In Argentina, Lolium spp. occur in 40% of winter cereal crops from the Pampas. Several years ago, cases of glyphosate-resistant perennial ryegrass (Lolium perenne L.) were detected, and the use of acetyl-CoA carboxylase (ACCase)-inhibiting herbicides to eradicate these plants has been considered. The aim of this study was to evaluate the sensitivity of a putative pinoxaden-resistant L. perenne population to ACCase-inhibiting herbicides. Around 80% of plants from the putative resistant population survived at a recommended dose of pinoxaden, and they produced viable seeds. The resistance indices (RIs) to pinoxaden were 5.1 and 2.8 for plant survival and seed production, respectively. A single point mutation that conferred a Asp-2078-Gly substitution in ACCase was the source of the resistance. To match the plant control achieved in the susceptible population, the resistant population required 5.4- and 10.4-fold greater doses of clethodim and quizalofop, respectively. RIs for viable seed production when treated with clethodim and quizalofop were 3.3 and 6.6, respectively. The Asp-2078-Gly mutation endowed significant levels of resistance to pinoxaden, clethodim, and quizalofop. For three herbicides, the level of resistance of a pinoxaden-resistant L. perenne population to ACCase inhibitors was evaluated, based on an evaluation of dose response for plant survival and seed production. The RIs were higher for plant survival than for seed production. In Argentina, the selection pressure associated with clethodim and haloxifop preplant application and pinoxaden use on wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) crops, would have favored the propagation of the Asp-2078-Gly mutation with its associated resistance.

An experiment was conducted to determine the interval needed between dithiopyr formulation (EC and G) treatments and seeding of tall fescue, perennial ryegrass, and creeping bentgrass. The cover of tall fescue 10 wk after seeding was not reduced when the EC formulation was applied at ≤ 0.84 kg ha−1 ≥ 2 wk before seeding. Dithiopyr G at 0.56 kg ha−1 did not reduce tall fescue cover when applied at ≥ 8 wk before seeding; whereas, 0.84 kg ha−1 required a 12-wk interval between treatment and seeding in one of two years. Perennial ryegrass cover 10 wk after seeding was not reduced with dithiopyr at 0.56 kg ha−1 applied from 2 to 12 wk before seeding. When the dithiopyr G was applied at 0.56 kg ha−1, an 8-wk interval before seeding was needed to prevent a reduction in turf cover. When rates of EC and G dithiopyr were increased to 0.84 kg ha−1 an 8-wk interval was needed for the EC and a 12-wk interval was needed for the G formulation to prevent a reduction in cover in one of two years. Creeping bentgrass cover was not reduced when dithiopyr EC was applied at 0.56 kg ha−1 ≥ 8 wk before seeding. When the EC rate was increased to 0.84 kg ha−1 a 12-wk interval was needed between treatment and seeding in one of two years. When dithiopyr G was applied within 12 wk of seeding creeping bentgrass, the cover was reduced to an unacceptable level regardless of application rate.

Field experiments were conducted in 1982 through 1986 to evaluate the effect of six postemergence grass herbicides, clethodim, the methyl ester of diclofop, the butyl ester of fluazifop-P, the methyl ester of haloxyfop, the methyl ester of quizalofop, and sethoxydim, on 31 seedling grass species. All herbicides except diclofop controlled most of the species tested. None of the herbicides controlled rattail fescue or red fescue. Only clethodim and haloxyfop controlled annual bluegrass.

Five perennial cool-season grasses: Kentucky bluegrass, creeping red fescue, tall fescue, orchardgrass, and perennial ryegrass were treated at the 2- to 3-leaf, 3- to 4-leaf, 4- to 6-leaf, and 6- to 7-leaf growth stages with four rates (0, 0.14, 0.28, and 0.56 kg ai/ha) of the butyl ester of fluazifop. Creeping red fescue was not injured by any rate at any growth stage. Reduction in shoot dry weight increased linearly with increasing fluazifop rate for the remaining four species, except for the 6- to 7-leaf stage in Kentucky bluegrass and the 4- to 6-leaf stage in tall fescue and perennial ryegrass. Significant differences between growth stages within each rate varied with species. In general, tolerance increased at the 4- to 6-leaf and 6- to 7-leaf growth stages.

The response of Persian darnel, green foxtail, wild oat, and volunteer cereals to clethodim applied at two growth stages, and the effect of these treatments on flax yield and quality were determined in a series of field experiments in which each weed species was evaluated separately. Clethodim applied at 45 g/ha provided excellent control of Persian darnel, green foxtail, wild oat, and volunteer wheat and barley. Sethoxydim applied at 150 g/ha provided excellent control of Persian darnel, green foxtail, wild oat, and volunteer barley. Rax was tolerant at five times the recommended rate of application and was unaffected by plant growth stage at time of application. When applied at an early (2- to 3-leaf) stage, control of Persian darnel, green foxtail, wild oat, and volunteer wheat and barley was excellent at 20 g/ha. Except for Persian darnel, weed control as determined by reduction in plant dry weight, was negatively affected by growth stage at time of application. Although visible control and reduction in plant dry weight were somewhat less when clethodim was applied at the 4-to 5-leaf stage, control of each weed species was acceptable. When weight of water in plant tissue (fresh weight – dry weight) was used as a measure of living plant tissue, wild oat and volunteer barley control were unaffected by growth stage at time of application.

Formulations of fenoxaprop enriched in the R(+) isomer, HOE 46360-08H EW (described as an emulsifiable water-based formulation) and HOE 46360-05H EC (emulsifiable concentrate), were compared with the commercial racemate EC for control of roughstalk bluegrass and tolerance of perennial ryegrass and tall fescue grown for seed. The enriched EW and EC formulations were more than twice as active as the racemate per kg total fenoxaprop, despite enrichment to only 92 and 89% R(+) isomer purity, which should have increased activity by 1.84- and 1.78-fold, respectively. Increased activity was evident both as improved weed control and as decreased crop tolerance. Although the 3.69-fold increase in activity of HOE 46360 EW on roughstalk bluegrass relative to the racemate may be partly due to changes in the emulsion, the 3.06-fold increase in activity of HOE 46360 EC suggests that the S(-) isomer somehow interferes wih the activity of the R(+) isomer. Use of 280 g ai ha–1 of the racemate could be replaced with 76 g ha–1 of the EW or 92 g ha–1 of the EC formulation of HOE 46360 for equivalent roughstalk bluegrass control, but with decreased crop tolerance in perennial ryegrass and tall fescue.

Italian ryegrass interference in broccoli was measured in field experiments and the data fit a rectangular hyperbolic competition model. The model predicted 58% of broccoli yield loss related to Italian ryegrass density when pooled over three y. An economic threshold value of 4.9 Italian ryegrass plants m−1 of crop row was determined to be the density required to cause a 3.6% yield loss, equal to postemergence weed control costs. Italian ryegrass densities of 600 to 1000 plants m1 of broccoli row caused 100% yield loss.

Pronamide was applied to a ‘Tifway’ bermudagrass golf green overseeded with perennial ryegrass during 1988 and 1989 to determine how herbicide rate and date affected ryegrass injury on transition back to a bermudagrass monostand. In most instances, perennial ryegrass was injured less when pronamide was applied at 0.28 or 0.56 kg ai/ha March 30 or April 13 than when application was delayed until April 28 or May 14. Perennial ryegrass injury lasted longer when 0.28 kg/ha was applied twice (March and/or April) than when applied singly at 0.28 kg/ha March 30 or April 13. The optimum time and rate of pronamide for effective transition was April 13 and 0.56 kg/ha, respectively.

Efficacy, movement, and degradation of dithiopyr and pendimethalin were monitored after application to perennial ryegrass turf in 1990 and 1991. Dithiopyr at 0.6 kg ai ha−1 reduced large crabgrass infestation by ≥ 90% up to 87 DAT in both years, while pendimethalin at 1.7 kg ai ha−1 provided similar control up to 59 DAT in 1 of 2 yr. Turf thinning due to disease and insect damage in 1990 allowed more herbicide to reach the thatch, mat, and underlying soil in 1991. In both years, more pendimethalin but less dithiopyr was found in thatch 7 DAT than 1 DAT, suggesting greater pendimethalin retention in verdure and thatch. Precipitation was greater after herbicide application in the second year, and concentration in the mat layer reached a maximum by 7 DAT compared to 14 DAT in 1990. Maximum concentrations of dithiopyr and pendimethalin in soil at 0 to 5 cm deep were 20 and 56 μg kg−1, respectively, measured 7 DAT in 1991. Herbicide concentration 5 to 10 cm deep was ≤ 5 μg kg−1 at all sampling dates in 1990. In 1991, dithiopyr and pendimethalin concentrations 5 to 10 cm deep did not exceed 4 and 20 μg kg−1, respectively, measured within 1 DAT. Neither herbicide was detected 10 to 20 cm deep, nor in samples collected 30 cm outside of the experimental plots in either year. At 126 DAT (final sampling date), little herbicide was detected in verdure, but residues were found in most thatch and all mat samples. The estimated time for a 50% reduction in detectable residues (DT50) of dithiopyr and pendimethalin from the sampling zone was 35 and 23 days, respectively.

Field experiments were conducted in 1989 and 1990 to evaluate the tolerance of ‘Penncross' creeping bentgrass and ‘Tifway’ bermudagrass fall-overseeded with perennial ryegrass to irrigation water containing simazine. Creeping bentgrass was more sensitive to simazine than the bermudagrass/ryegrass mixture. The predicted critical simazine concentration necessary to reduce creeping bentgrass quality ≤ 60% was 0.22 mg L–1 which corresponded to a cumulative simazine rate of 98 g ha–1. For bermudagrass/ryegrass, the predicted critical simazine concentration and cumulative simazine rate that reduced quality ≤ 60% was 0.68 mg L–1 and 306 g ha–1, respectively.

The metabolism of the herbicide 14C-glufosinate (GA) was studied in excised shoots and leaves of 20 weed and nonweed species; GA was fed through the xylem. After 24 or 48 h of incubation, the plant material was examined for phytotoxic symptoms, analyzed by autoradiography, and extracted. The extract was cleaned up and analyzed by high-performance liquid chromatography. GA and its metabolites were identified by cochromatography with authentic 14C-labelled reference compounds. 14C-GA was rapidly absorbed by the excised plant parts. Most of the radioactivity (91.3 to 99.7%) in the shoots and leaves was extractable. The main metabolite observed with all species was 3-(hydroxymethylphosphinyl)propionic acid (MPP); lower amounts of 2-hydroxy-4-(hydroxymethylphosphinyl)butanoic acid (MHB) were also found in 14 species. Metabolic rates of GA varied considerably ranging between 13.1 and 2,836.5 ng GA h−1 mg−1 protein. The species with the highest rates of metabolism of GA were Galium verum (2,836.5 ng GA h−1 mg−1 protein), Lythrum hyssopifolia (846.0 ng GA h−1 mg−1 protein), and Digitalis purpurea (494.8 ng GA h−1 mg−1 protein). The mean value across all species was 275.9 ng GA h−1 mg−1 protein.

Eighteen Lolium spp. (ryegrass) accessions collected in 1998 from several locations in Arkansas were tested for resistance (R) to diclofop in both seed and whole-plant response bioassays. Eleven accessions were L. temulentum and eight were L. perenne. Fourteen of eighteen accessions were confirmed resistant to diclofop in whole-plant assay. Three of the susceptible (S) accessions were L. temulentum. The GR50 (diclofop concentration that reduced shoot or root length by 50%) R/S ratios based on whole-plant response were greater than those of the seed bioassay in all test populations, indicating that the whole-plant bioassay was more sensitive than the seed bioassay for determining diclofop resistance in Lolium spp. The most resistant (#18) and most susceptible (#3) accessions of L. temulentum were used for multiple resistance and enzyme assay experiments. Based on whole-plant bioassay, accession #18 was 411 times more resistant to diclofop than the susceptible accession #3. Accession #18 exhibited cross-resistance to fenoxaprop and multiple resistance to chlorsulfuron applied preemergence or postemergence. Resistance to other herbicide families was not observed. Resistance to chlorsulfuron was not detected in the seed bioassay. Acetyl-CoA carboxylase (ACCase) from accession #18 was 833 times more resistant to diclofop and 10 times more resistant to sethoxydim than ACCase from accession #3. Cross-resistance to sethoxydim was not observed at the whole-plant level. Resistance to diclofop among Lolium spp. from Arkansas may be due to an alteration in the target enzyme, ACCase.