Field and greenhouse studies were conducted to evaluate mesotrione alone and in combinations with low rates of atrazine and bentazon for control of yellow and purple nutsedge. Mesotrione alone at rates of 105 to 210 g ai/ha controlled yellow nutsedge 43 to 70%. Mixtures of mesotrione with atrazine at 280 g ai/ha did not always improve yellow nutsedge control over that by mesotrione alone, but increasing atrazine to 560 g ai/ha in these mixtures generally provided more consistent control of yellow nutsedge. Mesotrione at 105 g ai/ha mixed with bentazon at 280 or 560 g ai/ha controlled yellow nutsedge 88% or greater which was similar to control from the standard halosulfuron at 36 g ai/ha. Mesotrione, atrazine, and bentazon alone did not control purple nutsedge. Mixtures of mesotrione plus bentazon, however, did improve control of purple nutsedge over either herbicide applied alone, but this control was not considered commercially acceptable.

It has been suggested that trifloxysulfuron might increase the efficacy of asulam for control of johnsongrass. Container and field studies were conducted to determine the efficacy of POST applications of trifloxysulfuron and asulam for johnsongrass control in sugarcane. Asulam was applied at 460 and 920 g ai/ha to container-grown johnsongrass plants, with and without 8 g ai/ha of trifloxysulfuron. Combinations of asulam and trifloxysulfuron generally reduced johnsongrass height, rhizome length, and biomass more than when either was applied alone. Results suggested that combinations of asulam and trifloxysulfuron were synergistic in their control of johnsongrass biomass 8 wk after treatment. In a sugarcane field heavily infested with rhizome johnsongrass, asulam was applied at 1,800, 2,800, and 3,700 g/ha with and without trifloxysulfuron at 16 g/ha. Asulam plus trifloxysulfuron generally controlled johnsongrass more effectively than either herbicide alone. The control of johnsongrass with asulam at 1,800 g/ha resulted in an increase in sugar yield of more than twice that in the nontreated control. Sugar yield increased further when asulam was applied at 2,800 g/ha or combined with trifloxysulfuron, but application of trifloxysulfuron alone did not increase yield. Combinations of asulam and trifloxysulfuron might slow the spread of rhizome johnsongrass enough to allow an increased number of ratoon crops before sugarcane fields need to be replanted.

Research was conducted to evaluate residual activity of herbicides applied postemergence (POST) and preemergence (PRE) for red morningglory control. Atrazine at 2.24 kg ai/ha controlled 30- to 60-cm red morningglory 78% 10 d after treatment (DAT) but control was greater for carfentrazone at 0.035 kg ai/ha, diuron plus hexazinone at 1.57 + 0.44 kg ai/ha, flumioxazin at 0.14 kg ai/ha, and sulfentrazone at 0.32 kg ai/ha (88 to 93%). At 28 DAT control with diuron plus hexazinone, flumioxazin, and sulfentrazone, reflective of both the initial POST control and soil residual activity, was equivalent to that of atrazine at 3.36 kg/ha (92%), but control was 34 to 66% with carfentrazone at 0.009, 0.018, and 0.026 kg/ha, hexazinone at 0.56 kg/ha, pyraflufen at 0.007 and 0.015 kg ai/ha, and trifloxysulfuron at 0.016 kg ai/ha. In another study to evaluate residual control with soil-applied herbicides, red morningglory was controlled at least 87% 35 DAT with atrazine at 2.24 kg/ha, diuron plus hexazinone at 1.57 + 0.44 kg/ha, flumioxazin at 0.14 kg/ha, sulfentrazone at 0.16 kg/ha, and metribuzin at 1.68 kg ai/ha. Control 35 DAT was 78% for atrazine at 1.12 kg/ha, 84% for diuron plus hexazinone at 1.05 + 0.30 kg/ha, and 63% for flumioxazin at 0.07 kg/ha. By 49 DAT only sulfentrazone controlled red morningglory 80% or more and by 63 DAT, control with sulfentrazone at rates of 0.21 kg/ha and higher was 83 to 88%. At 77 DAT sulfentrazone at 0.21 kg/ha controlled red morningglory 78% and no other herbicide treatment provided more than 46% control. In another study red morningglory control did not change from 49 to 63 DAT when sulfentrazone at 0.28 to 0.42 kg/ha was applied PRE following trifluralin preplant incorporated (PPI), but control decreased from 49 to 63 DAT when sulfentrazone was incorporated with trifluralin.

Palmer amaranth accessions were collected from 21 fields in northeastern Arkansas in the fall of 2006 to determine if they differed in response to increasing doses of glyphosate and to determine the survival frequency following treatment with the label rate (870 g ae/ha). The herbicide dose required to kill 50% of individuals in an accession (LD50) ranged from 41 to 339 g/ha glyphosate, with most accessions responding similarly to glyphosate. The AR18 and AR19 were the least-sensitive accessions, with LD50 rates of 312 and 339 g/ha glyphosate, respectively. The mean survival frequency was 2.2% across all accessions when 870 g/ha glyphosate was applied to five- to seven-leaf plants. Sixteen of the accessions had at least one plant survive glyphosate at 870 g/ha. AR18 and AR19 accessions had a survival frequency of 6.3 and 11.8%, respectively. Following an additional cycle of selection with glyphosate, 44.3% of the progeny from the AR19 accession survived glyphosate at 870 g/ha, and its LD50 value significantly increased to 646 g/ha glyphosate. This research shows that there is a low percentage of Palmer amaranth plants currently present in production fields throughout northeastern Arkansas that are capable of surviving a single glyphosate application at the labeled rate and that further selection with glyphosate can increase the frequency of survival.

Experiments were conducted to evaluate sharppod morningglory control with postemergence herbicides used in cotton and to determine the influence of diuron on glyphosate efficacy. Glyphosate plus diuron was one of the most efficacious herbicide treatments in the field experiment, providing up to 78% control of 10- to 20-cm stem length sharppod morningglory. In growth-chamber experiments, mixtures of either 420 or 840 g ai/ha diuron plus glyphosate potassium salt at 840 g ae/ha were needed to significantly reduce sharppod morningglory biomass. Compared to published results with other Ipomoea spp. and field bindweed, sharppod morningglory absorbed more and translocated less glyphosate after 72 h. Retention of glyphosate in treated leaves increased when the glyphosate was mixed with 420 g/ha diuron. Mixture with 420 or 840 g/ha diuron reduced the concentration of glyphosate in roots; however, only 2% of glyphosate alone was translocated to the roots. These results indicate that the combination of glyphosate with diuron improves aboveground sharppod morningglory desiccation, but limits glyphosate translocation.

Weed management in furrow-irrigated corn is challenging because of weed emergence associated with each irrigation event. Residual herbicides that provide extended in-season control of a broad spectrum of weeds would be beneficial to producers in this system. Field experiments were conducted in 2005 and 2006 in Yellowstone County, Montana, to evaluate KIH-485 for the control of velvetleaf, kochia, and wild buckwheat in furrow-irrigated corn. KIH-485 was applied at three rates (166, 209, and 250 g ai/ha) and two timings (PRE and POST) and compared to standard rates of S-metolachlor, acetochlor, and pendimethalin. All PRE treatments were applied alone, whereas POST treatments were combined with 1,261 g ae/ha of glyphosate. All rates of KIH-485 applied PRE controlled velvetleaf and kochia 88% or greater at 4 mo after planting (MAP). Wild buckwheat was controlled 89% or greater with the high rate of KIH-485 applied PRE, which was superior to control achieved with any other PRE herbicide treatment. Velvetleaf, kochia, and wild buckwheat were controlled 91% or greater when any herbicide treatment was combined with glyphosate. Corn treated with KIH-485 applied at 209 g ai/ha PRE produced yield that was similar to that produced by the weed-free control in both years.

Research was conducted along the Texas Gulf Coast in 1998 and 1999 to determine trifloxysulfuron soil persistence and potential injury to corn, grain sorghum, rice, and soybeans. Trifloxysulfuron was applied at 0, 7.5, and 60 g/ha to plots 0, 15, 30, 60, and 90 d prior to planting of crops. Corn and grain sorghum were more sensitive to trifloxysulfuron than rice and soybeans when planted 0 to 90 d after treatment (DAT). Trifloxysulfuron was more persistent at the San Patricio location than at Fort Bend, which had a lower soil pH. However, no phytotoxicity or plant-height reduction was observed at the four locations with corn, grain sorghum, rice, and soybeans planted 209 to 312 DAT. Greenhouse data showed that neither corn nor sunflower planted 209 to 312 DAT were adversely affected by either rate of trifloxysulfuron. Trifloxysulfuron applied to cotton up to 20 g/ha the previous year should not cause phytotoxicity to corn, grain sorghum, rice, or soybeans when grown in rotation under soil and weather conditions similar to those in these studies.

Acetolactate synthase (ALS)–inhibiting herbicides are often used to control Italian ryegrass in winter wheat in Texas. An Italian ryegrass biotype near Waco, TX was evaluated for resistance to mesosulfuron in field and greenhouse experiments. Control of the biotype in the field was less than 10% with the label rate of mesosulfuron (15 g ai/ha). Greenhouse studies confirmed that the biotype was resistant to mesosulfuron; control of the biotype was less than 35% at 120 g ai/ha mesosulfuron. The herbicide dose required to reduce plant biomass of a susceptible and the Waco biotype by 50% (GR50) was 1.3 and 31 g ai/ha, respectively, indicating a resistance level of 24-fold in the Waco biotype. However, the Waco biotype was controlled with the acetyl-CoA carboxylase inhibitors diclofop and pinoxaden.

Rattail fescue infestations are increasing in dryland conservation-tillage winter wheat cropping systems in the inland Pacific Northwest (PNW) region of Idaho, Oregon, and Washington. Rattail fescue typically is controlled with cultivation in conventional tillage farming systems. However, reduced soil disturbance has allowed infestations to increase significantly. The objectives of this research were to determine the effectiveness of glyphosate rates and application timings on control of rattail fescue during a chemical-fallow period in winter wheat cropping systems. Chemical-fallow field studies were conducted during two growing seasons at nine sites throughout the PNW. Glyphosate was applied early POST, late POST, or sequentially in early plus late POST timings. Additionally, paraquat + diuron was applied early and late POST alone or sequentially with glyphosate. Sequential application treatments (glyphosate followed by [fb] glyphosate, paraquat + diuron fb glyphosate, and glyphosate fb paraquat + diuron) controlled rattail fescue (∼ 94% in Idaho and Washington, ∼ 74% in Oregon) and reduced panicle number (∼ 85% in Idaho, ∼ 30% in Oregon and Washington) equivalent to or greater than one-time treatments. Rattail fescue control and panicle reduction generally increased with increasing rates of glyphosate within application timings. Paraquat + diuron usually provided similar control and reduced rattail fescue panicle number compared to glyphosate treatments applied at the same application timing. Although not completely effective, sequential applications of either glyphosate or paraquat + diuron, fb glyphosate will provide effective control during chemical fallow.

Field studies were conducted in different peanut-growing areas of Texas during the 1999 through 2001 growing seasons to evaluate yellow nutsedge control and peanut tolerance to diclosulam alone applied PRE, S-metolachlor alone applied POST, or diclosulam applied PRE followed by (fb) S-metolachlor applied POST. Yellow nutsedge control was > 80% at five of six locations when diclosulam at 0.018 or 0.026 kg/ha applied PRE was fb S-metolachlor applied POST at 0.56, 1.12, or 1.46 kg ai/ha. Peanut stunting was noted with diclosulam at the High Plains locations but not at the Rolling Plains or south Texas locations. This stunting with diclosulam was due to a combination of peanut variety and high soil pH. Peanut yield was not always increased where yellow nutsedge was controlled.

In 2005 a hailstorm struck a long-term dose–response study of irrigation requirements and corn plant populations. This misfortune occurred again in 2006 at approximately the same growth stage. Therefore, the objectives of the studies were redirected to measure the impact of actual hail events on corn leaf area index (LAI) and the competitive interaction of escaped Palmer amaranth populations induced by hail across different levels of irrigation and corn populations. In 2005, the study treatment with the lowest corn population and level of irrigation had twice the Palmer amaranth biomass (PABM) at corn harvest compared with the highest corn population and irrigation level. Corn LAI produced simple linear models that predicted both corn grain yield and PABM. In 2007, the nonhail year, PABM was depressed 4- to 15-fold compared with hail years. PABM declined linearly from 417 kg/ha at the lowest level of irrigation and corn population to 48 kg/ha at the highest level of irrigation and corn plant population. Although economic return per increment of irrigation declined in both hail years, the trends in economic returns were still positive. This suggests that a producer with similar conditions should continue to irrigate even though his or her rate of economic return is reduced.

The influence of carrier volume was evaluated in field experiments for glyphosate applied to wheat at rates representing 12.5 and 6.3% of the usage rate of 1,120 g ai/ha (140 and 70 g/ha, respectively). Wheat at first node and at heading was exposed to glyphosate applied in a constant carrier volume of 234 L/ha, where herbicide concentration declined with reduction in dosage, and in proportional carrier volumes of 30 L/ha for the 12.5% rate and 15 L/ha for the 6.3% rate, where herbicide concentration remained constant. At 28 d after treatment, glyphosate applied at first node in proportional carrier volume (an average for 30 and 15 L/ha adjusted proportionally to glyphosate rate) reduced wheat height 42% compared with 15% when glyphosate was applied in 234 L/ha. Height reduction was no more than 15% when glyphosate was applied at heading in 234 L/ha or in the proportional carrier volumes and at first node in 234 L/ha. Wheat yield was reduced 42% when glyphosate at 140 g/ha was applied in 234 L/ha but was reduced 54% for the same rate applied in proportional carrier volume. For 70 g/ha glyphosate, wheat yield was reduced 11% when applied in 234 L/ha, but was reduced 42% when the same rate was applied in proportional carrier volume. Wheat yield reduction was equivalent when glyphosate was applied in 234 L/ha at first node and at heading (29 and 24%, respectively), but yield reductions of 60% for first node application and 36% for heading application were observed when glyphosate was applied in a proportional carrier volume. When averaged across carrier volumes and glyphosate rates, the greater yield loss from application at first node was attributed to decreased number of spikelets per spike and seed weight per spike.

Field experiments were conducted to evaluate weed control provided by glyphosate, glufosinate, and MSMA applied alone or in mixture with residual and nonresidual last application (LAYBY) herbicides. Herbicide treatments included glyphosate early postemergence (EPOST) alone or followed by glyphosate, glufosinate, or MSMA late-postemergence (LPOST) alone or tank-mixed with one of the following LAYBY herbicides: carfentrazone-ethyl at 0.3 kg ai/ha, diuron at 1.12 kg ai/ha, flumioxazin at 0.07 kg ai/ha, fluometuron at 1.12 kg ai/ha, lactofen at 0.84 kg ai/ha, linuron at 0.56 kg ai/ha, oxyfluorfen at 1.12 kg ai/ha, prometryn at 1.12 kg ai/ha, or prometryn + trifloxysulfuron at 1.12 kg ai/ha + 10 g ai/ha. Residual herbicides were also applied alone LPOST. Weeds evaluated included barnyardgrass, broadleaf signalgrass, coffee senna, entireleaf morningglory, hemp sesbania, ivyleaf morningglory, johnsongrass, large crabgrass, Palmer amaranth, pitted morningglory, prickly sida, redroot pigweed, sicklepod, smooth pigweed, spiny amaranth, and velvetleaf. Treatments containing MSMA provided lower average weed control compared to those containing glyphosate or glufosinate, and residual herbicides applied alone provided inadequate weed control compared to mixtures containing a nonresidual herbicide. Across 315 of 567 comparisons (55%), when a LAYBY herbicide was added, weed control increased. The most difficult to control weed species at all locations was pitted morningglory. Barnyardgrass and hemp sesbania at the Mississippi location and hemp sesbania at the Louisiana location were collectively difficult to control across all treatments as well.

Studies were conducted in 2006 at Clinton and Kinston, NC, to determine the influence of halosulfuron POST (over the crop plant) or POST-directed (to the crop) on growth and yield of transplanted ‘Precious Petite’ and ‘Tri-X-313’ triploid watermelon. Treatments included a nontreated control, 39 g/ha halosulfuron applied POST-directed to 25% of the plant (distal or proximal region), POST-directed to 50% of the plant (distal or proximal; Precious Petite only), and POST. Watermelon treated with halosulfuron displayed chlorotic leaves, shortened internodes, and increased stem splitting. Vines were longest in the nontreated control (Tri-X-313 = 146 cm, Precious Petite = 206 cm) but were shortest in the POST treatment (Tri-X-313 = 88 cm, Precious Petite = 77 cm). Halosulfuron POST to watermelon caused the greatest injury (Tri-X-313 = 64%, Precious Petite = 67%). Halosulfuron directed to 25 or 50% (distal or proximal) of the plant caused less injury than halosulfuron applied POST. Stem splitting was greatest when halosulfuron was applied to the proximal area of the stem compared with POST-directed distal or POST. Internode shortening was greatest in treatments where halosulfuron was applied to the distal region of the stem. However, Tri-X-313 in the POST-directed 25% distal treatment produced similar total and marketable fruit weight as the nontreated control at Clinton. Fruit number did not differ among treatments for either cultivar. At Kinston, Precious Petite nontreated control and POST-directed 25% distal end treatment had greater marketable fruit weight than the POST-directed 50% proximal and POST treatments. The current halosulfuron registration allows POST application between rows or PRE. Limiting halosulfuron contact to no more than 25% of the watermelon plant will likely improve crop tolerance.

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.

Field trials were conducted in 2003/2004 and 2005/2006 at Reidsville, Georgia, to evaluate the effects of previously applied residual herbicides on onion growth and bulb production. Before transplanting onion, preplant applications of imazapic at 18 and 36 g ai/ha, diclosulam at 7 and 14 g ai/ha, pyrithiobac at 27 and 54 g ai/ha, trifloxysulfuron at 6.6 and 13.2 g ai/ha, diuron at 224 and 448 g ai/ha, and cloransulam at 22 g ai/ha were made. An untreated control was included for comparison. Trifloxysulfuron at 13.2 g/ha, diclosulam at 14 g/ha, pyrithiobac at 54 g/ha, and cloransulam at 22 g/ha injured onion 26, 73, 86, and 86% in 2003/2004, respectively, and 13 to 44% injury in 2005/2006. These same herbicides also reduced yield. Imazapic and diuron injured transplanted onion 6% during both seasons but did not reduce yield. This research suggests imazapic and diuron restrictions could possibly be reduced. However, the onion rotational restrictions for diclosulam, pyrithiobac, trifloxysulfuron, and cloransulam are accurate.

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.

In the semiarid northern Great Plains, the adoption of zero tillage improves soil water conservation, allowing for increased crop intensification and diversification. Zero-tillage crop production relies heavily on herbicides for weed management, particularly the herbicide glyphosate, increasing selection pressure for herbicide-resistant weeds. Barley is well adapted to the northern Great Plains, and may be a suitable herbicide-free forage crop in zero-tillage systems. A 2-yr field study was conducted to determine if planting date influenced crop and weed biomass, water use (WU), and water-use efficiency (WUE) of barley and weed seed production in three preplant weed management systems: (1) conventional preplant tillage with a field cultivator (TILL); (2) zero tillage with preemergence glyphosate application (ZTPRE); and (3) zero tillage without preemergence glyphosate (ZT). None of the systems included an in-crop herbicide. Planting dates were mid-April (early), late May (mid), and mid-June (delayed). Early planting of ZT barley resulted in excellent forage yields (7,228 kg/ha), similar to those from TILL and ZTPRE. Early planting resulted in a small accumulation of weed biomass, averaging 76 kg/ha, and no weed seed production regardless of preplant weed management system. Early planting resulted in higher WU than delayed planting, averaging 289 and 221 mm, respectively, across management systems and years. The WUE of crop and total biomass did not differ among preplant weed management systems at harvest from the early planting date. Delayed planting resulted in decreased forage yield with high amounts of weed biomass and seed production, especially in ZT. A pre-emergence glyphosate application was not necessary for early-planted ZT forage barley. Early planting of herbicide-free barley for forage can be an excellent addition to northern Great Plains cropping systems as part of a multitactic approach for improved weed and water management.

Open field production of fruit and nut-tree nursery stock depends upon preplant soil fumigation, extensive tillage, and hand-labor throughout the growing season for adequate weed control. Because methyl bromide, the favored fumigant, is being phased out because of environmental concerns and the costs of both fuel and labor continue to rise, herbicides are likely to become a more important weed management tool in the tree nursery industry. Two trials were conducted to evaluate weed control and crop safety with several herbicides applied following fumigation with methyl bromide or 1,3-dichloropropene in central California stone-fruit nurseries. PRE and POST-directed applications of several labeled and unlabeled materials were applied in a band over seeded peach rootstock or applied after emergence with a drop-nozzle spray boom. Crop productivity and weed control were monitored throughout the 1-yr growing season. PRE oryzalin and dithiopyr treatments provided the best weed control with very little crop injury. PRE applications of flumioxazin, rimsulfuron, and sulfentrazone did not have adequate crop safety at the rates and timings tested. However, POST-directed applications of flumioxazin and rimsulfuron were much safer to the peach and almond crops and should be evaluated in future trials. Additional herbicides and application techniques are needed to find acceptable, safe control measures for weeds, such as California burclover, common mallow, and redstem filaree, which often are poorly controlled with preplant fumigation in tree nurseries.

Smutgrass is a clump-type grassy weed that is commonly found in grazed pastures throughout the Southeast. Currently, the only herbicide registered for control of smutgrass is hexazinone and it is unknown if hexazinone negatively impacts bermudagrass forage growth and quality. Experiments were conducted to determine the impact of hexazinone rate on bermudagrass yield and crude protein. All rates of hexazinone reduced bermudagrass yield at 4 wk after treatment (WAT). From 6 to 12 WAT, no bermudagrass yield reduction was observed except for the 2 kg/ha treatment. The yield reducing effects of hexazinone were transitory at the 0.5 and 1.0 kg/ha rates; cumulative yield for the season was reduced by 17 and 23%, respectively. Although yield was negatively influenced by hexazinone, crude protein generally increased in a linear fashion with increased hexazinone rate.