Preemergence applications of mesotrione, an herbicide that inhibits 4-hydroxyphenolpyruvate dioxygenase (HPPD), have recently gained regulatory approval in soybean varieties with appropriate traits. Giant ragweed is an extremely competitive broadleaf weed, and biotypes resistant to acetolactate synthase inhibitors (ALS-R) can be particularly difficult to manage with soil-residual herbicides in soybean production. This study investigated control of giant ragweed from preemergence applications of cloransulam (32 g ai ha–1), metribuzin (315 g ai ha–1), and S-metolachlor (1,600 g ai ha–1) in a factorial design with and without mesotrione (177 g ai ha–1) at two different sites over 2 yr. Treatments with mesotrione were also compared with two commercial premix products: sulfentrazone (283 g ai ha–1) and cloransulam (37 g ai ha–1), and chlorimuron (19 g ai ha–1), flumioxazin (69 g ai ha–1), and pyroxasulfone (87 g ai ha–1). At 42 d after planting, control and biomass reduction of giant ragweed were greater in treatments with mesotrione than any treatment without mesotrione. Giant ragweed biomass was reduced by 84% in treatments with mesotrione, whereas treatments without mesotrione did not reduce biomass relative to the nontreated. Following these preemergence applications, sequential herbicide treatments utilizing postemergence applications of glufosinate (655 g ai ha–1) plus fomesafen (266 g ai ha–1) and S-metolachlor (1,217 g ai ha–1) resulted in at least 97% control of giant ragweed at 42 d after planting, which was greater than sequential applications of glufosinate alone in 3 of 4 site-years. Preemergence applications of mesotrione can be an impactful addition to soybean herbicide programs designed to manage giant ragweed, with the potential to improve weed control and delay the onset of herbicide resistance by providing an additional effective herbicide site of action.

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.

Five johnsongrass populations collected from corn grown in northern Greece were studied to elucidate the levels and mechanisms of resistance to acetolactate synthase (ALS)- and acetyl-CoA carboxylase (ACCase)-inhibiting herbicides. Whole-plant response assays indicated that two populations were highly cross-resistant to all ALS inhibitors tested (foramsulfuron, nicosulfuron, rimsulfuron, and imazamox) but were effectively controlled by the recommended rate of the ACCase-inhibiting herbicides propaquizafop and clethodim. The ALS gene sequence revealed a point mutation that resulted in the substitution of Trp574 by Leu in the ALS enzyme, suggesting that the resistance mechanism is target-site mediated. These findings highlight a serious threat against the sustainable use of the ALS-inhibiting herbicides in controlling johnsongrass and other grass weeds in cornfields, suggesting rotational use of herbicides with different modes of action, along with the use of nonchemical methods, for viable Johnsongrass management.

Field experiments were conducted in North Carolina from 1994 through 1998 to evaluate the effects of five weed management strategies and four corn (Zea mays)–peanut (Arachis hypogaea) rotation sequences on purple nutsedge (Cyperus rotundus) population development. Effects of these weed management programs on cotton (Gossypium hirsutum) and peanut production in following years were also investigated. Herbicide programs included a nontreated control, a carbamothioate preplant incorporated (PPI) combination treatment utilizing vernolate in peanut and butylate in corn, an early postemergence (EPOST) acetolactate synthase (ALS) inhibitor combination treatment utilizing imazapic in peanut and halosulfuron in corn, and EPOST treatments of imazapic and imazethapyr in both peanut and imidazolinone-resistant corn. Crop rotation sequences for the 3 yr included continuous corn (CCC), corn–peanut–corn (CPC), peanut–corn–peanut (PCP), and continuous peanut (PPP). The imazapic and ALS inhibitor combination treatments both provided excellent shoot and tuber control. After 3 yr, imazapic and the ALS inhibitor combination treatment reduced shoot and tuber population densities to less than 10% of the nontreated control. Imazethapyr provided variable but better control than the carbamothioate treatment with tuber densities (measured from 0 to 15 cm soil depth) and shoot densities increasing from 733 to 2,901 tubers/m3 of soil and 16 to 43 shoots/m2, respectively, after 3 yr. Tuber densities increased in the nontreated control from 626 to 9,145 tubers/m3 of soil and from 962 to 5,466 tubers/m3 of soil in the carbamothioate treatment during this same period. Also, shoot densities increased in the nontreated control from 22 to 159 shoots/m2 and from 8 to 92 shoots/m2 in the carbamothioate treatment. There was a 31% peanut yield reduction from 1994 to 1996 when peanut was continuously planted or rotated to corn for only 1 yr. Herbicide carryover effects were not observed in cotton during 1997.

Henbit is a facultative broadleaf winter annual in the Lamiaceae family. Acetolactate synthase (ALS) inhibitors are primarily used to control a broad spectrum of weeds, including henbit. During 2012 to 2013, field applications of ALS-inhibiting herbicides were ineffective in controlling a henbit population from Marion County, KS (MCK). To confirm field-evolved resistance to ALS inhibitors, response of MCK henbit and a known susceptible henbit population from Kansas (DPS) to varying doses of three different ALS inhibitors were examined: chlorsulfuron, imazamox, and propoxycarbazone. Results of the dose–response experiments suggest that the MCK population is highly resistant to chlorsulfuron (resistance index [R/S] > 1,000) and propoxycarbazone (R/S = 331) but is susceptible to imazamox. A full-length ALS gene sequence obtained using the 5′- and 3′- rapid amplification of complementary DNA ends approach revealed a Pro197 to Arg point mutation (a common mutation that confers resistance to sulfonylurea herbicides, e.g., chlorsulfuron) in the MCK henbit. No other known resistance-conferring mutations were found in the study. Evolved resistance to major classes of ALS inhibitors in the MCK henbit will reduce herbicide options for its control. To our knowledge, this is the first case of evolution of herbicide resistance in henbit.

Field experiments were conducted from 1994 through 1996 to evaluate the effects of imazapic applied postemergence (POST) at 0.05 and 0.07 kg ai/ha, imazethapyr applied preplant incorporated (PPI) or POST at 0.07 kg ai/ha, and metolachlor applied PPI at 1.7 kg ai/ha on yellow nutsedge control, nutsedge tuber density, and peanut yield. All herbicides controlled yellow nutsedge at least 75% after 3 yr. Imazapic at 0.05 and 0.07 kg/ha controlled yellow nutsedge more consistently than did imazethapyr or metolachlor. Imazethapyr applied POST controlled yellow nutsedge more effectively than did imazethapyr applied PPI. Yellow nutsedge tuber densities in herbicide-treated plots were 51 to 75% less than in untreated control plots. Plots treated with imazapic, imazethapyr applied POST, and metolachlor resulted in higher peanut yields than did untreated control plots.

Greenhouse experiments were conducted to evaluate the response to halosulfuron of several smooth pigweed populations that had been shown to be resistant to acetolactate synthase (ALS, EC 2.2.1.6)-inihibiting herbicides. Five ALS-resistant smooth pigweed populations (R1, R2, R3, R4, and R5) and one susceptible (S) population were treated with halosulfuron POST at 0.27, 2.7, 27, 270, and 2,700 g ai/ha. Percentage injury and dry weight were used to determine resistance of smooth pigweed populations to halosulfuron. Populations of smooth pigweed with previous reports of resistance to ALS-inhibiting herbicides showed varying degrees of resistance to halosulfuron compared with the susceptible population. Concentrations of halosulfuron required to reduce ALS-resistant smooth pigweed dry weights 50% were 2 to 12-fold higher than that of the susceptible population. One population, designated R2, had increased resistance to halosulfuron applications, requiring 97 g/ha halosulfuron to reduce shoot dry weight 50% compared with only 8 g/ha for S. Our results show that populations of smooth pigweed with a history of ALS-inhibiting resistance can have differing degrees of resistance to halosulfuron.

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.

Mayweed chamomile seeds were collected from six different fields across the Pacific Northwest. All populations (each collection site was considered a population) were suspected to have some level of acetolactate synthase (ALS) resistance. Greenhouse and laboratory studies were conducted to determine if these populations were resistant to three different classes of ALS inhibitors: sulfonylureas (SU), imidazolinones (IMI), and triazolopyrimidines (TP). A whole-plant dose–response and in vitro ALS activity studies confirmed cross-resistance to thifensulfuron + tribenuron/chlorsulfuron (SU), imazethapyr (IMI), and cloransulam (TP); however, resistance varied by herbicide class and population. Two ALS isoforms of the ALS gene (ALS1 and ALS2) were identified in mayweed chamomile; however, only mutations in ALS1 were responsible for resistance. No mutations were found in ALS2. Sequence analysis of the partial ALS gene identified four point mutations at position 197 (Pro197 to Leu, Gln, Thr, or Ser) in the resistant populations. This study demonstrates genotypic variation associated with cross-resistance to ALS inhibitors within and between populations.

This study was conducted to ascertain movement potential of imazethapyr resistance and to measure the relative growth and productivity of imazethapyr-resistant (IR) and imazethapyr-susceptible (IS) biotypes of common sunflower under noncompetitive and competitive conditions. Susceptible biotypes of common sunflower were planted in the field in concentric circles at distances of 5.5, 8.0, 15.0, and 30.0 m around a center of densely planted IR biotypes in four locations in northeast Kansas in 1998 and 1999. Pollen movement was analyzed by sampling the IS progeny for the presence of imazethapyr resistance. The distance in which resistance is first detected from the IR pollen source, first unnatural resistant distance (FURD), ranged from 12.1 to 15.5 m. Wind direction was highly correlated with FURD; the north sections had larger FURD. Greenhouse studies were conducted to study growth of IR and IS biotypes under noncompetitive and competitive conditions. Under noncompetitive conditions, leaf area and dry weight were slightly greater for the IR than the IS biotype at early growth stages, but photosynthesis and height were similar. Under competitive conditions, photosynthesis, leaf area, height, and dry weight of IR and IS biotypes were similar. As a result, IR–IR and IS–IS intracompetition equaled IR–IS intercompetition. Gene flow from IR to IS biotypes occurred with movement up to 15.5 m. The lack of differences between growth of the IR and IS biotype at late growth stages in noncompetitive conditions and similar growth of IR and IS biotypes under competitive conditions indicated no competitive advantage from imazethapyr resistance.