Glyphosate-based, ready-to-use weed control products frequently contain diquat (typically, 0.04 by weight relative to glyphosate) under the supposition that the diquat, “makes glyphosate work faster.” However, in light of the known modes of actions of glyphosate and diquat, we hypothesize that diquat may be antagonistic to glyphosate activity. Greenhouse experiments using longstalked phyllanthus were conducted to test this hypothesis. Glyphosate was applied at a series of rates, ranging from 0.11 to 1.12 kg ae/ha, either alone or tank-mixed with either 0, 0.02, 0.04, and 0.06 diquat. Onset of visual injury was more pronounced with the glyphosate + diquat tank mixtures compared with glyphosate alone. However, long-term control, as expressed by regrowth suppression, was greater with glyphosate alone. Regression analysis indicated that, at marginally effective glyphosate rates, the amount of glyphosate must be increased by approximately 60% to compensate for the diquat-based antagonism. Absorption and translocation studies using 14C-glyphosate revealed that the antagonism of diquat toward glyphosate can be attributed to reduced translocation of absorbed glyphosate.

Sulfosulfuron was recently registered for grassy weed control in creeping bentgrass, but turf sensitivity is a concern for intensively managed golf courses. Field and growth chamber experiments in New Jersey investigated creeping bentgrass growth responses and tolerance to sulfosulfuron. Creeping bentgrass chlorosis increased with sulfosulfuron rate but turf had less chlorosis from sequential sulfosulfuron applications compared to bispyribac–sodium. Herbicide-treated turf had similar root weight compared to untreated turf on six sampling dates. In growth-chamber experiments, creeping bentgrass treated with sulfosulfuron had chlorosis and clipping weight reductions exacerbated by reductions in temperature from 25 to 15 C. Overall, creeping bentgrass appears to tolerate sequential sulfosulfuron applications better than or comparable to bispyribac-sodium in early summer, whereas creeping bentgrass sensitivity to sulfosulfuron increases at cooler temperatures.

Studies were conducted at eight sites during a 3-yr period in Oklahoma and Arkansas to determine the effectiveness and safety of preemergence applications of halosulfuron both alone and in tank mixtures with bensulide, clomazone, ethalfluralin, and naptalam. Ethalfluralin, naptalam plus bensulide, and sulfentrazone also were applied alone. Although halosulfuron caused up to 20% seedling stunting, watermelon plants recovered by 5 to 7 wk after planting, and yield was similar to that of hand-weeded plots. Halosulfuron treatments controlled hophornbeam copperleaf, Palmer amaranth, carpetweed, and cutleaf groundcherry 80 to 100%. Control of goosegrass was at least 97% with clomazone plus ethalfluralin plus halosulfuron. Injury to watermelon treated with sulfentrazone ranged from 76 to 98% at 2 to 4 wk after treatment. This was reflected by yields that were lower than any other herbicide treatment in the studies.

The role of preplant glyphosate applications and residual herbicides in the efficacy of glyphosate for weed management in double-crop no-till glyphosate-resistant soybean (GRS) was investigated in the coastal plains of Mid-Atlantic United States. The experiment had a two- by two- by five-factorial treatment structure laid out in three or four randomized complete blocks at research centers in Delaware and New Jersey. The factors investigated were preplant weed management: preplant or no preplant glyphosate applications; postemergence (POST) herbicide treatments: 0.8 kg ae/ha glyphosate alone or 0.8 kg/ha glyphosate tank-mixed with 0.6 kg ai/ha clomazone plus 0.07 kg ai/ha imazethapyr; and GRS growth stage at herbicide application which ranged from cracking, 5 to 8 d after planting, (DAP) to the V6 stage (35 DAP). Preplant glyphosate applications did not influence the efficacy of POST glyphosate applications alone or with the residual herbicides. Glyphosate alone or with clomazone plus imazethapyr provided excellent control of horseweed and fall panicum irrespective of the time of herbicide application from GRS at cracking to the V6 stage. With other weed species, residual herbicide influence varied with year, weed species, and GRS growth stage at herbicide application. Generally, glyphosate alone was most effective when applied at the V2 to V6 stages (16 to 35 DAP). A tank-mix of glyphosate with clomazone plus imazethapyr extended this window to include applications at GRS cracking and the V1 stage. Herbicide treatments were safe on GRS at all stages of application up to the V6 stage (35 DAP).

Field experiments were conducted to study the effects of various tillage and mulching practices on fruit maturity and weed suppression in pumpkins. Conventional tillage (CT), disking, no tillage with rye removed (RR), no tillage with standing rye (SR), and strip tillage (ST) were evaluated with and without ethalfluralin plus halosulfuron (1.5 plus 0.036 kg ai/ha, respectively) applied preemergence. In 2001, when heavy rain after herbicide application caused significant crop injury, the herbicides delayed maturity and significantly reduced yields of mature pumpkins within each herbicide treatment, total yields did not differ with tillage. In 2002, weed populations were significantly greater than those in 2001, and in 2002, regardless of herbicides, yields of mature fruit were greater in tillage treatments with higher rye residues (SR, ST). Although weed populations were less in one year than the other, herbicides provided effective control in both seasons, and RR, ST, and SR effectively suppressed weeds compared with CT. Averaged over treatments, greater yield losses were attributable to weed competition (42%) in 2002 than to herbicide injury (32%) in 2001.

The effect of weather variation on pesticide losses was estimated with the Erosion-Productivity Impact Calculator (EPIC) model. Weather variations had little effect on pesticide loss from a hypothetical site near Memphis, TN, but the effect was more dramatic and in the expected direction at Des Moines, IA. Atrazine losses at Des Moines were reduced by lowering relative humidity or rainfall intensity. Increasing the CO2 level from 300 to 660 ppm slightly increased atrazine losses. Results from these two sites are very limited and only serve to demonstrate modeling potential for addressing weather/pesticide problems. Further, more comprehensive studies are needed to better estimate pesticide loss sensitivity to weather variation.

Recent trends favoring organic and sustainable turfgrass management practices have led to an increased desire for biologically based alternatives to traditional synthetic herbicides. Thaxtomin A, produced by the bacterium Streptomyces scabies, has been reported to have PRE efficacy on broadleaf weeds, but efficacy of thaxtomin A on annual grassy weeds and safety to newly seeded cool-season turfgrasses have not been reported. Field experiments were conducted to evaluate PRE efficacy of thaxtomin A on smooth crabgrass and annual bluegrass. Monthly applications of thaxtomin A from April to July controlled smooth crabgrass through July but did not provide season-long control equivalent to an industry standard PRE herbicide. An initial application of thaxtomin A at 380 g ai ha−1 followed by two applications at 190 or 380 g ha−1 at 4-wk intervals provided season-long annual bluegrass control similar to an industry standard PRE herbicide. At 380 g ha−1, thaxtomin A reduced tall fescue and perennial ryegrass cover when applied 1 wk before seeding, at seeding, or 1 wk after seeding but was safe at other application timings. Up to three applications of thaxtomin A at 380 g ha−1 at 4-wk intervals did not reduce perennial ryegrass cover. Applications to creeping bentgrass resulted in unacceptable turfgrass injury. These results suggest that thaxtomin A can suppress annual grassy weeds in tall fescue or perennial ryegrass turf when applied at least 2 wk before or after seeding. Furthermore, repeated applications of thaxtomin A can provide effective PRE control of annual bluegrass during overseeded perennial ryegrass establishment.

Adoption of conservation tillage practices has been slow in organic vegetable production, partially due to producers’ concerns regarding weed management. Integrating cover crops into a conservation tillage program may provide organic producers a viable weed management option enabling growers to practice conservation tillage. A four-year study was conducted to evaluate the influence of different tillage methods (two conventional and two conservation practices) jointly with a mixed winter cover crop for weed suppression, time required for hand weeding, and crop yield in organically managed eggplant (2012 and 2014) and sweet corn (2013 and 2015) production systems. Tillage treatments were conventional tillage without surface mulch (CT-BG) and with black polyethylene (plastic) mulch (CT-BP), strip-tillage (ST), and no-tillage (NT) with cover crop residue. At 2 and 7 WAT/P (weeks after transplanting/planting), intra-row weed density was higher in CT-BG and ST, and inter-row weed density was higher in CT-BG and CT-BP treatments. Time required for hand-weeding was greatest in CT-BG and least in CT-BP and NT treatments. Eggplant yield was lowest in NT treatment in 2012 but similar among treatments in 2014. Sweet corn yield was similar among treatments in 2013 but highest in ST in 2015. Though both CT-BP and NT treatments showed greater potential for weed suppression, production input was highest in CT-BP but least in NT. Implications of these findings suggest that there is a potential to use strip tillage integrating with stale seedbed tactic for weed management in organic vegetables, which reduces herbicide use, hand-labor, and overall weed management cost while maintaining high yield potential.

Chinese sprangletop, a C4 species, is one of the most important grass weeds of seeded rice in Asia. Chinese sprangletop biology was studied by growing it alone and in competition with 4 and 12 rice plants. Rice competition did not affect the height of Chinese sprangletop, and the weed grew taller than rice, regardless of the competition. Compared with Chinese sprangletop grown alone, competition from rice reduced Chinese sprangletop leaf number, leaf production rate, tiller number, tiller production rate, leaf area, shoot biomass, relative growth rate, and net assimilation rate. Leaf area and shoot biomass of Chinese sprangletop when grown in competition with 12 rice plants was only 16% and 13%, respectively, of the leaf area and biomass of the weed grown alone.

Diflufenzopyr is an auxin-transport inhibitor that can increase the phytotoxicity of certain auxin-mimicking herbicides such as dicamba on broadleaf species. Dicamba is commonly used alone and in combination with other auxin herbicides for broadleaf weed control in various species of turfgrass. Dicamba efficacy applied over a series of rates either alone or as an admixture with either 20 or 40% by weight of diflufenzopyr relative to the weight of dicamba was evaluated on purple cudweed and common lespedeza. The 20% admixture reduced the LD50 of dicamba on purple cudweed from 23 to 20 g/ha. Similarly, LD50 on common lespedeza was reduced from 36 and 27 g/ha. The 20% admixture was 13 and 25% more active than dicamba alone for these two weed species, respectively. However, the synergistic benefit was limited to a relatively narrow range of rates that are below the minimal registered rate of dicamba. Turfgrass injury, as expressed by the suppression of foliage growth, was similar whether dicamba was applied alone or with diflufenzopyr for all species evaluated except St. Augustinegrass. The admixture was less injurious than dicamba alone in St. Augustinegrass. The synergistic benefit with respect to weed control was obtained without a corresponding increase in injury on the turfgrasses.

Field trials were conducted during 2006 and 2007 and a container study was performed twice in 2007 at the Dean Lee Research and Extension Center in Alexandria, LA to evaluate the interaction of glyphosate and zinc coapplied to selected weeds. Across all experiments, no differences in either visible weed control or weed fresh weight were detected among glyphosate formulations. In the field studies, weed control was greatest when glyphosate was applied alone, in which case control of barnyardgrass, browntop millet, and Palmer amaranth ranged between 93 and 95%. When glyphosate was coapplied with formulations of zinc, control of the aforementioned weed species was reduced to 39, 39, and 45%, respectively. Visual estimates of weed control in the container studies showed glyphosate performance to be the highest (82 to 98%) in the absence of zinc for control of barnyardgrass, browntop millet, johnsongrass, ivyleaf morningglory, and redroot pigweed. Across all weed species, control was reduced 43 to 59% when zinc was coapplied with glyphosate. Similar results were noted in reduction of weed fresh weights. Results indicate that glyphosate-based weed control is reduced when coapplied with the zinc products at their current use rates. Producers should be aware of this antagonism and these coapplications should not be recommended.

Two biotypes of field bindweed differing in their susceptibility to glyphosate were used to determine if diluent or carrier volume and additional surfactant could overcome differences in intraspecific response to glyphosate. In greenhouse studies, glyphosate (formulated product) was applied at 1.68 kg/ha in three diluent volumes (142, 189, and 237 L/ha), with and without 1 % (v/v) additional amphoteric surfactant. Nonparametric and ordinal categorical analyses indicated that field bindweed biotype, diluent volume, and surfactant significantly increased glyphosate phytotoxicity 7 DAT. Only biotype and volume were significant 21 DAT. The tolerant biotype was less injured at the 189 and 237 L/ha volumes than the susceptible biotype. Field bindweed injury was similar at a diluent volume of 142 L/ha for both biotypes. These greenhouse studies suggest that control of field bindweed may be improved with glyphosate by using low spray volume in concert with additional surfactant.

Field-studies were conducted over two years to determine the area of influence and duration of interference of wild poinsettia in soybean. Soybean canopy width averaged across years was reduced approximately 10% beginning at 6 wk of interference for both the 0- to 10-and 10-to 20-cm distances from the weed. Soybean dry weights decreased from 14 to 38% within 20 cm of the weed for 12 through 18 wk of interference. In 1990, soybean yield within 10 cm of wild poinsettia was similar to distances of 10 to 20 and 20 to 40 cm, but was less than that for distances greater than 40 cm from the weed. Weed interference resulted in a 9.5% yield reduction in the 0- to 10-cm distance when compared with the 80-to 100-cm distance. In contrast, yield of soybean in 1991 growing within 10 cm of the weed was less than at greater distances, corresponding to an 18% yield reduction when compared with 80 to 100 cm. Differences in wild poinsettia dry weights when growing alone and when growing within the soybean row occurred after 6 and 8 wk of interference in 1990 and 1991, respectively. In 1991 when rainfall during the growing season was twice that of the previous year, canopy width and dry weight of wild poinsettia growing within the soybean row were reduced an average of 57 and 82%, respectively, compared with weeds growing alone.

Field and greenhouse studies examined glyphosate and several ACCase-inhibitor herbicides for control of wirestem muhly. A field study evaluated application rate and timing for control in glyphosate-resistant soybean. Herbicides were applied at two rates, 4 and 6 wk after planting (WAP) of soybean when wirestem muhly was about 30 and 50 cm tall, respectively. A complimentary greenhouse experiment with the same herbicides examined wirestem muhly rhizome viability 30 days after herbicide application. A second field study examined optimum glyphosate application timing for control of wirestem muhly during a fallow rotation period. Glyphosate was applied at 14-day intervals from mid-May through early October. In the first field study, glyphosate, fluazifop, and clethodim provided better control of wirestem muhly than sethoxydim and quizalofop. Clethodim, fluazifop, and glyphosate provided 74 to 92% control over both rates and application timings, whereas quizalofop and sethoxydim were the least effective, providing only 52 to 64% control regardless of rate or timing. In the greenhouse, glyphosate was the most effective herbicide, providing at least 97% control and an 87% reduction in shoot biomass; sethoxydim and fluazifop were the least effective herbicides. All of the herbicides reduced rhizome viability compared to nontreated plants up to 71%, but no difference between herbicides was observed. In the second field study, end-of-season ratings revealed that glyphosate provided 95 to 100% control of wirestem muhly from applications made between late June and early September. The year following application, glyphosate provided 91 to 99% control of wirestem muhly from applications made between mid-June and early September. Applications earlier than mid-June or after early September resulted in less than 90% control. Overall, results show that (1) glyphosate was the most effective herbicide; (2) the ACCase-inhibitor herbicides differ in their effectiveness; and (3) rate and application timing can impact control.

Greenhouse studies determined the dose-responses of cheat, downy brome, Japanese brome, jointed goatgrass, and winter wheat to preplant-incorporated MON 37500 and its residual effects on kochia. Concentrations of MON 37500 up to 60 ppbw did not affect winter wheat. MON 37500 did not prevent weed emergence, but increasingly inhibited weed growth as the dose was increased up to about 20 ppbw. GR50 values were 16, 16, 11, and 31 ppbw for cheat, downy brome, Japanese brome, and jointed goatgrass, respectively. Japanese brome was more susceptible than cheat or downy brome, and jointed goatgrass tolerated two to three times more MON 37500 than the Bromus species. Plant dry weights of kochia seeded after removal of the winter annual grasses decreased with increasing initial MON 37500 concentrations up to 20 ppbw. Kochia density was influenced by which winter annual grass was grown previously.

Field trials evaluated sequential applications of plant growth retardants (PGRs) to bahiagrass for a 3-yr period at two Florida locations: Newberry (north Florida) and Okeechobee (south Florida). Seven PGRs from various chemical families were used: glyphosate, fluazifop, sethoxydim, maleic hydrazide, trinexapac-ethyl, imazapic, and imazaquin. In 1991 and 1992, PGRs reduced turf color for 2 to 4 wk after treatment (WAT) at the Newberry location but remained above acceptable levels (> 5.0) and recovered by 6 WAT. In 1993, glyphosate, fluazifop, and maleic hydrazide reduced turf color below acceptable levels (< 5.0) 4 WAT, but turf color was acceptable by 6 WAT. Turf color was slightly reduced following single and sequential imazapic applications in Okeechobee; however, turf recovered by 8 WAT. Cumulative seedhead suppression at both locations increased during the 3 yr resulting in at least a 60% reduction in seedhead numbers. Seedhead production each year was 10 to 50% less at 12 WAT in treated plots than in the previous year. Single applications of imazapic and imazapic + imazaquin provided 85 to 100% seedhead suppression for 12 WAT and 100% suppression for up to 16 wk following sequential applications made 8 wk after the initial application (WAI). Single applications of glyphosate, fluazifop, and maleic hydrazide provided ≥ 70% seedhead suppression for 4 to 8 WAT. Sequential applications of fluazifop made 8 WAI provided > 70% control of seedheads through 8 to 16 WAI for 3 yr.

Field studies were conducted in 1992 and 1993 to determine the effect of atrazine and pendimethalin on velvetleaf competitiveness in corn. The three factors included in this research were herbicide, velvetleaf density (zero or nine plants per meter of row), and treatment (treated with herbicide or untreated). Herbicide treatments consisted of atrazine (0.6 and 1.1 kg/ha) or pendimethalin (0.6 and 1.1 kg/ha) applied preemergence. In 1992 and 1993, both atrazine and pendimethalin applied at 1.1 kg/ha greatly reduced velvetleaf growth and seed production and prevented velvetleaf from reducing corn yield. Both atrazine and pendimethalin applied at 0.6 kg/ha reduced velvetleaf growth and seed production in 1993, but only atrazine prevented yield loss in 1992. In the absence of velvetleaf, corn yield was not affected by either herbicide. These results indicate that sublethal exposure to atrazine or pendimethalin can significantly reduce velvetleaf competitiveness in corn.

Field studies were conducted in 1999, 2000, and 2001 to evaluate broadleaf weed control in glyphosate-resistant cotton by glyphosate plus CGA 362622 applied postemergence. Treatments included 560 and 1,120 g ai/ha glyphosate-isopropylamine alone or in mixtures with CGA 362622 at 3.8 and 7.5 g ai/ha, and CGA 362622 at 7.5 g/ha alone. Cotton injury 7 d after treatment (DAT) was 3 to 11% from glyphosate alone and 16 to 24% from glyphosate plus CGA 362622. Injury 28 DAT with CGA 362622 or herbicide mixtures did not exceed 6%. Broadleaf weed control by herbicide mixtures was generally more consistent than control from either herbicide applied alone. Glyphosate plus CGA 362622 controlled common cocklebur and smooth pigweed better than glyphosate alone. In most instances, the mixtures also controlled common ragweed, common lambsquarters, ivyleaf morningglory, pitted morningglory, and tall morningglory better than glyphosate applied alone. Common cocklebur and smooth pigweed were controlled at least 85% by all treatments. CGA 362622 did not control spurred anoda or jimsonweed. Cotton yields generally reflected weed control. According to these results, glyphosate plus CGA 362622 mixtures can consistently control many broadleaf weeds in cotton.

The use of triclopyr for the removal of woody and broad-leaf vegetation in right-of-ways and agricultural settings has been proposed for Alaska. Triclopyr concentrations in soil after application are of concern because residual herbicide may affect growth of subsequent vegetation. In order to measure triclopyr residues in soil and determine the amount of herbicide taken up by the plant, soil bioassays were developed. Four agricultural species, turnip, lettuce, mustard, and radish, were tested to determine sensitivity to triclopyr in a 1-wk bioassay. The sensitivity (I 50) of turnip, lettuce, mustard, and radish was 0.33 ± 0.05 kg ai ha−1, 0.78 ± 0.11 kg ai ha−1, 0.78 ± 0.07 kg ai ha−1, and 0.85 ± 0.10 kg ai ha−1 (mean ± SE), respectively. Mustard was the most consistent crop in the bioassay with a midrange response to triclopyr and lowest standard deviation for germination as compared to the other species. Thus, it was used in a bioassay to determine triclopyr concentrations in a field trial. The bioassay of mustard closely matched residual amounts of triclopyr in a field trial determined by chemical extraction. Estimates of residual triclopyr concentrations using the bioassay method were sometimes less than the triclopyr concentration determined using a chemical extraction. These differences in concentrations were most evident after spring thaw when the chemical extraction determined there was enough triclopyr in the soil to reduce mustard growth over 60%, yet the bioassay measured only a 10% reduction. The chemical extraction method may have identified nonphototoxic metabolites of triclopyr to be the herbicidal triclopyr acid. These methods, when analyzed together with a dose–response curve, offer a more complete picture of triclopyr residues and the potential for carryover injury to other plant species.

Annual, cylindric, and globe sedges were controlled > 90% with a single application of MSMA at 2.2 kg ai/ha in field studies. But this same treatment controlled fragrant and green kyllingas only 69 and 52%, respectively. Control was increased to 82 and 81%, respectively, with a repeat application. Other postemergence-applied (POST) herbicides evaluated included bentazon, halosulfuron, imazapic, imazaquin, and CGA-362622. Postemergence-applied herbicides were applied either once or twice, as well as alone and in combination with MSMA. In general, a sequential application of MSMA, either alone or in combination with any of the aforementioned herbicides, except bentazon, provided maximum control of the sedge and kyllinga species evaluated. Preemergence-applied (PRE) oxadiazon and S-metolachlor, controlled annual sedge ≥ 94% at 7 wk after treatment (WAT) in field studies and 96 and 70% at 9 WAT, respectively. Dithiopyr and prodiamine provided 86 to 80% control of annual sedge over the 9-wk rating period. In a hydroponic-type laboratory study, oxadiazon and S-metolachlor were more effective than atrazine, bensulide, imazaquin, oryzalin, or simazine, in reducing seedling development of annual, cylindric, and globe sedges, and green kyllinga.