Field and pot experiments were conducted in Greece to study the occurrence of resistance in silky windgrass to acetolactate synthase (ALS)- and acetyl-CoA synthase (ACCase)-inhibiting herbicides. Twenty-four populations of silky windgrass were examined in whole-plant response experiments. High levels of field-evolved resistance to chlorsulfuron (0% to 28% control in terms of fresh weight reduction) with the recommended field rates were confirmed in most silky windgrass populations. However, other ALS inhibitors, such as pyroxsulam and a premix of mesosulfuron-methyl and iodosulfuron, provided adequate control (76% to 100% in terms of fresh weight reduction) of most populations, except eight silky windgrass populations that were found to be cross-resistant to all ALS-inhibiting herbicides tested (i.e., chlorsulfuron, commercial mixture of mesosulfuron-methyl plus iodosulfuron, and pyroxsulam). Conversely, most silky windgrass populations were controlled effectively (90% to 100% in terms of fresh weight reduction) with the recommended field rates of ACCase inhibitors cycloxydim, clethodim, and pinoxaden, but five populations were also found to be resistant to clodinafop-propargyl (10% to 68% control in terms of fresh weight reduction). The ALS gene sequencing of the eight silky windgrass populations, with cross-resistance to ALS inhibitors, revealed a point mutation at the Pro-197 position, causing amino acid substitution by Ser or Thr in the ALS enzyme. Overall, chlorsulfuron and clodinafop-propargyl were selecting agents of field-evolved multiple resistance to ALS- and ACCase-inhibiting herbicides in five silky windgrass populations. As the available postemergence-applied chemistries/modes of action registered for grass weed control in cereals are rather limited, adopting integrated management practices and implementing proactive and reactive measures to delay the evolution of resistant populations is essential.

Itchgrass [Rottboellia cochinchinensis (Lour.) Clayton] is recognized as one of the most noxious and troublesome annual weeds in tropical and subtropical regions. Acetyl-coenzyme A carboxylase (ACCase)-inhibiting herbicides have been frequently used for managing R. cochinchinensis POST in a variety of crops, resulting in evolved resistance to these herbicides. Recently, resistance to fluazifop-P-butyl has been demonstrated for this weed, as the result of a G-to-C single-nucleotide polymorphism (SNP) that leads to the Trp-2027-Cys substitution in the ACCase enzyme. This study was conducted to develop a high-resolution melting analysis (HRMA) for the detection of the mutation underlying the Trp-2027-Cys substitution. The HRMA assay allowed differentiating between fluazifop-P-butyl–resistant (C mutant) and susceptible (G wild type) R. cochinchinensis plants. HRMA accuracy was confirmed with DNA sequencing of the target-site mutation, and no false positives or negatives were observed. Our results illustrated how HRMA is effective detecting the Trp-2027-Cys substitution in an R. cochinchinensis resistance, and how this technique can be of great value for developing high-throughput programs for monitoring evolution and dispersion of target site–based herbicide resistance at large scales.

A large crabgrass population (PW2) that demonstrated resistance to fluazifop-P and sethoxydim was identified in Wisconsin during 1992. Dose-response experiments were conducted in the greenhouse to determine the level of resistance of a PW2 large crabgrass accession to aryloxyphenoxypropionate (APP), cyclohexanedione (CHD), and other herbicide chemistries relative to a large crabgrass accession (A310) which was susceptible to APP and CHD herbicides. Based on shoot dry biomass reduction, the PW2 accession was 337- and 59-fold resistant to sethoxydim and fluazifop-P, respectively, relative to the A310 accession. Resistance of the PW2 accession to fenoxaprop, haloxyfop, quizalofop, and diclofop ranged from 18- to 29-fold. The PW2 accession was only 7-fold resistant to clethodim. Both large crabgrass accessions were susceptible to imazethapyr and linuron. These results suggest that APP and CHD herbicides will be ineffective for the management of the PW2 accession. An integrated approach including cultural, mechanical, and alternative chemical methods, should be implemented for the management of the PW2 accession and to prevent additional resistance problems.

Dose-response experiments were conducted in a greenhouse on a biotype of johnsongrass from Washington County, Mississippi, to determine the level of resistance to the aryloxyphenoxypropionate (AOPP) herbicide fluazifop-P-butyl and the cyclohexanedione (CHD) herbicides clethodim and sethoxydim. Both seedling and rhizomatous plants were evaluated. Resistant/susceptible (R/S) ratios were calculated based on GR50 values (the rate required to reduce shoot dry biomass, expressed as a percent of the control, 50%). The GR50 values for the resistant and susceptible seedling plants were 110 and 10 g ai/ha for clethodim, 193 and 34 g ai/ha for fluazifop-P-butyl, and 265 and 48 g ai/ha for sethoxydim, resulting in R/S ratios of 11.0, 5.7, and 5.5, respectively. The GR50 values for the resistant and susceptible rhizomatous plants were 609 and 39 g/ha for clethodim, 657 and 29 g/ha for fluazifop-P-butyl, and 668 and 30 g/ha for sethoxydim, resulting in R/S ratios of 15.6, 22.7, and 22.3, respectively.

Field and greenhouse dose-response experiments were conducted to investigate the potential for resistance in a johnsongrass biotype from New Kent County, VA that survived repeated applications of quizalofop and quizalofop-P. During 1996 and 1997, foliar injury (30 to 60%) was initially observed on the johnsongrass at the New Kent field site, but this biotype eventually recovered and survived applications of fluazifop-P, quizalofop-P, and sethoxydim at twice the recommended field use rates. However, applications of clethodim at twice the recommended field use rates during 1997 provided essentially complete control of the New Kent johnsongrass biotype. In greenhouse dose-response experiments, the amount of quizalofop-P required to inhibit shoot growth by 50% (GR50) was 13.6 g/ha in the New Kent johnsongrass biotype and 0.8 g/ha in the susceptible johnsongrass biotype. In response to sethoxydim, the GR50 for the New Kent biotype was 122.5 g/ha while that of the susceptible biotype was 21.6 g/ha. Additionally, the New Kent biotype was least sensitive to fluazifop-P, which provided a GR50 value of 148.7 g/ha for the New Kent biotype and 5.1 g/ha for the susceptible biotype. As in the field trials, the New Kent biotype was sensitive to clethodim, which provided a GR50 value of 49.8 g/ha and 69.1 g/ha for the susceptible biotype. These values indicate that the New Kent biotype was 17 times more resistant to quizalofop-P, 5.7 times more resistant to sethoxydim, and 29.5 times more resistant to fluazifop-P than the susceptible biotype, and that the New Kent and susceptible johnsongrass biotypes are equally sensitive to clethodim. These GR50 values for the New Kent johnsongrass biotype are inconsistent with the much higher GR50 values most commonly observed in graminicide-resistant weed biotypes, and suggest a mechanism of resistance other than an insensitive ACCase in the New Kent johnsongrass biotype.

Johnsongrass populations that are resistant to 5-enolpyruvyl-3-shikimate synthase (EPSPS)–, acetyl coenzyme A carboxylase (ACCase)–, or acetolactate synthase (ALS)–inhibiting herbicides are increasingly common throughout the midsouth. Three trials were conducted in 2012, 2013, and 2014 in Fayetteville, AR and Alexandria, LA to evaluate strategies with and without ALS- and ACCase inhibitors for management of rhizomatous johnsongrass in the absence of glyphosate. Fluometuron or fluometuron plus pyrithiobac applied PRE followed by (fb) EPOST, MPOST, and LAYBY tank mixtures containing multiple effective mechanisms of action (MOA) controlled johnsongrass at least 90%. Simplifying the program by removing a herbicide or eliminating an application timing reduced control, and increased vegetative and sexual reproduction of johnsongrass. To manage severe infestations or escapes glufosinate plus clethodim fb glufosinate plus clethodim or clethodim plus pyrithiobac fb clethodim) effectively controlled 15-cm johnsongrass. However, johnsongrass control was reduced when ALS and ACCase inhibitors were tank mixed, especially for the second POST application, compared to ACCase inhibitors alone. Effective herbicide programs are available to growers to control johnsongrass in the absence of glyphosate, but the use of PRE herbicides followed by multiple applications of POST herbicides is critical for successful management.

Clodionafop, an acetyl-coenzyme A carboxylase (ACCase) inhibitor, changed the shape of the chlorophyll fluorescence induction curve (Kautsky curve) in barley and oat in greenhouse experiments. Biomass ED50, based on log-logistic dose–response curves, for barley was considerably higher than that for oat in all experiments. Biomass ED50 and relative potency (ED50 [barley]/ED50 [oat]) were consistent among experiments when sprayed at the same phenological stage of plant development. Especially at high doses, clodinafop changed the shape of the Kautsky curve more for oat than for barley. From the numerous parameters that can be derived from the OJIP steps of the Kautsky curve, we found that (1) F vj, the relative changes at the J step [F vj = (F m − F j)/F m], (2) area between Kautsky curve and maximum fluorescence (F m), and (3) F v/F m, maximum quantum efficiency of Photosystem II [F v/F m = (F m − F 0)/F m], were closely linked to the biomass dose–response relationships for both species. The linkage between biomass and the fluorescence parameters may be used to shorten the screening period for ACCase inhibitors.

Italian ryegrass populations investigated in this study were harvested in an alfalfa-based cropping system. In that system, the agronomic practices and chemical weed management, based on the use of aryloxyphenoxy-propionates herbicides (i.e., quizalofop ethyl ester), were optimized to obtain a dual seed–forage production. Five of seven populations tested were confirmed resistant to quizalofop ethyl ester with resistance indexes ranging from 4.5 to >209. Both target- and nontarget-site resistance mechanisms were most likely involved. Three allelic variants were detected (Ile-1781–Leu, Trp-2027–Cys, and Ile-2041–Asn) in four resistant populations, whereas no known mutations were found in one resistant population. The herbicide treatment on Italian ryegrass plants at different phenological stages suggested that to control regrowth, it is necessary to use two to fives times the herbicide dose suitable for younger plants. This situation is encountered in fields when Italian ryegrass plants need to be controlled to maximize the alfalfa seed production, and it is comparable to using a sublethal herbicide dose, leading to the selection of herbicide-resistant biotypes. In such a situation, the cropping system is not sustainable, and integrated weed management should be implemented to deplete the soil weed seed bank and prevent new weed seed production.

A seedling bioassay was developed for the rapid diagnosis of resistance to clethodim and fluazifop-P in johnsongrass. The assay was based on differences in the coleoptile length of susceptible (S) and resistant (R) seedlings exposed to clethodim and fluazifop-P in petri dishes for 5 d. Bioassay concentrations of 0.09 mg/L clethodim and 0.18 mg/L fluazifop-P were chosen as discriminant based on rate responses of each biotype to increasing herbicide dose. At 5 d after treatment (DAT), the amounts of clethodim required to reduce coleoptile length by 50% (GR50) for the R and S seedlings were 462.5 and 24.8 mg/L, respectively, resulting in an R:S ratio of 18.7. The fluazifop GR50 values for the R and S seedlings were 618.7 and 17.5 mg/L, respectively, resulting in a R:S ratio of 35.4.