGR The highest observed PMGF was 5.3% at 0.23 m. The farthest distance at which PMGF was detected was 61 m and the highest PMGF at that distance was 0.25%. Higher levels and greater distances of PMGF were detected in commercial fields than in experimental plots Based on estimates from a generalized linear mixed model with a random location effect, the distance required to ensure 95% confidence that 95% of locations have PMGF less than 0.9% is 41.1 m for cultivars heading earlier than the pollen source and 0.7 m for cultivars heading later than the pollen source. These confidence limits should represent the highest levels of PMGF expected to occur in winter wheat in the west-central Great Plains and will be useful for wheat biotechnology regulation.

Varietal purity in wheat (Triticum aestivum L.) seed production is necessary for agronomic uniformity and to enable potential market segregation. We conducted a survey of certified and farm-saved seed samples using a nontransgenic imidazolinone-resistant (IR) wheat cultivar in 2004 and 2005 in eastern Colorado. The objective was to compare varietal purity based on type of seed producer and IR wheat history. Ninety-two samples of non-IR varieties were taken from certified and farm-saved seed growers, who either produced or had never produced IR wheat. Adventitious IR seeds were detected using a seed-soaking technique in samples from each producer type and each IR production history. Levels of IR seed ranged from 0 to 11.28%. One certified sample and three farm-saved samples exceeded the 0.1% threshold for off-types in certified wheat seed. Using a two-factor analysis, farm-saved production class and positive IR history increased the estimated proportion of adventitious seed. Based on grower interviews, higher levels of adventitious seed presence were associated with volunteer plants from previous crops of the resistant cultivar and mechanical mixture during harvesting. Production practices for certified seed address these factors and may need to be strengthened if more stringent purity criteria are adopted. This information is important for risk assessment and policy development for potential commercial release of transgenic wheat varieties.

Imidazolinone-resistant (IR) wheat (Triticum aestivum L.) was released for commercial production in portions of the USA in 2002 and has provided growers with a new technology to selectively control winter annual grass weeds. Imidazolinone herbicides inhibit acetolactate synthase (ALS) in susceptible plants; however, IR wheat has an altered target site which confers resistance to these herbicides. The mutation-derived resistance trait of most commercially available IR winter wheat cultivars is located on the D-genome; however, winter and spring wheat cultivars with the resistance trait on the A, B, or D genome or on multiple genomes are currently under development. Four groups of near-isoline wheat with spring or winter growth habit and resistance genes on the B, D, or both B and D genomes were compared for whole plant and ALS enzyme response to imazamox. Biomass accumulation after treatment was similar among B- and D-genome resistant winter wheat biotypes and was always higher than Band D-genome resistant spring wheat biotypes. D-genome resistant spring wheat was more resistant than B-genome resistant spring wheat and the two-gene resistant spring wheat had an additive level of tolerance to imazamox compared with single-gene resistant spring wheat. Growth habit (spring vs. winter) did not affect in vitro ALS activity among B- or among D-genome resistant cultivars; however, D-genome resistant cultivars had significantly higher in vitro ALS activity in the presence of imazamox compared with B-genome resistant cultivars regardless of growth habit. D-genome resistance appears to provide greater tolerance to imazamox compared with B-genome resistance; however, multiple-genome resistance likely will be required to consistently avoid crop injury in spring wheat from labeled U.S. rates. Although ALS extracted from winter wheat and spring wheat responded similarly to imazamox, whole plant responses demonstrates that tolerance is affected by factors other than resistance gene location., HERBICIDE RESISTANT (HR) crop cultivars exist for most major world crops and several minor crops and ornamentals (Devine, 2005). Growers in North America, in general, quickly accepted HR crop technology [...]