As a poor competitor with weeds, alfalfa seedling success appears to be strongly related to the critical period of weed control. The critical period of weed control consists of two complementary events that lead to a necessary weed-free period during the crop growth cycle. The first component of the critical period examines the effect of weeds that emerge with the crop that are then removed after a set duration, while the second component examines the effect of weeds that emerge some time after crop emergence and remain until crop harvest. Once these events are combined, a specific critical weed-free period can be identified during the crop growth cycle. The objective of this study was to determine the critical period of weed control in alfalfa by demonstrating how weed competition impacts alfalfa yield as well as the importance of proper herbicide application timing for optimum weed control. Alfalfa was seeded conventionally at two locations in Pennsylvania in the spring of 2004 and 2005; glyphosate and glyphosate-resistant alfalfa were used for weed management. Three weed densities were examined using Japanese millet (Echinochloa frumentacea) as a surrogate weed to simulate varied weed severity. Treatments included a control, four individual glyphosate applications based on time elapsed after planting, a weed-free treatment, and three delayed seedings of Japanese millet after alfalfa planting. The delayed Japanese millet plantings simulated new weed flushes that occur after crop emergence or failure with the initial weed management practice. Plot yields were converted to relative yield of the weed-free control. The relative yield was then regressed against the number of days for which the plots were kept weedy or weed free using nonlinear regression following a sigmoid model. Preliminary analysis suggests that the critical period of weed control for alfalfa in Pennsylvania is between 4 and 6 weeks after planting.
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Glyphosate-resistant alfalfa offers new weed control options for alfalfa establishment. Two field studies were conducted to determine the effect of establishment method and weed control method on forage production and alfalfa stand during the establishment year and following year. Seeding methods included clear seeding and companion seeding with oats. Herbicide treatments, including glyphosate, imazamox, or imazamox + clethodim, where applied in the establishment year in both studies. Glyphosate injury was minimal, short-lived and no longer evident at the first harvest in 2004. No glyphosate injury was observed in 2005. Weed control with glyphosate was more consistent than imazamox or imazamox + clethodim. In 2004, total seasonal forage yield was the highest where no herbicide was applied in the oat companion crop and was reduced where herbicides were applied in both establishment systems. In 2005, seeding method or weed control method did not affect total seasonal forage production. Alfalfa established by clear seeding methods with glyphosate applied for weed control yielded the highest alfalfa dry matter in both establishment years. Imazamox injury reduced alfalfa yield at the first harvest in the clear seeded system in both establishment years. When no herbicide was applied, alfalfa yield was highest in the clear seeded system. The oat companion crop suppressed alfalfa yield significantly in both studies during the establishment year. Alfalfa established with an oat companion crop had a lower weed biomass than the clear seeded system where no herbicide was applied in both studies.
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Experiments were conducted in 2005 and 2006 in Huntley, Montana to evaluate mesosulfuron-methyl for the control of wild oat in malt barley. Mesosulfuron-methyl is the active ingredient in two herbicides produced by Bayer CropScience that are registered for use in wheat. These herbicides, Silverado™ and Osprey™, contain mesosulfuron-methyl and the safener mefenpyr-diethyl at a ratio of 1:6 and 1:2, respectively. Silverado and Osprey were applied alone at two rates (low and high) or in combination with other small grain herbicides that are typically used for broadleaf weed control. These small grain herbicides included thifensulfuron plus tribenuron, clopyralid plus fluroxypyr, and bromoxynil plus MCPA. The experiments were conducted in a randomized complete block design with a factorial arrangement of treatments containing four replications. These treatments were compared to standard treatments of tralkoxydim and fenoxaprop. At 8, 21, and 56 days after treatment (DAT), the main effects of herbicide, rate, and the addition of other small grain herbicides were determined to be significant with respect to barley injury in both years. In 2005, Silverado applied alone at the low and high rate caused barley injury of 7 and 11% at 21 DAT and 4 and 7% at 56 DAT, respectively. Osprey applications resulted in 9 and 17% greater barley injury at 21 DAT compared to Silverado when these herbicides were applied alone at the low and high rate, respectively. At 56 DAT, barley injury from Osprey applied alone was 12 and 18% greater than injury from Silverado applied alone at the low and high rate, respectively. Barley injury was greater when either rate of Silverado or Osprey was combined with bromoxynil plus MCPA at 21 and 56 DAT compared to Silverado or Osprey applied alone. Similar barley injury was also observed in 2006 when these two herbicides were applied alone at the low and high rate or in combination with other small grain herbicides. In both years, wild oat was controlled between 86 and 93% at 21 DAT and between 89 and 98% at 56 DAT with all Silverado and Osprey treatments regardless of rate or the addition of other small grain herbicides. In both years, wild oat control with Silverado and Osprey was similar to that provided by tralkoxydim and fenoxaprop at 56 DAT. In 2005, no difference in barley yield was observed among treatments, however, percent plump kernels was lower in plots receiving an application of Osprey in comparison to those that received an application of Silverado. In 2006, barley yields were greater when Silverado and Osprey were applied alone at the low rate in comparison to when applied alone at the high rate. In both years, no difference in the percentage of plump kernels occurred between treatments of Silverado, tralkoxydim, or fenoxaprop. Results indicate that Silverado applied at the low rate alone or in combination with thifensulfuron plus tribenuron or clopyralid plus fluroxypyr are effective treatments for the control of wild oat in malt barley and result in barley yield and quality equivalent to the labeled treatments of tralkoxydim and fenoxaprop.
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The occurrence of transgenic herbicide-resistant canola (Brassica napus) in ruderal (non-crop disturbed) areas has not been investigated previously in Canada. The primary objective of this study was to document their occurrence along roads and railways in the province of Saskatchewan, where half of all canola is grown, and at the port of Vancouver, British Columbia on the west coast of Canada where most canola destined for export is transported by rail. During the 2005 growing season, leaf samples of canola plants were collected at randomly-selected sites in two main ruderal areas (along railways and roads) across Saskatchewan ecoregions and at Vancouver. The presence of the glyphosate and glufosinate resistance traits was determined using test strips. The infestation area of canola, averaged across 155 sampled sites in the Saskatchewan survey, was markedly smaller in populations along railways than roads; in contrast, infestation area averaged across 54 sites in the Vancouver survey was greater for populations along railways than roads. In both surveys, mean plant density was greater for populations found along railways than roads. Two-thirds of canola plants sampled across Saskatchewan ecoregions and at Vancouver were transgenic, although the relative proportion of plants with the glyphosate or glufosinate resistance trait varied between surveys. Frequency of occurrence of transgenic plants in ruderal areas is similar to the proportion of the canola area planted with transgenic cultivars. A single transgenic B. rapa x B. napus hybrid was found along a road in Vancouver, confirming the relatively high probability of hybridization between these two Brassica species. A greater effort is needed to manage transgenic plants in these ruderal areas to limit their persistence and spread.
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Eight field trials were conducted over a two year period (2005 and 2006) in Ontario and Michigan to study the efficacy of five sulfonylurea herbicides for the control of wirestem muhly in field corn. Treatments consisted of a weedy check, a weed-free check, and postemergence (POST) applied rimsulfuron (15 g ai/ha), nicosulfuron (25 g ai/ha), nicosulfuron plus rimsulfuron (25 g ai/ha), foramsulfuron (35 g ai/ha), and primisulfuron (25 g ai/ha). All treatments included 141 g ai/ha of dicamba for broadleaf weed control. Rimsulfuron provided up to 28% control of wirestem muhly but had no effect on density, dry weight, and corn yield compared to the weedy check. Nicosulfuron caused up to 44% control of wirestem muhly and reduced density 44%, dry weight 70%, and increased corn yield 18%. Nicosulfuron plus rimsulfuron controlled wirestem muhly up to 38%, had no effect on density, decreased dry weight 46%, and increased corn yield 14%. Foramsulfuron provided up to 89% control of wirestem muhly, decreased density 76%, decreased dry weight 94%, and increased corn yield 14%. Primisulfuron provided up to 25% wirestem muhly control but had no effect on density, dry weight or corn yield. Based on these results, foramsulfuron POST has potential for the control of wirestem muhly in corn. However, rimsulfuron, nicosulfuron, nicosulfuron plus rimsulfuron, and primisulfuron applied POST do not provide adequate control of wirestem muhly in corn.
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Cultural alternatives such as altering plant spacing is thought to increase weed control by increasing the interspecific competition. The objectives of this study were to determine the effect of spatial pattern on corn above and belowground biomass, grain yield, weed aboveground biomass and soil water and photosynthetically active radiation (PAR) dynamics. Field studies were conducted in La Plata (Argentina) in 2003 and 2004. Three corn hybrids with two spatial pattern (square: 0.35 x 0.35 m and rectangular: 0.70 m x 0.20 m) under weed-free and weedy conditions were tested. The resources complementarity between corn and weeds was measured by the relative yield total (RYT). Corn grain yield was greater in the square than in the rectangular pattern. This increase was related to better resources use i.e. soil moisture and PAR interception while reduces the competition from weeds (RYT > 1.0). RYT value was greater than 1.0 at any sampling date in corn sowed at uniform spatial pattern in both years. Early in the growing season, there was greater moisture content in the intrarow in square pattern while a lower soil water content was noted at maturity. Conversely, the square pattern registered lower soil water content during the whole corn growing season in the interrow. This soil water profile was an outcome of the uniform corn belowground biomass distribution. A greater PAR interception with a lower weed aboveground biomass was obtained in square plant arrangement. The use of uniform spatial pattern appeared as an interesting alternative to increase both the grain yield potential and the corn suppressive ability against weeds in dry-land Argentinean production systems.
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In 2006, 108 herbicide drift complaints were filed in Mississippi, most of them involving glyphosate. Crops suffering from herbicide drift often do not manifest extensive visual injury symptoms; however yields may be reduced. Developing methods for detection of glyphosate drift on corn would allow producers to determine potential crop damage and make appropriate management decisions. Field studies were conducted in 2005 and 2006 at the Black Belt Branch Experiment Station near Brooksville, MS to determine the level of glyphosate drift detectable on non-glyphosate resistant corn utilizing multispectral aerial imagery. Corn was planted on three fields measuring 3, 2.5, and 2.4 hectares each. ‘Pioneer 31G98’ was planted on April 22, 2005 and ‘Pioneer 31G68 Bt’ was planted on April 20, 2006. Both hybrids were planted at 77,000 seeds/ha. Season long weed control was achieved with 1.6 kg ai/ha atrazine, 1.6 kg ai/ha s-metolachlor and 0.20 kg ai/ha mesotrione in 2005 and 2.8 kg ai/ha atraine, 1.4 kg ai/ha s-metolachlor, and 0.04 kg ai/ha nicosulfuron in 2006. All fields received 100 to 112 kg N/ha approximately one month after planting. Approximately six weeks after planting, glyphosate drift rates were applied. Applications were made in 2005 with ground application equipment with a 18.3 m boom width. Applications were made in 2006 with a shielded spray boom covering 3.9 m. Drift rates were applied at 1/2, 1/4, 1/8, 1/16, 1/32, 1/64, and 1/128 of the full labeled rate of 0.87 kg ae/ha. Plant height and growth stage were taken at four to eight samples points within each replication. Each treatment was replicated three to five times per year. Plant height and growth stage were taken prior to application and weekly up to four weeks after treatment. Multispectral images at 0.5 m spatial resolution containing reflectance data at 450 ± 10 nm, 550 ± 10 nm, 650 ± 10 nm, and 850 ± 10 nm were collected prior to application and weekly up to four weeks after treatment. Yields were collected using a combine equipped with a global positioning system and yield monitor equipment. Plant height and yield data were subjected to analysis of variance and means were separated using Fishers Protected LSD at p = 0.05. Normalized difference vegetation indices (NDVI) were calculated from the multispectral imagery and means were separated with Fishers Protected LSD at p = 0.05. Preliminary data analysis indicates that glyphosate drift rates up to as low as 1/32 of the full labeled rate can result in yield reductions with extreme yield reductions observed at 1/2 and 1/4 of the full labeled rate. Reductions in plant height were observed at drift rates of 1/16 of the full labeled rate and greater. Reductions in NDVI, indicating reduction in plant growth, were observed at drift rates greater than 1/16 of the full labeled rate. Preliminary data analysis indicates that reductions in corn yield due to glyphosate drift up to 1/16 of the full labeled rate can be detected using plant height and remote sensing data. Further analysis is needed to determine if corn yield reductions due to drift rates lower than 1/16 of the full labeled rate can be detected and quantified.
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Studies were conducted in 2004, 2005 and 2006 to evaluate programs for managing stale seedbeds for Roundup Ready corn. The experimental designs were Randomized Complete Blocks with factorial treatment arrangements. Factor A was stale seedbed management approach. Management approaches included removal of weeds 2 weeks before planting, mechanical removal of weeds by dragging at planting, and to plant as is and remove weeds after corn emerged. Glyphosate at 1 lb ai/A plus 0.75 lb ai/A 2,4 -D was applied 2 weeks prior to planting to remove weeds in plots receiving the preplant treatment. Factor B was postemergence weed management options and included 1 lb ai/A glyphosate plus 0.75 lb ai/A 2,4-D or glyphosate plus 0.5, 1 or 1.5 lb ai/A atrazine applied at V2. A layby treatment of 1 lb ai/A glyphosate plus 1.0 lb ai/A atrazine was applied at V6. Despite excellent weed control from all treatments, corn yields were dramatically affected by stale seedbed management programs. In 2004, the best corn yields were observed in plots where weeds were removed before planting. Dragging beds prior to planting was better than doing nothing, but still resulted in an average yield reduction of 20%. On average, planting as is and removing weeds at V2 resulted in a 30% yield reduction. In 2005 and 2006, corn yields were similar for removing weeds before planting and the dragging. Planting as is and removing weeds at V2 resulted in an average yield reduction of 65% in 1995 and 30% in 2006. 2, 4-D applications at V2 reduced corn yields from 10 to 55% compared to atrazine treatments. These results indicate that removing weeds at or before planting is necessary to achieve maximum corn yields, even when weeds can be effectively removed after corn emergence.
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Cotton production has increased in the central and northern High Plains regions of Texas over the last 3 to 5 years. These areas have traditionally produced large acreages of wheat, corn and sorghum, and include large grassland areas where the use of 2,4-D and dicamba are common. These expanding cotton areas are at high risk of exposure to drift of 2,4-D and dicamba. Little information is available that clearly identifies the relationship between exposure level, crop injury, and cotton yield reductions following 2,4-D or dicamba drift. The objectives of this study were to determine the effects of simulated drift rates of 2,4-D and dicamba on cotton growth and yield, and to correlate injury levels and effects on lint yield and fiber quality. Studies were initiated at the Texas Agricultural Experiment Station in Halfway, TX in 2005 and repeated in 2006. 2,4-D or dicamba were made at four growth stages including: cotyledon to 2 leaf, 4 to 5 leaf, pinhead square, and first bloom. Rates of 2,4-D included: 0.25 (1/2X), 0.125 (1/4X), 0.063 (1/8X), 0.025 (1/20X), 0.0025 (1/200X), and 0.00025 lbs ai/A (1/2000X). Dicamba rates included: 0.125 (1/2X), 0.063 (1/4X), 0.032 (1/8X), 0.0125 (1/20X), 0.00125 (1/200X), and 0.00013 lbs ai/A (1/2000X). Visual injury was recorded at 7 and 14 days after treatments, and end of season. Cotton was harvested and ginned to determine lint yield and fiber quality. In both years, 2,4-D caused more visual injury than dicamba at all growth stages. Early-season injury did not always result in yield loss, and visual injury tended to over estimate yield loss. Applications made at the pinhead square growth stage reduced lint yield and fiber quality dramatically. While dicamba at 1/8X reduced yield, similar yield loss resulted with 2,4-D applied at rates as low as 1/20X.
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Browntop millet (Brachiaria ramosa (L.) Stapf) is becoming a dominant weed in late season and post-harvest fields of soybean, corn, and cotton in the Mississippi Delta. Though readily controlled by preemergence herbicides and glyphosate, browntop millet continues to germinate after preemergence herbicide activity diminishes. Browntop millet can establish beneath, and grow through, the canopy in cotton. Browntop millet may interfere with mechanical harvest, reduce lint quality and color grade, and increase trash content. The objectives of the study were to measure the effects of tillage (subsoiling and hipping versus no tillage) and planting date (April 1 versus May 1) on browntop millet biomass after cotton defoliation. In addition, the effects of tillage, a rye (Secale cereale L.) cover crop, and herbicide programs, consisting of glyphosate, applied alone or in combination with preemergence herbicides, were investigated on browntop millet biomass after defoliation in glyphosate-tolerant cotton. Early planting and no tillage resulted in the greatest browntop millet biomass. The rye cover crop provided sufficient control of most summer annuals such that preemergence herbicides could be eliminated. However, rye cover crop and no tillage resulted in high browntop millet biomass indicating the rye provided an environment favorable to browntop millet survival. Glyphosate reduced browntop millet populations to near zero between applications. However, in the time from the last glyphosate application to cotton defoliation, browntop millet was re-established beneath the cotton canopy and became the most prevalent weed. These results demonstrate the need for residual herbicides along with postemergence herbicides at layby to manage late season browntop millet.
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Three field experiments were conducted over a three year period (2003, 2004, 2005) in Ontario to determine if reduced rates of imazathapyr tank-mixed with dimethenamid applied pre-emergence (PRE) can be used as an efficacious, environmentally acceptable, and economically feasible weed management strategy for broad spectrum weed control in white and kidney beans. There was no injury in white or kidney beans with the imazethapyr plus dimethenamid tank-mix treatments evaluated. The rate of imazethapyr required to provide a minimum of 80 and 95% control of green foxtail, common lambsquarters, common ragweed, wild mustard and redroot pigweed was generally reduced when tank-mixed with dimethenamid (1000 g ha-1). There was no adverse effect on the yield of white and kidney beans with the highest rates (75 g ai ha-1) of imazethapyr evaluated. The low application rate of imazethapyr compared to dimethenamid (75 vs. 1000 g ai ha-1, respectively) resulted in an environmental impact (EI) of imazethapyr that was seven-times less than dimethenamid. Other than the weedy check, the lowest profit margins occurred in dimethenamid (1000 g ai ha-1) and imazethapyr alone (15 g ai ha-1) treatments. Higher rates of imazethapyr alone and tank-mixes of dimethenamid with imazethapyr increased the profit margins for both white and kidney beans. Profitability generally increased as the rate was increased. Tank-mixes of imazethapyr with dimethenamid will provide growers with a weed management strategy that causes only a minor increase in environmental impact, acceptable weed control and increased net returns.
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Trials were established in 2003 and 2004 north of Ontario, OR to evaluate corn and dry bean rotations for controlling yellow nutsedge and reducing tuber numbers in the soil. Treatments in dry bean included metolachlor plus EPTC or metolachlor applied PPI and PPI metolachlor followed by one POST application of halosulfuron or two POST applications of bentazon. Treatments in corn included PRE metolachlor alone or followed by POST treatments of bentazon, glyphosate, or halosulfuron. Halosulfuron was applied once and bentazon and glyphosate were applied twice. Pendimethalin was applied PRE to control other annual weeds in the dry bean and corn trials. Treatments were arranged in a randomized complete block design and were replicated four times. All trials were furrow irrigated. Data collected included yellow nutsedge control, percent change in yellow nutsedge tubers in the soil, and dry bean and corn yields. All data were combined over years with the exception of yellow nutsedge control ratings in corn which exhibited significant year by treatment interactions. In dry beans, combinations of PPI metolachlor plus halosulfuron or bentazon POST provided greater yellow nutsedge control than metolachlor or metolachlor plus EPTC. Tuber numbers increased 1% in the metolachlor alone treatment and decreased 2% in the metolachlor plus EPTC treatment. Tuber numbers were reduced 17% by PPI metolachlor followed by halosulfuron POST and 35% by PPI metolachlor followed by two POST bentazon applications. In the untreated dry bean plots, yellow nutsedge tubers increased by 79%. All herbicide treatments increased dry bean yields compared to the untreated check. Herbicide treatments in corn provided 57 to 97% control of yellow nutsedge. PRE metolachlor or two POST applications of bentazon alone controlled yellow nutsedge less than PRE metolachlor followed by two POST bentazon applications in both years. The number of yellow nutsedge tubers was reduced 32 to 55% by herbicide treatments. Yellow nutsedge tubers increased by 36% in the untreated control. All herbicide treatments except for metolachlor followed by halosulfuron significantly increased corn yield compared to the untreated control. This research demonstrates that combinations of soil-applied and postemergence herbicides provide the most consistent control of yellow nutsedge and herbicide treatments in dry bean and corn crops can significantly reduce the number of yellow nutsedge tubers in the soil.
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Six classes of dry beans and adzuki beans were planted in 2005 and 2006 at St. Charles, Michigan to determine the tolerance of dry beans and adzuki bean to halosulfuron applied preemergence (PRE) and postemergence (POST). Varieties of the different dry bean classes are as follows: ‘Vista’ navy beans, ‘Jaguar’ black beans, ‘Merlot’ small red beans, ‘Othello’ pinto beans, ‘Chinook 2000’ light red kidney beans, ‘Matterhorn’ great northern beans, and ‘Erimo’ adzuki beans. The plots were kept weed-free throughout the season. Visual injury, maturity ratings, and yield for PRE and POST applications of halosulfuron were compared with an untreated control using contrast statements. Injury from PRE applications of halosulfuron ranged from 6 to 19%, 30 d after planting (DAP) for all classes in 2005. Adzuki beans and the ‘Chinook 2000’ light red kidney beans exhibited the greatest injury. In 2006, only the adzuki beans were injured (9%). Injury consisted of stunting compared with the untreated control. Even though some stunting occurred with all classes in 2005 and only the adzuki beans in 2006, yields were only lower than the untreated control with ‘Jaguar’ black beans and ‘Chinook 2000’ light red kidney beans in 2005 (" = 0.1 level of significance). In both years, POST applications of halosulfuron caused stunting and chlorosis to all classes. Injury 4 to 6 d after treatment (DAT) ranged from 21 to 48% in 2005 and 12 to 56% in 2006. By 12 to 14 DAT, most dry bean classes started to out grow the injury; however injury in the adzuki beans increased. In 2005, injury was also greater for the light red kidney beans, 12 DAT. In 2005 yield was lower than the untreated control with adzuki beans (" = 0.05), light red kidney beans, and black beans (" = 0.1). However, in 2006 adzuki bean was the only bean were a significant reduction of yield was observed (>45% yield reduction). Differences in precipitation between 2005 and 2006 demonstrated the differences in recovery rates from POST halosulfuron injury to these different classes. Of all of the beans tested POST applications caused the greatest injury and yield reductions for adzuki beans, therefore POST applications of halosulfuron should be avoided to this class. Caution should also be taken when applying halosulfuron to black beans and light red kidney beans if conditions are not conducive for recovery from initial halosulfuron injury.
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Morningglories are among the most troublesome weeds in row crops and other agricultural and non-agricultural areas throughout the U.S. In 2005 and 2006, plants of a fertile purported hybrid between sharppod and pitted morningglory (Ipomoea x leucantha Jacq.), cypressvine morningglory (I. quamoclit L.), ivyleaf morningglory [I. hederacea (L.) Jacq.], pitted morningglory (I. lacunosa L.), palmleaf morningglory (I. wrightii Gray), purple moonflower (I. turbinata L.), red morningglory (I. coccinea L.), sharppod morningglory (I. cordatotribola Dennst.), and smallflower morningglory [Jacquemontia tamnifolia (L.) Gresb.] were grown in greenhouses at Stoneville, MS. Ten progeny from each of 74 accessions of these morningglories were evaluated for leaf shape, size, dry weight, and pubescence; corolla size and color; calyx length, color and pubescence; stem color, pubescence, and number of nodes to first internode elongation; and other morphological characteristics with the goal of developing baseline data to correlate with herbicide efficacy and herbicide tolerance/resistance issues. The greatest morphological diversity within accessions was observed among pitted, sharppod, and the hybrid morningglory accessions. Corolla color was white, lavender, and varied from light pink to lavender in pitted, sharppod, and hybrid morningglory populations, respectively. Leaf shape and pubescence varied more among pitted and the hybrid morningglory accessions than among sharppod morninglory and other morningglory accessions. Leaves and petioles were smooth for cypressvine, palmleaf, and red morningglory accessions and pubescent for ivyleaf and smallflower morningglory accessions. However, for pitted, sharppod, and the hybrid morningglories, pubescence on leaves and petioles varied among and within accessions. Flower diameter and length was more variable for the hybrid morningglory accessions compare to other accessions within species. In the future, these morphological traits will be compared to determine if any of these traits or group of traits may be correlated with herbicide efficacy and herbicide tolerance. cbryson@ars.usda.gov
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Our objectives were to evaluate peanut yield response and weed response to various weed-free timings, weed removal timings, and determine the critical periods of weed control for peanut in the presence of broadleaf weeds as well as the critical period of weed interference in peanut with only grass weeds. Separate trials were conducted near Lewiston and near Rocky Mount, NC in 2005. Grass or broadleaf weeds were removed using chemical and hand weeding throughout the season for the respective studies. Treatments included weed competition periods of 0 (Weed-free), 3, 5, 7, 9, and 16 weeks after planting (WAP) where weeds were allowed to compete with the peanut crop then removed and plots maintained weed-free for the remainder of the season; weed-free periods of 0 (Full season weedy), 3, 5, 7, 9, and 16 WAP where plots were maintained weed-free until weeds were allowed to infest and compete with the crop for the rest of the season, and weedy intervals of 3 to 7, 3 to 9, 3 to 11, 5 to 9, 5 to 11, and 7 to 11 WAP where plots were maintained weed-free for a period of 3, 5, or 7 WAP and weeds were then allowed to grow for a period of up to 8 weeks before being removed until harvest. Peanut yields were modeled using a Gompertz equation. North Carolina experienced dry weather in 2005 which was evident in Rocky Mount, but not at Lewiston where irrigation was available. Peanut yield responses based on percent of weed-free yield were quantified for both locations and a critical period of weed interference in peanut for both broadleaf and grass weeds were determined. The critical period of broadleaf weed interference to avoid a 5% yield loss was from 2 to 9 and 2 to 11 WAP at both locations, respectively while the critical period of grass weed interference was shorter at both locations ranging from 5 to 8 and 3 to 8 WAP at Rocky Mount and Lewiston, respectively. The weedy intervals supported the critical period of interference determinations at both locations for grass and broadleaf weeds. We observed greater than 10% yield loss when broadleaf weeds were allowed to compete with peanut from 3 to 9 or 3 to 11 WAP at both locations. However, we only observed a yield loss of 10% or greater due to grass weed interference from 3 to 11 WAP at both locations. The critical period of weed control varied for broadleaf and grass weeds in peanut and was influenced by weed biological characteristics such as plant architecture, germination capacity as influenced by environmental conditions, and rate of vegetative growth. At both locations, the critical period of broadleaf weed interference was greater than the critical period of grass weed interference. Based on the data collected, we observed peanut are sensitive to weed interference during the majority of the growing season. Grass control is more important during mid-season to maximize yield potential. Broadleaf weed control is important throughout the growing season, which is evident by yield losses due to early-, mid- and late-season interference. Management systems that provide early and mid-season weed control will usually maximize yield potential; however, late season control may be required to minimize yield losses from reduced harvesting efficiency.
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Since the initial discovery of acetolactate synthase (ALS) inhibiting herbicide resistance for Palmer amaranth (Amaranthus palmeri S. Wats) in 2002, there have been numerous reports from growers of the decreasing efficacy of imazapic in Georgia peanut. In 2005, a study was initiated to establish the extent, distribution, dose response, and heritability for ALS resistance in Palmer amaranth. Seed samples were collected from 61 field locations in 21 counties ranging from the extreme southwest to the east central area of Georgia. Samples were randomly sampled from suspect populations, the entire female thyrses was clipped from the plant, and then stored at 4 C for 45 d. Seed were then hand thrashed, seed were then sown in pots containing standard potting media which were then placed in a growth chamber with diurnal settings of 30 C for 16 h and 20 C for 8 h. Upon seedling germination and emergence, pots were moved to a greenhouse, placed under growth lights set on 16-8 hour cycle with diurnal settings of 32 and 25 C, respectively. Plants were screened for resistance between the 4 and 5 leaf stage with imazapic at 0.071 and a 0.71 kg ai ha-1. Plants were visually rated for injury using a scale of 0 (plant death) to 100% (survived) at 7 and 14 d after treatment. All samples showed some level of resistance to imazapic (45-97% survival). Given these results a dose response trial was initiated to establish the levels of resistance among the different sites. For the dose response study, rates were 0.00071, 0.0071, 0.071, and 0.71 kg ha-1. For the dose response study, plant weight and vigor were affected by the varying levels of imazapic, but plants did not die. Susceptible checks died at .0071 kg ha-1. The plants that had been treated with the .71 kg ha-1 rate in the screening trial were grown to reproductive maturity and allowed to cross creating an F2 generation. This generation was then subjected to a dose response trial using the same rates as for the previous dose response study. Results indicated that the F2 generation were more resilient in there resistance to imazapic. To corroborate these results, a lab assay was conducted to determine resistance. Leaf tissue from the treated plants was evaluated using an acetoin assay and spectrometer and compared to the acetoin levels of a known susceptible plant. This assay confirmed the results of the screening trial.
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Field studies were conducted between 2003 and 2006 at the Plant Sciences Farm near Athens, GA on a Cecil sandy loam soil (pH 6.1, OM = 0.9%) to assess large crabgrass control and pearl millet tolerance to mesotrione. Mesotrione was applied PRE and POST when the pearl millet was approximately 5 to 8 cm tall and weeds were < 5 cm tall. Greenhouse experiments were conducted to examine varietal tolerance of pearl millet to mesotrione applied PRE. These studies indicate that pearl millet exhibits excellent tolerance to mesotrione applied PRE (<10% injury at 110 g ai/ha). Pearl millet is not as tolerant to foliar applications of mesotrione. Significant (>50%) visual injury symptoms were observed for ~14 DAT following a 110 g/ha application of mesotrione. Foliar injury declined to less than 10% after 14 DAT. In an examination of 23 varieties of pearl millet, forage pearl millet seems to be more sensitive than the grain pearl millet varieties. Visual injury ED10 values ranged from 15 (‘Tift 23DB’) to 159 g/ha (‘Tif 99B’). Visual injury ED50 values ranged from 52 (‘Tift 23DB’) to 267 g/ha (‘Tif 99B’). Additional studies indicate that injury increases if mesotione is applied PRE greater than one day after planting (ED10 ranged from 13 to 43 g/ha). Large crabgrass control was >90% 21 DAT from mesotrione applied at 110 g ai/ha. These studies indicate that mesotrione has the potential to provide annual grass weed control in pearl millet.
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Late watergrass (Echinochloa phyllopogon) is a major weed of rice in California, and several populations showed resistance to multiple herbicides of differing modes of action. Low level of resistance to clomazone was found in dose response studies with three late watergrass biotypes collected in rice fields of the Sacramento Valley. This level of resistance corresponds to escapes seen in the field under conventional treatment in farms heavily infested with a resistant biotype of this weed. The dose-response studies were conducted under flooded conditions, with a four inch flood, and the weed at the one-leaf stage of growth. Fresh weight was harvested 20 days after treatment. Clomazone rates were: 0, 1/4X, 1/2X, X, 2X, and 4X; X is the field rate = 673 g ha-1; a commercial formulation of clomazone (CERANO) was used. Growth reduction (50%) values were significantly lower for the susceptible biotype compared to the resistant biotypes. Application of clomazone in combination with disulfoton or oxydemeton methyl (organophosphate insecticides and also cytochrome P450 inhibitors) reduced clomazone toxicity to resistant late watergrass biotypes, suggesting that an oxidative step is required for activation and toxicity of this herbicide. Studies are under way to clarify the mechanism of resistance.
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Efficacy of V-10142 for weed control in drill-seeded rice was evaluated in several trials at the Northeast Research Station near St. Joseph, LA on a Sharkey Clay soil in 2005 and 2006. Rice was seeded at 100 kg/ha to plots measuring 2 by 4.5 m. Permanent floods were established 4 to 5 weeks after planting. Nitrogen, in the form of prilled urea, was applied at 126 kg/ha just before permanent flood. At panicle initiation an additional 42 kg/ha of nitrogen was applied. Herbicide treatments were applied in 140 L/ha of water using a CO2 pressurized backpack sprayer. V-101142 demonstrated good postemergence activity on several weeds including sesbania (Sesbania exaltata (Raf.) Rydb. ex A.W.Hill), Texasweed (Caperonia palustris (L.) St. Hil.), eclipta (Eclipta prostrata L.) and rice flatsedge (Cyperus iria L.) when they were small, but struggled on larger weeds. It did, however, demonstrate excellent residual activity on these weeds. When applied preemergence with clomazone V-10142 controlled sesbania (80%) and Texasweed (99%). V-10142 applications made at the 2-3 leaf rice stage with other herbicides like propanil and bispyribac resulted in season-long control of sesbania, Texasweed and eclipta. V-10142 did not demonstrate any activity on barnyardgrass (Echinochloa crus-galli (L.) Beauv.). Results with Amazon sprangletop (Leptochloa panicoides (Presl) Hitchc.) were inconsistent. In four trials, V-10142 at high rates controlled sprangletop preemergence but had very little postemergence activity. In one trial, V-10142 did not appear to have any sprangletop activity. Overall, V-10142 appears to be a very promising herbicide for early season weed control in drill-seeded rice.
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Alligatorweed (Alternanthera philoxeroides (Mart.) Griseb.) is becoming more problematic as Louisiana producers adopt conservation tillage practices. Studies were established in the fall of 2003 and 2005 near Monroe, La to evaluate fall applications of herbicides for alligatorweed control. The field used for the 2003 study was planted to rice in 2002 and 2004 and fallowed in 2003 and 2005. The field used for the 2005 study was planted to rice in 2004 and 2006 and fallowed in 2005. During the fallow years the fields were disked 2-3 times, leveled and cultivated. The fields were not tilled during the cropping years. The effect application timing on alligatorweed control with glyphosate, glyphosate plus 2,4-D and glyphosate plus triclopyr was evaluated. Glyphosate applied alone resulted in excellent alligatorweed control for as much as two years after application. Control was best from mid-September to early-October, and was considerably lower with mid-October application timings. Tank mixing glyphosate with either 2,4-D or triclopyr did not improve alligatorweed control. In many cases, especially with a mid-October application, alligatorweed control was reduced when glyphosate was mixed with 2,4-D or triclopyr compared to glyphosate alone. This research suggests that fall applications of glyphosate can effectively control alligatorweed when applications are made between September 15 and October 15.
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Witchweed (Striga sp.) infestations are the greatest obstacle to food production in many sorghum producing areas. Witchweeds are obligate root parasitic weeds that infested many sorghum fields in West Africa. Although host-plant resistance and cultural management strategies have been shown to provide good control of these parasites, new management tools are needed to further improve the level of control. The objective of this project was to evaluate herbicide seed treatments to control Striga infestation in sorghum. Seeds from an experimental sorghum hybrid (ATx623 x Tailwind) with resistance to acetolactate synthase (ALS) inhibiting herbicides were treated with varying rates of imazapyr (0.018, 0.037, 0.075 mg ai seed-1) and metsulfuron (0.003, 0.006,0.012 ai seed-1). These treatments were compared to an untreated control in Cinzania, Mali and Konni, Niger in 2005. Striga infestation was variable in Mali and no significant differences were detected among treatments. In Niger, significant differences were detected between herbicide treatments. Striga emergence was delayed in plots produced using herbicide treated seeds compared to the untreated controls. Striga counts also were lower in plots produced from seed treated either with imazapyr or metsulfuron as compared to the control. These studies suggest that herbicide seed treatments may provide another tool for suppressing or delaying Striga infestation of sorghum.
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Postemergence (POST) control of common lambsquarters with glyphosate in glyphosate-resistant crops has generally been effective. In 2002, a common lambsquarters biotype from Westmoreland County, Virginia was identified after not being controlled with a POST glyphosate application in glyphosate-tolerant soybean. F1 and F2 progeny from a common lambsquarters plant collected at this site exhibited significantly reduced susceptibility to glyphosate relative to common lambsquarters collected in Montgomery County, Virginia. Because the Westmoreland population was not challenged with glyphosate in the grower’s normal rotation in 2003, no collections or observations were made during this growing season. In 2004, mature plants from the Westmoreland location which survived glyphosate application were harvested, and two generations of progeny from these plants were also screened for glyphosate sensitivity. All comparisons were made relative to the susceptible biotype from Montgomery County. Vigor reduction of F1 progeny from three 2004 Westmoreland common lambsquarters plants with 1.12 kg ai/ha of glyphosate ranged from 66 to 85% at 28 days after treatment (DAT), compared to 90% for the Montgomery biotype. In the F1 line for which vigor was reduced 66%, 0% mortality was observed, relative to 63% mortality for the Montgomery biotype. Fifteen F2 progeny lines were created from each of the three 2004 Westmoreland F1 lines. Response to glyphosate varied significantly among these populations, and among the 15 F2 lines within some populations. Vigor reduction of one Westmoreland F2 population among the 15 lines ranged from only 24 to 37% at 28 DAT when treated with 2.24 kg ai/ha glyphosate. Sequential applications of 0.56, 1.68, and 2.24 kg ai/ha of glyphosate were made at three week intervals to 400 individual seedlings from one 2002 Westmoreland F2 line, and to the same number of Montgomery seedlings. The same sequential treatments were made to 360 seedlings from three 2004 Westmoreland F2 lines, and to the same number of Montgomery seedlings. In the 2002 F2 line, vigor reduction at three weeks following the third application was 61.0 and 99.7% for Westmoreland and Montgomery biotypes, respectively. For the 400 plants treated, 13 and 98% mortality was observed in Westmoreland and Montgomery common lambsquarters, respectively. Similar results were observed with sequential treatments to 2004 Westmoreland F2 lines. In the least susceptible 2004 Westmoreland F2 line, vigor reduction averaged 36% at three weeks following the third application, and no mortality was observed among the 360 treated plants.
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Field and greenhouse studies were conducted in 2006 to determine the response of four giant ragweed populations from Ohio and Indiana to foliar application of glyphosate. These populations were mostly from continuous glyphosate-resistant soybean fields that had been treated with glyphosate exclusively for a number of years. Problems with the control of giant ragweed had been observed in these fields in 2004 or 2005. The field study included two glyphosate-sensitive populations, and four populations from fields where glyphosate failed to adequately control giant ragweed (suspect populations). Glyphosate was applied at 0.8 and 2.5 kg ae ha-1 to plants 5 to 51 cm tall. Glyphosate was applied again at 1.7 kg ha-1 three weeks later to half of the plants treated with 0.8 kg ha-1 initially. A total of 54 to 85 plants per population were treated with each initial rate. Glyphosate applied twice, or once at the higher rate, controlled at least 93% of the sensitive populations. Control of the four suspect populations ranged from 50 to 76% for two applications of glyphosate, and from 45 to 83% for the single application at 2.5 kg ha-1. A dose response study was conducted in the greenhouse with two sensitive and four suspect giant ragweed populations. Glyphosate was applied at eight rates ranging from 0.08 to 20 kg ha-1 when plants were in the three- to four-node stage. Results of the greenhouse study were subjected to analysis of variance and a log-logistic function was fit to the data to estimate the herbicide dosage required to reduce giant ragweed biomass by 50%. Results of the greenhouse study will be presented.
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A 2-yr non-irrigated field experiment was conducted on a Sharkey clay soil at the Delta Branch Experiment Station, Stoneville, MS to evaluate the effect of residual herbicides on grass control in wide- (102-cm) and narrow- (51-cm) row glyphosate-resistant soybean as influenced by planting date in the early soybean production system of the midsouth USA. Planting dates were 5 April (early), 18 April (standard), and 11 May (late) in 2005 and 27 March, 19 April, and 14 May in 2006. Herbicide programs evaluated were 1) no herbicide, 2) 0.84 kg ae ha-1 glyphosate early postemergence (EPOST) followed by (fb) 0.94 kg ha-1 glyphosate mid-postemergence (MPOST), 3) 0.84 kg ha-1 glyphosate (EPOST) fb + 0.94 kg ha-1 glyphosate plus 1.6 kg ai ha-1 pendimethalin (MPOST), and 4) 0.84 kg ha-1 glyphosate (EPOST) fb + 0.94 kg ha-1 glyphosate plus 1.26 kg ai ha-1 S-metolachlor (MPOST). All herbicides were applied over-the-top of weeds and soybean in 140 L ha-1 water. Herbicide treatments for each planting date were applied to 2.5- to 7.6-cm-tall grasses (EPOST and MPOST) and soybean in the V1 (EPOST) and V3 (MPOST) growth stages. Treatments were arranged in a split-split plot design with row spacing as the main-plot, planting date as the sub-plot, and herbicide treatment as the sub sub-plot. Treatments were replicated four times in 4- by 12-m plots. End-of-season control of barnyardgrass [Echinochloa crus-galli (L.) Beauv.] just prior to soybean harvest was better in 2006 vs. 2005 due to overall drier environmental conditions in 2006. Barnyardgrass control was higher in narrow-rows (77%) vs wide-rows (67%) across all planting dates and herbicide treatments except the untreated check. Planting date had little effect on end-of-season barnyardgrass control, with 66 to 77% control across all planting dates. Herbicide treatment had the most impact on end-of-season barnyardgrass control regardless of planting date and row spacing, with control levels of 51, 78, and 87%, respectively, for treatments 2, 3, and 4. Soybean yields were impacted by planting date, row spacing, and herbicide treatment. Delaying planting date decreased soybean yields from 2127 (late) to 2639 (standard) kg ha-1. Planting soybean in narrow-rows (2536 kg ha-1) resulted in higher yields compared to wide-rows (2358 kg ha-1). The addition of metolachlor to glyphosate MPOST increased soybean yields (2664 kg ha-1) when compared to glyphosate alone (2468 kg ha-1) or glyphosate plus pendimethalin (2443 kg ha-1), and the untreated (2214 kg ha-1). The addition of a residual herbicide to glyphosate improves overall control of problematic grasses and soybean yield when compared to glyphosate alone regardless of planting date and row spacing. Optimal yields and weed control were obtained with the addition of a residual herbicide (metolachlor) to a glyphosate-based weed control program in narrow-row soybean. Potential residual weed control benefits obtained with the addition of pendimethalin to glyphosate were offset by injury to soybean caused by over-the-top applications of pendimethalin.
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Advances in statistical software allow both standard and more complex statistical methods for non-linear regression analysis of dose response curves to be carried out conveniently by non-statisticians. One such statistical software is the freely available program R with the drc extension package. The drc package can: (1) simultaneously fit multiple dose-response curves, (2) compare curve parameters for significant differences, (3) calculate any point along the curve as the response level of interest, commonly known as an effective dose (eg. ED30, ED50, ED90), and determine its significance, (4) generate graphs for publications or presentations. We believe that when it comes to dose response data, the drc package has advantages over many currently available statistical software programs for non-linear regression analysis. Therefore, our objectives are to: (1) provide a review of few common issues in dose response curve fitting, (2) facilitate the use of up-to-date statistical techniques for analysis of dose response curves and (3) invite further debate on the subject.
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Field trials were conducted in East Lansing, MI in 2004 and 2005 and in St. Charles, MI in 2004, 2005, and 2006 to compare weed control and sugar beet tolerance from the addition of s-metolachlor and dimethenamid-P to sugarbeet micro-rate herbicide applications. Herbicide treatments consisted of a base micro-rate herbicide treatment of desmedipham and phenmedipham at 90 g/ha + triflusulfuron-methyl at 4.4 g/ha + clopyralid at 26 g/ha + methylated seed oil at 1.5% v/v applied four times at 125 growing degree days (base 1.1 C) intervals. Micro-rate treatments were applied alone and in different combinations with s-metolachlor or dimethenamid-P. Total s-metolachlor and dimethenamid-P application rates were 1.4 and 0.84 kg/ha, respectively. The different treatments consisted of the full s-metolachlor or dimethenamid-P rate applied PRE or in one of each of four micro-rate herbicide application timings, split-applications of each herbicide at PRE and the third micro-rate, first and third micro-rate, or in the second and fourth micro-rate herbicide application. Additional treatments included s-metolachlor or dimethenamid-P applied at a quarter of the full-rates in each of the four micro-rate herbicide applications. All treatments resulted in sugarbeet injury. In 2004 and 2006, full-rates of both s-metolachlor and dimethenamid-P applied PRE or in the first micro-rate resulted in significantly more injury compared with the base micro-rate treatment alone. Splitting the application rates of s-metolachlor or dimethenamid-P PRE and in the third micro-rate or in the first and third micro-rate applications also injured sugar beets greater than the base micro-rate treatment. Giant foxtail control 14 days after the fourth micro-rate was greater in all treatments including either s-metolachlor or dimethenamid-P compared with the base micro-rate. In 2004, all treatments containing an application of either s-metolachlor or dimethenamid-P increased late-season grass control compared with the base micro-rate herbicide treatment alone. S-metolachlor applied at a full-rate PRE, or in the first, second, or third micro-rate application provided greater late-season giant foxtail control compared with applications of dimethenamid-P at the same time. All treatments containing either s-metolachlor or dimethenamid-P provided greater common lambsquarters control compared with the base herbicide micro-rate treatment alone.
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The development of kochia populations resistant to triflusulfuron and other acetolactate synthase inhibitors poses a serious risk to sugar beet production. Trials were conducted in 2004 and 2005 under furrow irrigation at the Oregon State University, Malheur Experiment Station in Ontario, OR to evaluate the contribution of various herbicides to kochia control in sugar beet. The experiments were three-factor factorials with treatments arranged in a randomized complete block design with 4 replicates. One factor was herbicide program; consisting of either a standard or micro-rate postemergence herbicide program. The second factor was the presence or absence of triflusulfuron. The third factor was PRE ethofumesate at 0, 1120, 1680, or 2242 g/ha. Each possible combination of factors was evaluated. Triflusulfuron was omitted from selected treatments to simulate ALS resistance and to better evaluate the contribution of the other herbicide components to kochia control. The standard rate program was applied three times on 7 to 10 day intervals. The standard herbicide program included ethofumesate/phenmedipham/desmedipham applied at 280, 378, and 469 g/ha in the first, second, and third applications, respectively, triflusulfuron applied at 18 g/ha in all three applications, and clopyralid applied in the second and third applications at 105 g/ha. The micro-rate program consisted of four applications on 5 to 7 day intervals. In the micro-rate program, ethofumesate/phenmedipham/desmedipham was initially applied at 90 g/ha and increased to 140g/ha in the last two applications. The micro-rate also included triflusulfuron at 5.6 g/ha, clopyralid at 35 g/ha, and methylated seed oil at 1.5% v/v. Weed control was visually evaluated and sugar beet yields were determined by harvesting the center two rows of each plot. Data were analyzed for significant main effects and factor interactions and means were separated using Fishers protected LSD (P = 0.05). Data were combined over years. For kochia control, herbicide program by triflusulfuron and ethofumesate by triflusulfuron interactions were significant. The removal of triflusulfuron from herbicide combinations caused greater reductions in kochia control with the micro-rate than with the standard herbicide program. Similarly, ethofumesate effects on kochia control were negligible when triflusulfuron was included in the postemergence herbicide combinations. When triflusulfuron was removed, the addition of ethofumesate provided increased kochia control. A significant herbicide program by triflusulfuron interaction for sugar beet yield illustrated that sugar beet yields were similar among the standard rate and micro-rate herbicide programs when triflusulfuron was included in the mixtures, but when triflusulfuron was removed the standard rate program produced significantly greater yields than the micro-rate program. This data supports recommendations made in areas of the Midwest where kochia has become resistant to triflusulfuron, that include using preemergence application of ethofumesate followed by standard rates of postemergence herbicides.
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Sugarcane has been one of the most important crop for the Brazilian agriculture. Due to the importance of the industrialization process and extensive cultivated area in the country, the use of herbicides is widely spread among growers; being herbicide selectivity a big concern. Therefore, studies on the differentiation of the varietal tolerance are required in order to identify endogenous plant components, capable of protecting the plants from phytoxic effects of the herbicides. Glutathione-S-Transferase (GST) plays a main role in plant natural tolerance to herbicides in many plant species; although in sugarcane it is not well known this metabolism. The aim of this research was to evaluate sugarcane cultivar responses to commercial herbicides and their possible correlation with GSTs level in the plant. Thirty days seedlings of the varieties SP90-3414, SP80-1842, SP87-396 and SP83-2847 were used and the herbicides tested were ametryn (2.5 kg/ha), diuron (2,5 kg/ha), imazapic (0.077 kg/ha) and isoxaflutole (0.090 kg/ha). Shoot dry weight derived from two plants of each cultivar and chlorophyll content, measured using a SPAD-502 (Minolta), were measured from five replications of the last leaf with a visible dew lap. Two measurements were taken at 1, 8, 13, 19 and 23 days after herbicide treatments (DAA). It was observed in the varieties SP90-3414 and SP87-396 a growth inhibition, 8 DAA for all herbicides, while for the varieties SP80-1842 and SP83-2847, only at 19 and 23 DAA growth inhibition were observed, respectively. There was a reduction in the total content of chlorophyll in the SP87-396, 8 DAA, for all herbicides, while for the SP83-2847 there was no alteration in the chlorophyll content, showing a great tolerance to the herbicides imazapic and diuron, 19 DAA. Similarly, the SP90-3414 did not present reduction in the chlorophyll content, 13 DAA, with ametryn, diuron and imazapic. The variety SP80-1842 was tolerant to all herbicides at 19 DAA. It was possible to observe a great reduction in the content of chlorophyll, 23 DAA, induced by isoxaflutole in all varieties studied. With regard to GST content, it was observed in the SP87-396 and SP83-2847 a very low constitutive activity, while in the SP90-3414 and SP80-1842 was observed twice as much GST activity. It can be inferred from these results that there is a differential response of sugarcane varieties in relation to the herbicide applied, and that these responses might be correlated to the level of GSTs, as possible classes or specific isoforms, that should be investigated in future researches in the project.
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Approximately 60,000 acres of sugarcane in Florida are produced on sandy soils. Sandland sugarcane acreage has been increasing due to the unavailability and subsidence of high organic matter, muck lands in the Everglades Agricultural Area (EAA). However, the majority of herbicide research conducted in the EAA has been on muck soils. In 2005, trials were conducted at 5 sandland sugarcane locations to evaluate weed control and crop safety with hexazinone at 0.15 kg ha-1 + diuron at 0.53 kg ha-1, hexazinone at 0.3 kg ha-1 + diuron at 1.05 kg ha-1, hexazinone at 0.45 kg ha-1 + diuron at 1.6 kg ha-1, atrazine at 4.5 kg ha-1, pendimethalin at 3.7 kg ha-1, flumioxazin at 0.14 kg ha-1, flumioxazin at 0.21 kg ha-1, and pendimethalin 3.7 kg ha-1 + atrazine at 4.5 kg ha-1. All herbicide treatments were applied preemergence (PRE) and early postemergence (EPOST) in a randomized complete block design. Weed control and sugarcane phytotoxicity were evaluated visually. Four weeks after treatment (WAT) hexazinone at 0.45 kg ha-1 + diuron at 1.6 kg ha-1 applied PRE controlled tropical crabgrass (Digitaria bicornis) 86 to 89% and fall panicum (Panicum dichotomiflorum) 90% to 94%. EPOST applications of hexazinone at 0.45 kg ha-1 + diuron at 1.6 kg ha-1 controlled fall panicum 80 to 90% 6 WAT. With hexazinone + diuron treatments, crop injury tended to be less severe with PRE applications compared to EPOST applications.
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Two field trials at two different locations were conducted in the Willamette Valley, OR, to assess ACCase-inhibiting herbicide efficacy for Italian ryegrass control. At both sites, winter wheat has been grown for several years with a long history of ACCase-inhibiting herbicide use. Herbicide treatments included the aryloxyphenoxypropionates (APP) diclofop-methyl, fenoxaprop-ethyl, and clodinafop-propargyl; the cyclohexanediones (CHD) tralkoxydim and clethodim; and the new ACCase inhibitor, pinoxaden. At site 1, Italian ryegrass control at 113 days after treatment (DAT) was 10% for diclofop-methyl and clodinafop-propargyl, 0% for tralkoxydim, 85% for clethodim, and 43% for pinoxaden. At site 2, Italian ryegrass control at 128 DAT was 0% for fenoxaprop-ethyl, 25% for clodinafop-propargyl, 32% for tralkoxydim, 62% for clethodim, and 42% for pinoxaden. Based on these results, both populations are resistant to APP herbicides and the CHD tralkoxydim, but not to clethodim. Although this was the first time these Italian ryegrass populations were treated with pinoxaden, the percentage of control was low. Seed from plants that survived the herbicide treatments were collected in order to confirm herbicide resistance and to further investigate the mechanisms of resistance.
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