Palmer amaranth is the latest pigweed species documented in Connecticut; it was identified there in 2019. In a single-dose experiment, the Connecticut Palmer amaranth biotype survived the field-use rates of glyphosate (840 g ae ha−1) and imazaquin (137 g ai ha−1) herbicides applied separately. Additional experiments were conducted to (1) determine the level of resistance to glyphosate and acetolactate synthase (ALS) inhibitors in the Connecticut-resistant (CT-Res) biotype using whole-plant dose-response bioassays, and (2) evaluate the response of the CT-Res biotype to POST herbicides commonly used in Connecticut cropping systems. Based on the effective dose required for 90% control (ED90), the CT-Res biotype was 10-fold resistant to glyphosate when compared with the Kansas-susceptible (KS-Sus) biotype. Furthermore, the CT-Res biotype was highly resistant to ALS-inhibitor herbicides; only 18% control was achieved with 2,196 g ai ha−1 imazaquin. The CT-Res biotype was also cross-resistant to other ALS-inhibitor herbicides, including chlorimuron-ethyl (13.1 g ai ha−1), halosulfuron-methyl (70 g ai ha−1), and sulfometuron-methyl (392 g ai ha−1). The CT-Res Palmer amaranth was controlled 75% to 100% at 21 d after treatment (DAT) with POST applications of 2,4-D (386 g ae ha−1), carfentrazone-ethyl (34 g ai ha−1), clopyralid (280 g ae ha−1), dicamba (280 g ae ha−1), glufosinate (595 g ai ha−1), lactofen (220 g ai ha−1), oxyfluorfen (1,121g ai ha−1), and mesotrione (105 g ai ha−1) herbicides. Atrazine (2,240 g ai ha−1) controlled the CT-Res biotype only 52%, suggesting the biotype is resistant to this herbicide as well. Here we report the first case of Palmer amaranth from Connecticut with multiple resistance to glyphosate and ALS inhibitors. Growers should proactively use all available weed control tactics, including the use of effective PRE and alternative POST herbicides (tested in this study), for effective control of the CT-Res biotype.

Field studies were conducted in 1988 and 1989 to examine the interrelations of cultivation and herbicide band width in controlling pitted morningglory and sicklepod in soybean. Alachlor + imazaquin PRE followed by imazaquin + surfactant POST were not applied or were applied on bands 15, 30, 45, 60, 75 cm wide or were broadcast. Plots were cultivated zero, one, two, or three times. Without cultivation, at least a 60-cm wide band was needed to achieve maximum soybean seed yields. A slight linear increase in soybean seed yield in response to increasing band width was observed with one cultivation. With two cultivations, soybean seed yields were similar with and without herbicides, but a 15-cm wide herbicide band was needed to achieve maximum production with three cultivations. Greatest gross returns exclusive of weed management costs were realized with two or three cultivations and a 15-cm wide band.

Greenhouse experiments were conducted to evaluate effects of sicklepod planting depths and chlorimuron, imazaquin, and metribuzin incorporation depths on sicklepod seedling emergence and growth. There was 35 and 90% sicklepod seedling emergence from 0- and 2-cm planting depths, respectively, in untreated soil 26 days after planting; emergence decreased with each additional increment below 2 cm. Chlorimuron at 63 g ai ha reduced the number of plants present 11 days after planting at 1- and 2-cm planting depths except when unincorporated. However, deeper sicklepod planting required chlorimuron incorporation to at least 6 cm for maximum reduction of plants. Imazaquin at 140 g ai ha was not as effective as chlorimuron, but a reduction in the number of plants was noted at 1- and 2-cm planting depths when imazaquin was incorporated to 4, 6, or 8 cm. Metribuzin at 420 g ai ha consistently reduced the number of plants at all incorporation depths except when unincorporated but had little effect on emergence. Chlorimuron and imazaquin reduced sicklepod dry weight most for all planting depths when incorporated 4 to 10 and 6 to 10 cm, respectively. Metribuzin reduced sicklepod biomass at all incorporation depths.

In field studies trifluralin and pendimethalin at 1.7 kg ai ha-1 applied PPI controlled itchgrass 87 and 78%, respectively, in late season when averaged over a 2-yr period. Clomazone at 1.1 kg ai ha-1 applied PRE to the soil surface in one year controlled 80% of itchgrass compared with 51% when applied PPI. POST application of 0.15 kg ai ha-1 fluazifop-P, 0.21 kg ai ha-1 haloxyfop, 0.10 kg ai ha-1 quizalofop, and 1.1 kg ai ha-1 diclofop controlled at least 90%. Itchgrass control was less effective with 0.43 kg ai ha-1 sethoxydim and 0.07 kg ai ha-1 clethodim POST and poor with 0.15 kg ai ha-1 imazaquin and 0.07 kg ai ha-1 imazethapyr.

Broomrape is a severe problem in the continuous cropping of bidi tobacco in India. Fifteen herbicides were evaluated for postemergence control, at different dates and rates, in field experiments in Nipani, Karnataka State. The effects of the herbicides on Orobanche control and growth and yield of tobacco were compared to the effects of handweeding and an untreated check. Following this screening experiment, four herbicides were selected for further testing. Glyphosate at 500 g ai/ha applied at 60 days after transplanting (DAT) and imazaquin at 10 g ai/ha applied at 30 DAT reduced the number of aboveground broomrape spikes by 75 to 80% and increased the tobacco yields by 80 to > 100% compared to the untreated plots. Imazethapyr and EPTC were found to be less effective for broomrape control, but they still led to substantially higher tobacco yield. None of the herbicide treatments resulted in yields higher than those in the hand-weeded plots. These plots probably did not yet reflect the potential yield of tobacco under broomrape-free conditions, due to nonemerged spikes. Toxicity to tobacco of all herbicides was higher for the higher rates tested. It was lowest with imazaquin.

Field trials were conducted to evaluate weed control and crop safety with postemergence-applied herbicides in two cultivars of low-tannin seedling sericea lespedeza (Lespedeza cuneata). Two applications of either 2,4-D amine at 0.28 kg ae/ha or 2,4-DB amine at 0.56 kg ae/ha controlled sicklepod (Senna obtusifolia) ≥ 90%. Equivalent control of smallflower morningglory (Jacquemontia tamnifolia) required either two applications of 2,4-D or three of 2,4-DB. Three applications of either herbicide were required for comparable spotted spurge (Euphorbia maculata) control. Sericea lespedeza was not adversely affected when either herbicide was applied up to three times per season over two consecutive seasons. Tolerance to 2,4-D and 2,4-DB was equal between two sericea lespedeza cultivars. Absorption of foliar-applied 14C-2,4-D was greater in sicklepod and smallflower morningglory than in sericea lespedeza. Translocation of absorbed 14C-2,4-D was equivalent between sicklepod and sericea lespedeza but greater in smallflower morningglory than in sericea lespedeza. Absorption and translocation of foliar-applied 14C-2,4-DB was equivalent between sicklepod and sericea lespedeza. Furthermore, 14C-2,4-DB absorption was less in smallflower morningglory than in sericea lespedeza.

Torpedograss has invaded fine turfgrasses in the Southern U.S. due to its tolerance to most commonly used turfgrass herbicides. Research was conducted to determine torpedograss and ‘Tifway’ bermudagrass tolerance to quinclorac and tank-mix combinations with other turf herbicides. Quinclorac rates in excess of 2.2 kg ai/ha were required for control (≥ 80%) of torpedograss. Sequential applications were more effective than single applications. Generally, sequential applications made 3 wk after the initial were more effective than delaying sequential applications until 4 wk after the initial application. Tifway bermudagrass injury from quinclorac applied at 4.5 kg/ha, or at 2.2 kg/ha followed by a sequential application of 1.1 or 2.2 kg/ha, was observed at 4 to 10 wk; however, turf recovered thereafter. Sequential applications of quinclorac plus imazaquin (1.1 + 0.6 kg/ha) increased torpedograss control (approximate 35%) as compared to quinclorac plus MSMA (1.1 + 1.1 kg/ha) but also decreased turf quality versus quinclorac applied alone or in combination with MSMA. Quinclorac applied at 2.2 kg/ha followed by 1.1 kg/ha applied 3 or 6 wk after the initial controlled torpedograss best with least reduction in bermudagrass quality. Quinclorac is the first herbicide reported to selectively control torpedograss in bermudagrass turf.

Studies were established in 1988, 1989, and 1990 at two locations in Missouri to study the influence of fall tillage and herbicides on carryover of several residual soybean herbicides to grass and legume forage crops. Chlorimuron, clomazone, imazaquin, imazethpyr, and metribuzin plus chlorimuron were applied at their registered and 2X-registered rates in soybeans. Forage crops were planted the following fall and spring after herbicide application and evaluated for carryover effects. Fall tillage did not influence the carryover potential of these herbicides. However, herbicides injured several of the rotational crops. This injury was crop species and herbicide specific.

Extension workers are sensing pressure to use soils information and chemical characteristics data to guide farmers in selecting pesticides least prone to leach into groundwater. Our objective was to estimate differences in herbicide migration to groundwater under conditions typical for the Southeast Coastal Plain, and to consider how a farmer might be advised to use such knowledge in selecting herbicides. We used a simple computer code for microcomputers to predict persistence and migration of 17 herbicides through a hypothetical, coarse-textured soil typical of the Southeast Coastal Plain. Appropriate herbicides were selected for several common crop-weed problems, such as sicklepod in soybean and Palmer amaranth in corn. Groundwater was assumed to be 3.15 m below the soil surface. Herbicides selected covered a broad range of half-lives and organic carbon partition coefficients. Only after the first-order degradation rate constant was reduced by a factor of five did predicted soil water concentrations of several herbicides at the groundwater interface reach normal detection limits. Still, predicted concentrations were below the level established for health effects advisory purposes. Due to the large number of uncertainties and the inability to estimate practical benefits, we conclude that data relating to soil and herbicide characteristics cannot be used at this time to override cost effectiveness, efficacy, and other factors normally considered by farmers and Extension professionals in herbicides for weed control.

Quizalofop controlled 2- to 3-leaf and 5- to 6-leaf red rice better than fluazifop-P, haloxyfop, or sethoxydim applied alone. Red rice control increased when acifluorfen was tank mixed with haloxyfop, fluazifop-P, or sethoxydim. Antagonism was most severe when imazaquin was tank mixed with any of the grass herbicides. The efficacy of sethoxydim, fluazifop-P, and haloxyfop was reduced when applied with chlorimuron or bentazon. In greenhouse experiments, all of the grass herbicides, except for fenoxaprop, reduced fresh weight of red rice 71% or more. Based on fresh weight reduction, acifluorfen, fomesafen, and lactofen only reduced the activity of fluazifop-P, while imazaquin decreased the activity of all grass herbicides.

Sicklepod control with seven chemical treatments was evaluated in 25-cm and 97-cm soybean row widths. Metribuzin applied preplant incorporated followed by metribuzin applied preemergence, and metribuzin applied preplant incorporated followed by either imazaquin or chlorimuron applied postemergence controlled sicklepod over 90% early in the season, and row spacing did not influence control at that evaluation. In late season ratings, all herbicide treatments controlled sicklepod better in 25-cm rows than in 97-cm rows. However, only imazaquin applied sequentially or metribuzin followed by chlorimuron in 25-cm rows controlled sicklepod greater than 80% late in the season. Sicklepod control in 25-cm row soybean with either imazaquin or metribuzin plus chlorimuron applied preplant incorporated was equivalent to or better than sequential preemergence and postemergence treatments of these three herbicides in 97-cm rows. All treatments resulted in lower yields than the weed-free check in the 97-cm rows, whereas all herbicide treatments except the sequential application of metribuzin gave yields equivalent to the weed free check in 25-cm rows.

Preplant incorporation of alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] at 3.6 kg ai/ha, metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] at 2.2 kg ai/ha, and tank mixtures of imazaquin {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid} with either alachlor at 0.14 + 2.2 kg ai/ha or metolachlor at 0.14 + 1.1 kg ai/ha controlled >90% red rice (Oryza sativa L. # ORYSA) in soybeans [Glycine max (L.) Merr.]. Also, FMC-57020 [2-(2-chlorophenyl)methyl-4,4-dimethyl-3-isoxazolidinone] at 1.7 kg ai/ha applied preplant incorporated controlled > 80% of the red rice. DPX-Y6202 {2-[4-[(6-chloro-2-quinoxalinyl)oxy]-phenoxy]-propionic acid, ethyl ester} was the best and most consistent postemergence herbicide treatment for red rice control when applied once at 0.28 kg ai/ha or sequentially at 0.14 kg ai/ha each time. Haloxyfop {2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl]oxy] phenoxy] propanoic acid} applied at 0.16 kg ai/ha followed by another application at 0.14 kg ai/ha gave excellent red rice control in 2 of 3 years. Soybeans were uninjured by the herbicide treatments and yield was higher from treated than untreated soybeans.

Greenhouse studies were conducted to evaluate potential interactions among glyphosate mixtures with five acetolactate synthase (ALS)-inhibiting herbicides (chlorimuron, imazamox, imazaquin, MON 12,000, or pyrithiobac) for the control of purple nutsedge and sicklepod at two growth stages. Herbicides were tested alone at 0.5X and 1X rates (1X being suggested use rate for these herbicides) and in combination with glyphosate at 560 (0.5X) and 1,120 (1X) g ai/ha on 3-wk-old plants and at 1,120 g/ha on 6-wk-old plants. Glyphosate alone at 1,120 g/ha gave complete control of purple nutsedge and at least 78% control of sicklepod regardless of growth stage. In 3-wk-old purple nutsedge plants, three of the 20 herbicide combinations were antagonistic and 17 combinations were additive, whereas all five combinations were additive in 6-wk-old plants. In sicklepod, eight combinations were antagonistic and 12 combinations were additive in 3-wk-old plants, and all five combinations were antagonistic in 6-wk-old plants. In 3-wk-old plants, the glyphosate (0.5X) plus imazaquin (0.5X) combination resulted in highest antagonism in purple nutsedge control (79%), and the combination of glyphosate (0.5X) plus imazamox (0.5X) resulted in highest antagonism in sicklepod control (54%). These results indicate that mixing chlorimuron, imazamox, imazaquin, MON 12,000, or pyrithiobac with glyphosate does not increase glyphosate efficacy on purple nutsedge or sicklepod.

Field studies were conducted from 1993 to 1995 to evaluate MON-12051 for turfgrass tolerance and control of yellow and purple nutsedges. The availability of herbicides for selective control of these weeds in turfgrass is limited. A sulfonylurea compound, MON-12051, has recently been developed for selective control of the nutsedges in turfgrass. When MON-12051 was applied at 0.07 to 0.14 kg ai/ha, the injury to Kentucky bluegrass, tall fescue, bermudagrass, and zoysiagrass was slight, with a maximum of 10% injury. At these rates, MON-12051 outperformed both bentazon and imazaquin in controlling yellow and purple nutsedges. Averaged over all tests 6 wk after treatment, yellow nutsedge control with MON-12051 was 83%. Control averaged 44% during the same period when treated with bentazon, whether applied once at 2.24 kg ai/ha or twice at 1.12 kg ai/ha. Purple nutsedge control averaged 96% when treated with MON-12051 in Kentucky bluegrass, while control was 42% with imazaquin applied at 0.19 and 0.43 kg ai/ha.

The effects of chlorimuron, imazaquin, imazethapyr, nicosulfuron, primisulfuron, and thifensulfuron were evaluated on a population of smooth pigweed in Painter, VA with no history of treatment with acetolactate synthase (ALS)-inhibitor herbicides. Imazethapyr and nicosulfuron gave the greatest smooth pigweed control, and subsequently were used in field and greenhouse studies to investigate susceptibility of smooth pigweed and livid amaranth populations to ALS-inhibitor herbicides. Approximately 5 million smooth pigweed plants from Painter were treated with imazethapyr or nicosulfuron from 1992 to 1994 and no ALS-inhibitor-resistant plants were identified. In the greenhouse, the response of smooth pigweed from Painter, VA, Marion, MD, and Oak Hall, VA and livid amaranth from Warren County, NJ to imazaquin or imazethapyr and nicosulfuron was investigated. Smooth pigweed from Marion and Oak Hall and livid amaranth from NJ had histories of treatment with ALS-inhibitors. Painter smooth pigweed control was 81 to 97% by imazethapyr and nicosulfuron while control of the Marion and Oak Hall populations was 3 and 18% by imazaquin at 560 and 1120 g ai/ha, respectively, and control by nicosulfuron at 35 g ai/ha was 50 to 73%. Control of livid amaranth from Warren County, NJ was 8 to 15% by imazethapyr at 560 g ai/ha, and was 30 to 58% by nicosulfuron at 35 g/ha.

Field experiments were established in 1991 and 1992 on silty clay and sandy loam soils to evaluate combinations of non-selective and selective herbicides for stale seedbed soybean weed control. Metribuzin PRE controlled sicklepod and pitted morningglory more consistently than other treatments. At 9 wk after planting, antagonism occurred in most cases on sicklepod control when metribuzin was tank-mixed with a non-selective herbicide. Other selective herbicides required addition of a non-selective herbicide at planting to effectively control sicklepod and pitted morningglory. Sicklepod and pitted morningglory control was better with POST selective herbicides when following glufosinate or paraquat than when following glyphosate or SC-0224. In most instances a follow-up POST treatment was needed to maintain weed control from non-selective herbicides applied PRE. Metribuzin and metribuzin plus chlorimuron increased soybean yields when tank-mixed with paraquat compared with yields obtained with paraquat alone.

Experiments were conducted to evaluate imazaquin and selected herbicides for POST control of wild garlic. At the same dosages, imazaquin applied in late November/early December (late fall) controlled wild garlic better than when applied in early March (late winter) when assessed in May. Imazaquin provided control equivalent to 1120 g ae ha–1 2,4-D plus imazaquin. Imazapyr, imazamethabenz, or imazethapyr at 280, 1120, or 560 g ai ha–1, respectively, provided control equivalent to 560 g ai ha–1 imazaquin. Thifensulfuron at 70 g ai ha–1 provided better control applied in late winter (85%) than late fall (45%). All herbicides consistently provided better control than 2,4-D plus mecoprop plus dicamba, except thifensulfuron applied in late fall, at the final evaluation. Common bermudagrass growth in early spring was delayed 2 mo by ≥ 560 g ha–1 imazaquin, 70 g ha–1 thifensulfuron, imazethapyr, and imazapyr or 2,4-D plus mecoprop plus dicamba applied in late winter. Imazamethabenz did not delay common bermudagrass growth. Late fall applications of 1120 g ha–1 imazaquin did not delay growth while ≥ 560 g ha–1 imazethapyr delayed common bermudagrass growth through April.

Greenhouse experiments were established to investigate the effects of tank-mixing glyphosate, paraquat, or glufosinate with metribuzin plus chlorimuron, imazaquin, or metribuzin on entireleaf morningglory, sicklepod, and johnsongrass control. Antagonism was the most frequent interaction, and usually occurred when the lower rates of non-selective foliar-active herbicides were used in tank mixtures with selective soil-active herbicides. Antagonism occurred on all species when 180 g ai/ha paraquat was tank-mixed with 90 g ai/ha metribuzin plus 15 g ai/ha chlorimuron. When the rates of non-selective herbicide were increased, antagonism was usually overcome. Antagonism also occurred on entireleaf morningglory control when 210 g ai/ha glyphosate was tank-mixed with 90 g/ha metribuzin plus 15 g/ha chlorimuron or 36 g ai/ha imazaquin. When lower rates of paraquat or glufosinate were tank-mixed with 210 g/ha metribuzin, antagonism also occurred. Less antagonism was noted with glufosinate.

Field studies were conducted at the Main Experiment Station, Fayetteville, AR, in 1994 and 1995 to evaluate the effect of postemergence herbicides applied at late-season timings on sicklepod seed production and viability. AC 263,222, dicamba, glyphosate, glufosinate, and paraquat were applied at five rates (zero; labeled rate; one-half, one-fourth, and one-eighth the labeled rate) to three sicklepod growth stages (bud formation, flowering to 9-cm pods, and 15- to 30-cm pods). The greatest reduction in seed production occurred when herbicides were applied at the flowering to 9-cm pod growth stage. All herbicides applied at one-half the labeled rate to sicklepod at bud formation and at the flowering to 9-cm pod growth stage reduced seed production greater than 80%, except glufosinate. Regardless of herbicide, rate, or application timing, viability of remaining seed was ≥ 90% with all treatments.

Field and greenhouse studies were conducted in 1995 and 1996 in Kansas to determine the efficacy of 21 herbicide treatments for control of tumble pigweed, Palmer amaranth, redroot pigweed, and two biotypes of common waterhemp in soybean. In field studies, nine of eleven preemergence treatments controlled all four species 90% or more. However, pendimethalin and trifluralin controlled Palmer amaranth, redroot pigweed, and tumble pigweed less than the other preplant incorporated and preemergence treatments. With the exception of flumiclorac and NAF-75, postemergence treatments controlled 75 to 90% of all four species. A biotype of common waterhemp collected in Iowa was not controlled by acetolactate synthase-inhibiting herbicides. Greenhouse results were similar to field experiments. Results suggest at least 90% control of these Amaranthus species is possible with proper herbicide selection.