Rice in Mississippi is often in early seedling growth stages when paraquat-based herbicide treatments are commonly applied to corn, cotton, and soybean; therefore, off-target movement of the herbicide onto adjacent rice fields may occur. After an off-target movement event has occurred, weed management in the rice crop is still necessary. Field studies were conducted from 2019 to 2021 in Stoneville, MS, to evaluate rice injury and barnyardgrass control with labeled herbicides after exposure to a sublethal concentration of paraquat. Herbicide treatments were label-recommended rates of imazethapyr, quinclorac, propanil, bispyribac-sodium, cyhalopfop, and florpyrauxifen-benzyl applied following rice exposure to a sublethal concentration of paraquat. Rice injury was detected 7 and 28 d after treatment (DAT) and was ≥35% and ≥14%, respectively, for all herbicides. Florpyrauxifen-benzyl and imazethapyr caused the greatest rice injury at 28 DAT. Following paraquat exposure, barnyardgrass control was similar for all labeled herbicide treatments at 7, 14, and 28 DAT except for florpyrauxifen-benzyl and no herbicide (paraquat alone) at 7 DAT. Across all evaluations, barnyardgrass control was at least 12% greater following paraquat exposure and labeled herbicide treatments than with no paraquat exposure. The current research demonstrates that labeled rates of herbicides applied following exposure to a sublethal concentration of paraquat resulted in <36% injury and provided as great as 95% control of barnyardgrass, depending on the herbicide treatment. Therefore, the labeled herbicides choice following rice exposure to a sublethal concentration of paraquat should be based on weed spectrum.

In Iran, monochoria is a noxious weed in fields of transplanted rice. Two field experiments were conducted to assess the efficacy of soil-applied and foliar-applied herbicides to control monochoria in transplanted rice. Prepackaged herbicides (triafamone plus ethoxysulfuron applied at 40 g ai ha−1, pyrazosulfuron-ethyl plus pretilachlor applied at 382.5 g ai ha−1, and pendimethalin plus clomazone applied at 1,200 g ai ha−1) reduced monochoria biomass by 100%, 100%, and 14%, respectively; and a single application of flucetosulfuron at 30 g ai ha−1, pendimethalin at 990 g ai ha−1, thiobencarb at 2,750 g ai ha−1, and pretilachlor at 1,000 g ai ha−1 reduced monochoria biomass by 100%, 99%, 75%, and 56%, respectively, compared with a nontreated control. Tank-mixed bensulfuron-methyl at 45 g ai ha−1 applied with pretilachlor, thiobencarb, or pendimethalin provided 100% control of monochoria. Rice height, and straw and grain yield were greater after herbicide treatments than those of the nontreated and hand-weeded controls, indicating the advantages of chemical control of monochoria over manual weeding. Full-season monochoria interference reduced rice grain yield by 32%. In the second study, the herbicides triafamone plus ethoxysulfuron, flucetosulfuron, 2,4-D at 1,080 g ai ha−1, dicamba plus 2,4-D at 928 g ai ha−1, bispyribac-sodium at 31.25 g ai ha−1, bentazon plus MCPA at 1,150 g ai ha−1, pyribenzoxim at 30 g ai ha−1, and propanil at 5,400 g ai ha−1 applied to foliage at 4- to 5-leaf seedlings of monochoria provided ≥97% control and prevented 100% of its regrowth, with the exception of propanil. This study shows that monochoria control can be achieved by using a variety of residual and foliar-applied herbicides with different mechanisms of action.

Bispyribac-sodium, a herbicide that inhibits acetolactate synthase (ALS), is frequently used in rice fields in India to control weeds, including the most common noxious weed, barnyardgrass. However, rice growers have recently reported reduced control of barnyardgrass with bispyribac-sodium. Hence, a large-scale survey was carried out to assess bispyribac-sodium resistance in Chhattisgarh and Kerala, two rice-growing states. Open-field pot experiments were conducted for 2 yr to confirm resistance to bispyribac-sodium. Of the 37 biotypes tested, 30% (11) survived the recommended label rate of bispyribac-sodium (25 g ai ha−1). The effective rate of bispyribac-sodium required to achieve 50% control (ED50) of putative resistant biotypes ranged from 18 to 41 g ha−1, whereas it was about 10 g ha−1 for susceptible biotypes. This suggests that putative biotypes were two to four times more resistant to bispyribac-sodium. At 6 d after herbicide application, an in vitro enzyme assay demonstrated higher ALS enzyme activity in putative resistant biotypes (66% to 75%) compared with susceptible biotypes (48% to 52%). This indicates the presence of an insensitive ALS enzyme in those biotypes and a target site mutation as a possible mechanism for resistance. Whole-plant bioassays also suggested that the resistance problem is more widespread in Chhattisgarh than in Kerala. This study confirmed the first case of evolved resistance in barnyardgrass to bispyribac-sodium in rice fields of India.

Pondweed is a rhizomatous perennial weed of aquatic habitats that recently adapted to rice ecosystems in northern Iran. Two field experiments were conducted at the Rice Research Institute of Iran to determine the impact of pondweed on rice yield and identify effective herbicides for pondweed control. The focus of the first study was to evaluate the herbicides commonly used in Iranian rice, including butachlor, pretilachlor, oxadiargyl, pendimethalin, thiobencarb, and bensulfuron-methyl. None of these herbicides effectively controlled pondweed, except bensulfuron, which reduced pondweed biomass by ≥95% and produced 26% higher rough rice grain yield than the nontreated plots. The second experiment evaluated the performance of acetolactate synthase–inhibiting herbicides on pondweed control, rough rice yield, and pondweed regrowth. Herbicide efficacy on pondweed varied from 36% to 100%. Five preemergence herbicides, bensulfuron at 45 g ai ha−1, flucetosulfuron at 30 g ai ha−1, triafamone plus ethoxysulfuron at 40 g ai ha−1, and metsulfuron-methyl at 15 g ai ha−1, provided ≥98% control of pondweed. Use of postemergence herbicides penoxsulam at 35 g ai ha−1, bispyribac-sodium at 30 g ai ha−1, and pyribenzoxim at 35 g ai ha−1 provided 36%, 89%, and 93% pondweed control, respectively. Rough rice yields ranged from 107% to 124% in herbicide-treated plots compared with the nontreated plots. Soil-applied herbicide treatments produced higher (≥119%) yield than the hand-weeded control or foliar-applied herbicides. Pondweed regrowth was affected by herbicides and was variable. Soil-applied residual herbicides metazosulfuron, flucetosulfuron, and metsulfuron provided complete control of pondweed and prevented regrowth. In contrast, pondweed regrowth in other soil- and foliar-applied herbicide treatments occurred, indicating their lesser translocation to underground vegetative rhizomes. This study shows that although most sulfonylurea herbicides can control pondweed effectively to achieve high rough rice yield, only a few soil-applied herbicides were able to prevent pondweed regrowth.

Smallflower umbrella sedge is one of the most problematic weeds in direct-seeded rice in India. Bispyribac-sodium (acetolactate synthase [ALS]-inhibiting herbicide) is commonly used in rice, but growers have recently reported lack of smallflower umbrella sedge control with this herbicide. An extensive survey was carried out in two rice-growing states, Chhattisgarh and Kerala, where 53 putative bispyribac-sodium-resistant (BR) biotypes were collected. Studies were conducted to confirm resistance to bispyribac-sodium and to test the efficacy of the newly developed synthetic auxin herbicide florpyrauxifen-benzyl on putative BR biotypes. A whole-plant bioassay revealed that bispyribac-sodium is no longer effective. Of 53 putative BR biotypes, 17 biotypes survived the recommended label rate of 25 g ai ha−1. The effective bispyribac-sodium rate required to control 50% of the plants in most of the BR biotypes (ED50) ranged from 19 to 96 g ha−1, whereas it was 10 g ha−1 in a susceptible biotype. In two highly resistant biotypes, the ED50 was beyond the maximum tested rate, 200 g ha−1. This suggests 2- to >20-fold resistance in BR biotypes. An ALS enzyme activity assay suggests an altered target site as mechanism of resistance to bispyribac-sodium. This study confirms the first case of evolved resistance to bispyribac-sodium in smallflower umbrella sedge in India. However, the newly developed synthetic auxin florpyrauxifen-benzyl effectively controlled all BR biotypes at the field use rate of 31.25 g ai ha−1.

In Bangladesh, weeds in transplanted rice are largely controlled by labor-intensive and costly manual weeding, resulting in inadequate and untimely weed control. Labor scarcity coupled with intensive rice production has triggered increased use of herbicides. These factors warrant a cost-effective and strategic integrated weed management (IWM) approach. On-farm trials with transplanted rice were conducted during monsoon (‘Aman’) season in 2016 and 2017 and winter (‘Boro’) season in 2016 to 2017 in agroecological zones 11 and 12 with ten treatments—seven herbicide-based IWM options, one mechanical weed control-based option, and two checks (farmers’ current weed control practice and weed-free)—to assess effects on weed control, grain yield, labor use, and profitability. Compared to farmers’ practice, herbicide-based IWM options with mefenacet + bensulfuron-methyl as preemergence followed by (fb) either bispyribac-sodium or penoxsulam as postemergence fb one hand-weeding were the most profitable alternatives, with reductions in labor requirement by 11 to 25 person-days ha–1 and in total weed control cost by US$44 to 94 ha–1, resulting in net returns increases by US$54 to 77 ha–1 without compromising on grain yield. In contrast, IWM options with bispyrbac-sodium or penoxsulam as postemergence application fb one hand-weeding reduced yields by 12% to 13% and profits by US$71 to 190 ha–1. The nonchemical option with mechanical weeding fb one hand-weeding performed similarly to farmers’ practice on yield and profitability. We suggest additional research to develop feasible herbicide-free approaches to weed management in transplanted rice that can offer competitive advantages to current practices.

Ludwigia prostrata is a problematic weed in rice fields in China, where acetolactate synthase (ALS)-inhibiting herbicides (e.g., bensulfuron-methyl) are widely used for the management of broadleaf weeds. Recently, an L. prostrata biotype (JS-R) that failed to be controlled with ALS-inhibiting herbicides was found in Jiangsu Province, China. This study aims to determine the level and molecular mechanism of resistance to bensulfuron-methyl in this JS-R biotype and to evaluate its spectrum of cross-resistance to other ALS-inhibiting herbicides. The dose–response assays indicated that the JS-R L. prostrata biotype had evolved 21.2-fold resistance to bensulfuron-methyl compared with the susceptible biotype (JS-S). ALS gene sequencing revealed that a nucleotide mutation (CCA to TCA) at codon 197, resulting in a Pro-197-Ser mutation, was detected in the resistant plants. Moreover, while the JS-R biotype contained the Pro-197-Ser resistance mutation and showed cross-resistance to pyrazosulfuron-ethyl (12.0-fold), it was sensitive to penoxsulam, bispyribac-sodium, and imazethapyr, which may serve as alternative herbicides to control the resistant L. prostrata biotype. This is the first confirmation of an L. prostrata biotype resistant to bensulfuron-methyl due to a Pro-197-Ser resistance mutation in the ALS gene.

The increased use of insecticide seed treatments in rice has raised many questions about the potential benefits of these products. In 2014 and 2015, a field experiment was conducted near Stuttgart and Lonoke, AR, to evaluate whether an insecticide seed treatment could possibly lessen injury from acetolactate synthase (ALS)–inhibiting herbicides in imidazolinone-resistant (IR) rice. Two IR cultivars were tested (a hybrid, ‘CLXL745’, and an inbred, ‘CL152’), with and without an insecticide seed treatment (thiamethoxam). Four different herbicide combinations were evaluated: a nontreated control, two applications of bispyribac-sodium (hereafter bispyribac), two applications of imazethapyr, and two applications of imazethapyr plus bispyribac. The first herbicide application was to two- to three-leaf rice, and the second immediately prior to flooding (one- to two-tiller). At both 2 and 4 wk after final treatment (WAFT), the sequential applications of imazethapyr or bispyribac plus imazethapyr were more injurious to CLXL745 than CL152. This increased injury led to decreased groundcover 3 WAFT. Rice treated with thiamethoxam was less injured than nontreated rice and had improved groundcover and greater canopy heights. Even with up to 32% injury, the rice plants recovered by the end of the growing season, and yields within a cultivar were similar with and without a thiamethoxam seed treatment across all herbicide treatments. Based on these results, thiamethoxam can partially protect rice from injury caused by ALS-inhibiting herbicides as well as increase groundcover and canopy height; that is, the injury to rice never negatively affected yield.

A field study was established to evaluate symptoms, growth, yield, and nut quality of walnut trees subjected to multiple exposures of simulated bispyribac-sodium drift. Nut yield the year following simulated drift treatment was also evaluated because tissue differentiation for future fruiting position occurs in the prior season. Bispyribac-sodium was applied four times, at weekly intervals, at 0.5% and 3% of the use rate in rice (45 g ai ha-1). Injury from the 0.5% rate exceeded 5% after three applications. In general, the severity of the symptoms peaked 14 d after last application (23% and 40% injury for 0.5% and 3% rate, respectively) and subsequently remained nearly constant over the duration of the study. Growth of shoots treated with the 0.5% rate was initially delayed during the treatment regime but recovered after treatments ended; however, walnut shoots exposed to the higher rate had fewer internodes than nontreated trees at the end of the season. No measurable reduction in walnut yield or average nut weight either in the year of exposure or in the subsequent year was observed. However, both rates negatively affected walnut kernel color in the year of exposure.

A field study was established to study symptoms, growth and yield of 2-year-old walnut trees exposure to simulated drift of several herbicides commonly used in rice production. Bispyribac-sodium, bensulfuron-methyl, and propanil were applied at four rates representing 0.5%, 1%, 3% and 10% of the normal use rate in rice (45, 70, and 6725 g ai ha−1, for the three herbicides respectively). Symptoms started to appear approximately 7 days after application (DAT) and peaked 28 DAT. At that time, bispyribac-sodium caused greater injury at low drift rates (6% and 15% visual injury for 0.5% and 1% rate, respectively) compared to bensulfuron-methyl and propanil. Bispyribac-sodium also appeared to slow walnut shoot elongation compared to nontreated trees; however, no yield reductions were observed either in the year after drift exposure. The effect of bispyribac-sodium simulated drift on the yield and nut quality in the year of drift exposure was evaluated in a separate study on 3-year-old walnut trees. While no yield or nut quality reductions were observed, a linear correlation between rate of bispyribac-sodium and color, an important quality factor, was found: higher herbicide rates tended to be associated with darker kernel color. Bispyribac-sodium may damage nearby walnut orchards if drifted at significant amounts. However, it is unlikely that in a field situation bispyribac-sodium would drift at high enough levels to cause the symptoms observed from the 10% use rates in this study.

Methiozolin is an isoxazoline herbicide that selectively controls annual bluegrass in cool-season turf and may control roughstalk bluegrass, another weedy Poa species that is problematic in many turfgrass systems. However, the majority of research to date is limited to evaluating methiozolin efficacy for annual bluegrass control in creeping bentgrass putting greens. Research was conducted comparing various application regimes of methiozolin and other herbicides for long-term roughstalk bluegrass control in creeping bentgrass golf fairways. Methiozolin-only treatments did not injure creeping bentgrass or reduce normalized difference vegetative index (NDVI) at 2 golf course locations based on 20 evaluation dates over a 2.5-yr period. The 2.5-yr average turf quality generally declined as roughstalk bluegrass control increased due to transient turf cover loss. At 1 yr after last treatment, methiozolin at 1500 g ai ha-1 applied four times in fall reduced roughstalk bluegrass cover 85%. This was equivalent to methiozolin at 1000 g ha-1 applied four times in fall, but greater than low rates of methiozolin applied four times in spring or twice in fall and spring. Amicarbazone, primisulfuron, and bispyribac-sodium alone either did not effectively reduce roughstalk bluegrass cover, or did so at the expense of increased creeping bentgrass injury. Results of this study suggest that methiozolin alone or tank-mixed with amicarbazone or primisulfuron is an effective long-term approach for selectively controlling roughstalk bluegrass in creeping bentgrass.

Eclipta, widespread in tropical, subtropical, and temperate regions, is one of the main malignant broadleaf weeds and thrives in moist and dryland fields. Field rates of acetolactate synthase (ALS) inhibitors have failed to control eclipta in some farmlands in China. One ALS inhibitor–resistant population (R) collected from Jiangsu province in China was confirmed in the greenhouse in our preliminary work. Whole-plant assays revealed that this R population was highly resistant to four sulfonylureas (pyrazosulfuron-ethyl, 134-fold; bensulfuron-methyl, 172-fold; metsulfuron-methyl, 30-fold; and tribenuron-methyl, 195-fold), two triazolopyrimidines (pyroxsulam, 98-fold; penoxsulam, 30-fold), and one pyrimidinylthio-benzoate (bispyribac-sodium, 166-fold) and was moderately resistant to two imidazolinones (imazethapyr, 10-fold; imazapic, 19-fold). ALS enzyme-activity assays showed insensitivity of the ALS from the R population (resistance index values ranged from 12 to 293) to all of the above ALS inhibitors in vitro. Chromatograms from ALS gene sequence analysis detected a homozygous Pro-197-Ser amino acid substitution in the R population. These results confirmed that the Pro-197-Ser substitution results in broad-spectrum cross-resistance to ALS inhibitors in the eclipta R population. To our knowledge, this study is the first to report broad cross-resistance to ALS inhibitors in eclipta and to obtain the full-length ALS gene sequence.

Basic factors contributing to the rapid evolution and broad distribution of acetolactate synthase (ALS)-inhibiting herbicide resistance in smallflower umbrella sedge L. have not yet been investigated. The objectives of this study were to examine patterns of cross-resistance to ALS herbicides and genetic diversity within and among smallflower umbrella sedge populations in California rice fields to provide insight into the processes contributing to resistance spread. Twelve different patterns of herbicide cross-resistance were found across the 56 populations sampled. The frequency of populations with at least one resistant individual in the North, Central and South Sacramento Valley, and the San Joaquin Valley were 76, 86, 67, and 50%, respectively. Analysis of the genetic diversity of 29 populations using 73 sequence-related amplified polymorphism molecular markers revealed little genetic diversity within populations, with estimates of Nei's gene diversity index, h, ranging from 0 to 0.049, and Shannon's information index (I) ranging from 0 to 0.079. Hierarchical analyses of molecular variance indicated that the majority of genetic variation was partitioned among populations, rather than within populations or among regional groups. No isolation by distance was evident. Unweighted pair group method with arithmetic averages analysis indicated that population clustering was not region specific. The results suggest that resistance to ALS-inhibiting herbicides in smallflower umbrella sedge populations from California rice fields appears to have evolved independently multiple times rather than spread from a single population where resistance originated. Consequently, prevention and management of smallflower umbrella sedge in California rice fields should emphasize in-field strategies that focus on decreasing the selection pressure caused by ALS-inhibiting herbicides.

Biotypes of smallflower umbrella sedge and ricefield bulrush resistant to acetolactate synthase (ALS)-inhibiting herbicides have been reported in several rice areas of the world. Here, we present results of a study conducted on whole plants of seven smallflower umbrella sedge and four ricefield bulrush biotypes collected in Italian, Spanish, and Californian rice fields to evaluate cross-resistance to ALS herbicides in these important weeds of temperate rice. The following herbicides were tested: bensulfuron-methyl, halosulfuron, cinosulfuron, imazamox, and bispyribac-sodium. The smallflower umbrella sedge and ricefield bulrush biotypes studied exhibited different cross-resistance patterns, some of which have not been previously reported. The Italian smallflower umbrella sedge biotype was cross-resistant to bensulfuron-methyl, cinosulfuron, imazamox, and bispyribac-sodium, but was susceptible to halosulfuron. One smallflower umbrella sedge biotype from California was also resistant to bensulfuron-methyl, imazamox, and bispyribac-sodium, but had a lower level of resistance to halosulfuron. In contrast, the second smallflower umbrella sedge biotype from California was strongly resistant to halosulfuron and was also resistant to bensulfuron-methyl and bispyribac-sodium, but moderately resistant to imazamox. The Spanish smallflower biotype was resistant to the sulfonylurea herbicides bensulfuron-methyl and halosulfuron. Different responses were observed in ricefield bulrush. The Italian biotype was resistant to the sulfonylureas only, whereas the biotype from California exhibited broad cross-resistance to all the ALS herbicides tested. Knowledge on cross-resistance is needed to formulate herbicide use and weed management strategies for delaying the evolution of resistance to ALS herbicides in rice systems.

Two field studies were conducted in 2007 and 2008 to evaluate at-planting burndown and POST herbicide applications targeting volunteer glyphosate-resistant (GR) soybean in rice. In the burndown study, paraquat, glufosinate, and a thifensulfuron plus tribenuron mixture were applied immediately after rice seeding. Paraquat controlled volunteer GR soybean at least 95% at all evaluations both years. Control with glufosinate was greater in 2007 than 2008 due to rainfall that occurred following application the second year. The thifensulfuron plus tribenuron mixture provided similar control in both years, but control never exceeded 71%. Additionally, a study was conducted evaluating POST-applied rice herbicides including propanil (4,480 and 2,240 g ai/ha), triclopyr (420 and 210 g ai/ha), bispyribac-sodium (38 and 19 g ai/ha), penoxsulam (40 and 20 g ai/ha), and halosulfuron (70 and 35 g ai/ha). Control across all POST herbicides and application rates was equivalent (> 95%) 28 and 56 d after application except for propanil, which controlled volunteer GR soybean less than other treatments. Volunteer GR soybean can be effectively managed in a rice production system with at-planting burndown or POST herbicide applications in rice.

Bispyribac-sodium selectively controls annual bluegrass in cool-season turf but efficacy may be influenced by management practices, such as plant growth regulator use. Experiments were conducted in New Jersey to investigate efficacy and absorption of bispyribac-sodium applied with trinexapac-ethyl for annual bluegrass control and turfgrass tolerance. In laboratory experiments with annual bluegrass, creeping bentgrass, and perennial ryegrass, tank-mixing trinexapac-ethyl with 14C-bispyribac-sodium increased presumed foliar absorption of 14C-bispyribac-sodium compared with nontrinexapac-ethyl treated; absorption increased with trinexapac-ethyl rate. Differences in 14C-bispyribac-sodium absorption were not detected among emulsifiable concentration, microencapsulated concentration, and wettable powder trinexapac-ethyl formulations. In field experiments, sequential bispyribac-sodium applications controlled annual bluegrass 93%, but trinexapac-ethyl did not affect efficacy. Tank-mixing all trinexapac-ethyl formulations with bispyribac-sodium provided similar annual bluegrass control and creeping bentgrass quality compared with bispyribac-sodium alone. Applications of bispyribac-sodium reduced dollar spot cover in both years, whereas trinexapac-ethyl reduced dollar spot cover only in 2005.

Alligatorweed is a perennial, invasive weed in southern United States rice production, but knowledge on effective management of this weed is limited, especially in conventional (non-imidazolinone-resistant) rice fields. Field studies were conducted in multiple environments in southeastern Texas to evaluate different herbicide options involving penoxsulam, propanil, triclopyr, halosulfuron, bispyribac-sodium, bensulfuron, and quinclorac for alligatorweed control in conventional drill-seeded rice when applied at early POST (EPOST), late POST (LPOST), or both. Among the herbicide options evaluated, penoxsulam alone (up to 83%), penoxsulam plus triclopyr (up to 87%), or bispyribac-sodium plus triclopyr (92%) provided superior alligatorweed control. Plots treated with penoxsulam plus triclopyr EPOST produced the highest yields (9,550 kg ha−1), which were comparable to plots receiving penoxsulam plus triclopyr LPOST (9,320 kg ha−1), penoxsulam alone EPOST (9,280 kg ha−1), and penoxsulam plus halosulfuron LPOST (9,180 kg ha−1). Considering both weed control and rice grain yields, penoxsulam plus triclopyr applied EPOST was found to be the best option among the treatments tested. The treatments bensulfuron alone, bensulfuron plus propanil, penoxsulam plus propanil, triclopyr plus propanil, and bispyribac-sodium plus propanil provided poor (≤ 65%) alligatorweed control. Results also suggest the likelihood for antagonistic interactions when tank-mix combinations tested in this study included propanil.

Annual bluegrass is a troublesome weed of Kentucky bluegrass because of a lack of selective POST herbicides for control. Field experiments were conducted in New Jersey to investigate the potential of bispyribac-sodium, primisulfuron, and sulfosulfuron for selective annual bluegrass control in Kentucky bluegrass. Primisulfuron provided the best combination of Kentucky bluegrass safety and annual bluegrass control with greater efficacy in summer than fall. Bispyribac-sodium also provided substantial annual bluegrass control, especially in the summer, but caused unacceptable (> 20%) Kentucky bluegrass injury, whereas annual bluegrass control with sulfosulfuron was inconsistent. Growth chamber experiments confirmed that greater primisulfuron efficacy in summer than fall may be attributed to higher temperatures. Overall, primisulfuron has promising implications for future use in Kentucky bluegrass for annual bluegrass control whereas bispyribac-sodium will likely be more applicable in other cool-season turfgrasses.

Annual bluegrass is the most problematic winter annual weed in managed turfgrass. Acetolactate synthase (ALS)-inhibiting herbicides are effective for annual bluegrass control, but reliance on this mode of action can select for herbicide-resistant biotypes. Two annual bluegrass biotypes not controlled with ALS-inhibiting herbicides were reported at golf courses in South Carolina and Georgia. Research was initiated at Clemson University to verify the level of resistance of these biotypes to ALS inhibitors. Two ALS-susceptible (S) and suspected resistant (SCr, GAr) annual bluegrass biotypes were established in a greenhouse. Dose-response experiments were conducted on mature annual bluegrass plants using trifloxysulfuron, foramsulfuron, and bispyribac-sodium, all ALS-inhibiting herbicides. Additionally, a rapid diagnostic ALS activity assay was optimized and conducted using the same herbicides. For dose-response experiments, the rate of herbicide that reduced shoot biomass 50% (I50) values for the S biotypes were 13.6 g ai ha−1 for trifloxysulfuron, 7.0 g ai ha−1 for foramsulfuron, and 38.3 g ai ha−1 for bispyribac-sodium. Fifty percent shoot biomass reduction was not observed in either the SCr or GAr biotypes at eight times the labeled field rate of all ALS-inhibiting herbicides tested. For in vivo tests of ALS activity, the SCr biotype yielded I50 (concentration of herbicide that reduced ALS activity 50%) values 3,650, 3,290, and 13 times the S biotypes following treatment with trifloxysulfuron, foramsulfuron, and bispyribac-sodium, respectively. Similarly, I50 values for the GAr biotype were 316, 140, and 64 times greater than the S biotypes following the same herbicide treatments. This research indicates high levels of annual bluegrass resistance to multiple ALS-inhibiting herbicides in South Carolina and Georgia. Future research should focus on the mechanisms of ALS resistance in these annual bluegrass biotypes as well as alternative options for control not targeting the ALS enzyme.

Bispyribac-sodium is an efficacious herbicide for annual bluegrass control in creeping bentgrass fairways, but turf tolerance and growth inhibition may be exacerbated by low mowing heights on putting greens. We conducted field and greenhouse experiments to investigate creeping bentgrass putting green tolerance to bispyribac-sodium. In greenhouse experiments, creeping bentgrass discoloration from bispyribac-sodium was exacerbated by reductions in mowing height from 24 to 3 mm, but mowing height did not influence clipping yields or root weight. In field experiments, discoloration of creeping bentgrass putting greens was greatest from applications of 37 g/ha every 10 d, compared to 74, 111, or 222 g/ha applied less frequently. Chelated iron effectively reduced discoloration of creeping bentgrass putting greens from bispyribac-sodium while trinexapac-ethyl inconsistently reduced these effects. Overall, creeping bentgrass putting greens appear more sensitive to bispyribac-sodium than higher mowed turf, but chelated iron and trinexapac-ethyl could reduce discoloration.