No-till production systems coupled with decreased use of soil residual herbicides has led to increased populations of winter annual weeds. Therefore, research was conducted to quantify the effect of henbit interference and crop stand loss on soft red winter wheat grain yield and grain volume weight. The main-plot effect consisted of weed-free versus weedy plots. The subplot effect was winter wheat stand loss treatments of 0, 20, 40, 60, 80, and 100%. Henbit interference did not affect test weight; however, test weight decreased as percentage of stand loss increased. Henbit did not reduce crop yield at 18 plants/m2; however, at 82 and 155 plants/m2, crop yield was reduced 13 and 38%, respectively, when averaged across all stand loss treatments. Yield estimates based on wheat tiller or spike density decreased linearly as crop stand loss increased. Our results indicated that henbit interference coupled with crop stand loss may significantly decrease crop yield and profitability of wheat production systems.

A field study was conducted in 2002 and 2003 to evaluate tolerance of eight rice cultivars to clomazone at 896 g ai/ha impregnated onto urea fertilizer. Rice foliar bleaching was 16 to 20% at 14 d after rice rooting (DAR) for long-grain cultivars ‘Ahrent’, ‘Cheniere’, ‘Cocodrie’, ‘Cypress’, ‘Francis’, and ‘Wells’ and 23 and 30% for medium-grain ‘Bengal’ and short-grain ‘Pirogue’, respectively, when clomazone was impregnated onto urea. Clomazone reduced the number of tillers/ m2 21 DAR for all cultivars. Early season height reductions occurred for all cultivars; however, Pirogue was the only cultivar shorter than the nontreated at harvest. There were no yield reductions with regard to the medium and long-grain cultivars when compared with respective nontreated cultivars. However, yield of short-grain Pirogue treated with clomazone was 1,740 kg/ha less than the nontreated.

Japanese stiltgrass is a nonnative invasive grass that occurs in a variety of habitats and is widely distributed throughout the eastern United States. In natural areas such as forests, herbicide options that selectively control Japanese stiltgrass while preserving native herbaceous and woody vegetation may be desired. The efficacy of three selective postemergence herbicides (fenoxaprop-P, imazapic, and sethoxydim) applied early season, midseason, or late season on monoculture understory stands of Japanese stiltgrass in forests was examined in an experiment conducted at a site in North Carolina and a site in Virginia from 2002 to 2004. The herbicides, averaged across application timings, controlled Japanese stiltgrass at the end of the growing season 83 to 89% and seedhead production 79 to 94% compared with nontreated plants. Seedling emergence was reduced in the spring of 2004 by 89, 70, and 78% by fenoxaprop-P, imazapic, and sethoxydim, respectively, applied in 2003. In another experiment at the North Carolina site in 2002 and 2003, fenoxaprop-P or sethoxydim applied twice (4 wk apart) at half-registered rates controlled Japanese stiltgrass. This study demonstrates that land managers have multiple POST herbicide and application timing, rate, and frequency options for Japanese stiltgrass control.

The effect of application technology on efficacy has been thoroughly investigated for sprays of chemical pesticide but not biological pesticides. This study investigates the effect of applying conidia of Microsphaeropsis amaranthi, a candidate bioherbicide for Amaranthus spp., with a range of different hydraulic nozzle tips. The nozzle tips were selected to deliver sprays with different spectra of droplet sizes deployed at different angles. We found that sprays of large droplets gave poor coverage of the target and resulted in low levels of disease severity on common waterhemp. The most effective nozzle tip tested was a hollow cone nozzle tip, such as is commonly used for the application of fungicides. This nozzle tip deployed large numbers of fine droplets that swirled within the plant canopy, provided good coverage of all plant parts, and resulted in the highest levels of disease severity, particularly on stems.

The value of glyphosate has been compromised in some fields where weed populations have developed resistance or tolerant species increased. Three case studies related to reduced control from glyphosate are: (1) a weed population that has become resistant to glyphosate, with horseweed in Tennessee as an example; (2) a weed population increases due to lack of control in “glyphosate only” systems, with tropical spiderwort in Georgia cotton used as an example; and (3) the hypothetical resistance of common waterhemp to glyphosate in Illinois. For each of these case studies, an economic analysis was performed using a partial budget approach. This economic analysis provides the cost of control to the farmer when glyphosate fails to control these weeds and gives a critical time in years to compare different glyphosate resistance management philosophies (applicable only before resistance has evolved). The cost of glyphosate-resistant horseweed in cotton-soybean-corn rotation in Western Tennessee was calculated to be $30.46/ha per year. The cost of tropical spiderwort in cotton in southern Georgia was calculated to be $35.07/ha per year. The projected cost if common waterhemp were to develop glyphosate resistance in a corn-soybean rotation in southern Illinois was projected to be $44.25/ha per year, and the critical time was determined to be greater than 20 yr, indicating that a resistance management strategy would extend the value of glyphosate-resistant crops.

Field experiments in 2001 and 2002 at Stuttgart, AR, demonstrated cyhalofop applied POST controlled propanil-susceptible barnyardgrass, propanil-resistant barnyardgrass, and broadleaf signalgrass 67, 65, and 71%, respectively 28 days after treatment (DAT). Halosulfuron applied 1 d before cyhalofop or applied in a tank-mixture reduced broadleaf signalgrass and propanil-resistant or propanil-susceptible barnyardgrass by 35 to 59 percentage points. Triclopyr applied 5, 3, or 1 d before, or mixed with, cyhalofop reduced control of propanil-resistant and propanil-susceptible barnyardgrass, 16 to 41 percentage points, respectively. Triclopyr applied 1 d before cyhalofop or combined with cyhalofop reduced broadleaf signalgrass control 38 and 18 percentage points, respectively compared with cyhalofop alone. Propanil reduced cyhalofop grass control only when mixed with cyhalofop. Application timings, averaged across halosulfuron, propanil, and triclopyr, made 1 d before cyhalofop or tank-mixed with cyhalofop reduced grass control more compared with other application timings made before or after cyhalofop. Barnyardgrass absorption of 14C-cyhalofop alone or mixed with propanil was 86 and 75%, respectively, with 14C-cyhalofop translocation being reduced by 10-fold when propanil was mixed with cyhalofop. Reductions in cyhalofop translocation in combination with propanil may have partially explained the reductions in grass control observed in the field.

Glyphosate-resistant soybean at 494,000 seeds/ha was drill seeded with conventional soybean at 0 to 1,976,000 seeds/ha, and conventional soybean controlled at 2, 4, or 6 wk after soybean emergence with glyphosate at 0.84 kg ae/ha. Weed control increased exponentially with soybean groundcover prior to treatment with glyphosate and was maximized at 69 and 84% for Palmer amaranth and Florida pusley, respectively. When glyphosate was applied 2 wk after soybean emergence, yields were improved 390 kg/ha at 494,000 glyphosate-resistant soybean plus 247,000 conventional soybean seeds/ha compared with glyphosate-resistant soybean alone. Further increases in seeding rate of the conventional soybean often had a negative effect on soybean yields due to interference between glyphosate-resistant and conventional soybean prior to applying glyphosate. Glyphosate applied 6 wk after soybean emergence was generally less effective in controlling Palmer amaranth and Florida pusley than at earlier timings because of weed size at application and the overlying soybean canopy. This research indicates the mix-seeded soybean system may suppress early-season weeds, but glyphosate needs to be applied no later than 4 wk after soybean emergence to ensure optimum yields and prevent interference between glyphosate-resistant and conventional soybean.

Studies were conducted at the University of Florida, West Florida Research and Education Center to determine the effect of glyphosate on purple nutsedge control and nutsedge tuber production when glyphosate was applied to the same plots over 3 y in glyphosate-resistant soybean and cotton. Greater than 90% control of purple nutsedge foliage was achieved with a single POST application of glyphosate at 0.9 kg ai/ha in soybean or a sequential glyphosate application of 1.1 kg/ha POST followed by 0.6 kg/ha POST-directed in cotton. By the end of the third year of the study, these same treatments reduced purple nutsedge tuber density to less than 0.2% of the nontreated. In cotton, cultivation alone reduced tuber numbers by greater than 90%. Viability of tubers was also reduced by 80% in soybean and by 65% in cotton in the glyphosate-treated plots. Comparison treatments of imazaquin PRE followed by imazaquin POST in soybean or norflurazon PRE followed by cyanazine plus MSMA POST-directed in cotton also reduced purple nutsedge tuber density by ≥85% after three consecutive years of treatment.

Field trials were conducted with sulfentrazone at 53, 80, and 105 g ai/ha combined with 0, 420, and 560 g ai/ha metribuzin in a 3 by 3 factorial arrangement. Sulfentrazone and metribuzin combinations improved control of redroot pigweed, common lambsquarters, hairy nightshade, and volunteer oat compared with sulfentrazone applied alone. Kochia control was more than 90% regardless of metribuzin presence in the treatment. Sulfentrazone at 105 g/ha was needed for greater than 90% hairy nightshade control. Sulfentrazone alone or in combination with metribuzin did not provide greater than 89% volunteer oat control. Potato crop injury was less than 5% and total tuber yields increased as metribuzin rate increased from 0 to 560 g/ha, reflecting improved weed control as metribuzin rate increased.

Field studies were conducted in 2003 and 2004 near Scottsbluff and Sidney, NE, to identify efficacious chemical weed-control options for irrigated and dryland chickpea production. Weed control had a greater relative effect on chickpea yield in the irrigated system than the dryland system, with yield from the hand-weeded check exceeding the nontreated check by 1,500% in the irrigated system and 87% in the dryland system. Imazethapyr, applied preemergence at the rate of 0.053 kg ai/ha, reduced plant height, delayed plant maturity, and caused leaf chlorosis. At Scottsbluff, preplant-incorporated ethalfluralin caused significant crop injury in 2003, but the ethalfluralin treatment also maintained weed densities 4 wk after crop emergence that were not significantly different than the hand-weeded check at both locations in 2003 and 2004. Treatments containing sulfentrazone provided a similar level of weed control but without any evidence of crop injury. Pendimethalin and pendimethalin + dimethenamid-P applied preemergence provided acceptable weed control in the irrigated system, where water was applied within 4 d after herbicide application, but did not provide acceptable control in the dryland system.

Treatments of dimethenamid-p at 0.7 kg ai/ha applied PRE in tank mixtures with EPTC (3.4 kg ai/ha), metribuzin (560 g ai/ha), pendimethalin (1.1 kg ai/ha), or rimsulfuron (26 g ai/ha) were compared with the same herbicides applied PRE alone in field efficacy trials in Idaho, Oregon, and Washington. Common lambsquarters, kochia, and redroot pigweed control was generally improved with dimethenamid-p tank mixtures compared with control by any herbicide applied alone except metribuzin. Hairy nightshade control at two locations was generally improved with tank mixtures compared with control by any herbicide applied alone. At Washington, where row spacing was narrower than at other locations, dimethenamid-p alone or in tank mixtures provided similar hairy nightshade control, and this control was greater than control by EPTC, metribuzin, or pendimethalin applied alone. ‘Alturas’, ‘Bannock Russet’, ‘Ranger Russet’, ‘Russet Burbank’, ‘Russet Norkotah’, and ‘Shepody’ potato tolerance to dimethenamid-p PRE at 0, 0.7, or 1.4 kg ai/ha was assessed in weed-free field trials conducted at Aberdeen, ID, in 2002 and 2003. Little or no crop injury was evident during the growing seasons and there were no reductions in U.S. No. 1 and total tuber yields regardless of dimethenamid-p rate or potato variety.

Plant competition studies designed to quantify interference between species provide valuable information on competitive interactions and on the effects of agronomic practices on those interactions. The effect of each species' density on the growth of itself and on the growth of the other species is quantified in a series of regression models. Traditionally, the models' regression coefficients have been combined in a series of ratios to quantify relative competitive ability and niche differentiation. Coefficients that are negative (positive interference—facilitation, mutualism) or zero (neutral interference or nonsignificant coefficient) do not lend themselves well to ratio-based methodology because of sign cancellation or undefined values, respectively. As a result, ratio-based methodology is limited to using only positive coefficients (negative interference—amensalism, competition). Rather than using ratios, the absolute-log method uses addition and subtraction of coefficients converted to a pseudologarithmic scale, thus allowing for use of coefficients with values that are negative or zero. As a result, the absolute-log method can be used to quantify relative competitive ability and niche differentiation involving all types of interference—negative, positive, and neutral. The absolute-log method includes an optional statistical procedure constructing confidence intervals for the estimates of relative competitive ability and niche differentiation.

Herbicides and herbicide prepackaged mixtures registered for use on established bermudagrass turf may cause significant injury to recently seeded bermudagrass cultivars, delaying full establishment. Research was conducted to evaluate the use of 12 herbicide treatments applied at onset of uniform stolon development (4 to 8 wk after seeding) to recently seeded ‘Princess 77,’ ‘Riviera,’ ‘Savannah,’ and ‘Yukon’ bermudagrass cultivars. In general, Yukon was more susceptible to herbicide injury than other cultivars. Atrazine at 1.1 kg ai/ha injured all cultivars 55 to 59% 14 d after initial treatment (DAIT), which lead to reduced bermudagrass cover 21 DAIT. Triclopyr + clopyralid at 0.63 + 0.21 kg ae/ha, respectively, injured Savannah and Yukon greater than other broadleaf weed herbicides (2,4-D + mecoprop + dicamba or 2,4-D + clopyralid + dicamba). Foramsulfuron did not injure or reduce bermudagrass cover of any cultivar evaluated at any rating date. The only adverse effect of trifloxysulfuron was a reduction in Riviera and Yukon ground cover at 21 DAIT. MSMA applied sequentially and quinclorac treatments did not injure or reduce ground cover of Savannah or Princess 77. MSMA applied sequentially and/or quinclorac injured and reduced ground cover of Riviera and Yukon; however, both cultivars completely recovered from MSMA or quinclorac injury by 42 DAIT.

Field studies were conducted in 1999 to 2000 on a clay soil and a sandy-loam soil in Londrina and Palmeira, PR, Brazil, respectively, to determine the persistence and carryover effect of a mixture of imazapic and imazapyr, applied to imidazolinone-tolerant corn, on rotational crops of soybean, edible bean, wheat, and corn in two different planting systems (no till and tillage). Main plots were herbicide treatments (0, 52.5 + 17.5, and 105 + 35 g ai/ha for imazapic and imazapyr, respectively) and subplots were five intervals (0, 30, 60, 90, and 120 d) between the herbicide application and rotational crop planting. Soil samples were collected for a cucumber bioassay and chemical residues analysis at each time interval. The dissipation time (DT50) of the herbicides in the soil was greater in Londrina than Palmeira, for both imazapic (54 d vs. 27 d, respectively) and imazapyr (40 d vs. 33 d, respectively), probably due to the lower pH and greater clay content of the soil in Londrina compared with Palmeira. The DT50 for both herbicides tended to increase slightly in no-till compared with conventional tillage but the differences were not great. Soybean was the least sensitive rotational crop, with a period for no yield drag (PINYD) of 87 d in Londrina and 88 d in Palmeira. Wheat and edible bean showed intermediate sensitivity. The PINYD for wheat and edible bean was 99 and 98 d for Londrina and 91 and 97 d for Palmeira, respectively. Corn was the most sensitive, with a PINYD of 117 d in Londrina and 97 d in Palmeira. Cucumber was more sensitive to imazapic and imazapyr residues than the rotational crops and should be an effective bioassay to indicate when rotational crops can be safely planted.

Pendimethalin plus atrazine (1,120 + 2,240 g ai/ha) or metolachlor plus atrazine (2,170 + 1,790 g ai/ha) applied PRE over 2 yr controlled broadleaf signalgrass and itchgrass 78 to 88% 30 d after treatment (DAT). The imazethapyr plus imazapyr prepackaged mixture (PREMIX) applied PRE at 47 + 16 g ai/ha controlled these weeds no more than 35%. With pendimethalin, metolachlor plus atrazine, or atrazine PRE followed by (fb) imazethapyr plus imazapyr POST, broadleaf signalgrass was controlled 89 to 98% and itchgrass 76 to 83% 28 d after POST application. When only imazethapyr plus imazapyr was applied POST, broadleaf signalgrass control was 88% the first year, but only 64% the second year. Itchgrass was controlled 71% when imazethapyr plus imazapyr was applied alone POST, but control was nearly 86% when pendimethalin or nicosulfuron was applied POST with imazethapyr plus imazapyr. Consistently high corn yields were obtained when pendimethalin was applied POST with imazethapyr plus imazapyr, and these corn yields were greater than those following POST-only applications of imazethapyr plus imazapyr in 1 of 2 yr. When imazethapyr plus imazapyr was applied alone POST, corn yield was at least 1.3 times that when imazethapyr plus imazapyr was applied alone PRE. Weed control with imazethapyr plus imazapyr POST at 47 + 16 g/ha was not improved when ammonium sulfate was added with nonionionic surfactant or when methylated seed oil with or without ammonium sulfate or organosilicone adjuvants were used in the spray solution compared with using only nonionic surfactant in the spray solution. When imazethapyr plus imazapyr plus the various adjuvants was applied to weeds 5 to 8 cm tall, broadleaf signalgrass was controlled 93%, itchgrass 74 and 88%, and pitted morningglory 87 and 93% 28 DAT over 2 yr.

Field studies were conducted from 2000 through 2002 to evaluate the effects of atrazine, pendimethalin, and trifluralin applied alone or in combination followed by cultivation when weeds and grain sorghum were less than 7 cm tall (early postemergence [EPOST]) or when weeds and grain sorghum were 10 to 15 cm tall (late postemergence [LPOST]). Atrazine plus pendimethalin applied EPOST caused 9 to 14% sorghum stunting all 3 yr while atrazine plus trifluralin applied EPOST caused 1 to 4% grain sorghum stunting. When applied LPOST, atrazine plus pendimethalin or trifluralin resulted in no greater than 3% stunting. Tumble pigweed was controlled at least 99% with atrazine plus pendimethalin or trifluralin applied EPOST or LPOST, whereas Texas panicum was controlled at least 97% with atrazine plus pendimethalin or trifluralin applied EPOST, and 76 to 100% with LPOST application. Sorghum yields were reduced with atrazine plus pendimethalin applied EPOST when compared with all herbicide combinations in one of 3 yr.

Greenhouse experiments were conducted in the summer of 2002 to determine the effect of root-knot nematodes on purple nutsedge suppression and chile pepper response after applications of halosulfuron. Purple nutsedge and chile pepper plants were grown together in 20-cm-diam pots, inoculated with root-knot nematodes, and treated with halosulfuron 2 or 4 wk after inoculation. Root-knot nematode infection had no effect on purple nutsedge control or chile pepper injury after treatment with halosulfuron, regardless of application timing. However, root-knot nematode reproduction was lower in pots that received a halosulfuron treatment, particularly halosulfuron applied 2 wk after inoculation. Controlling purple nutsedge using halosulfuron in a field infested with root-knot nematodes could also decrease the amount of nematode inoculum that is available for infecting chile plants later in the season.

A study was conducted in 2004 to determine the effect of coapplication of the insecticides acephate, acetamiprid, bifenthrin, cyfluthrin, cypermethrin, dicrotophos, dimethoate, emanectin benzoate, imidacloprid, indoxacarb, lambda-cyhalothrin, methoxyfenozide, spinosad, thiamethoxam, and zeta-cypermethrin; the plant growth-regulator mepiquat pentaborate; a foliar sodium calcium borate micronutrient solution; and a foliar nitrogen fertilizer solution with glyphosate on the efficacy of weeds that commonly infest cotton. Barnyardgrass, hemp sesbania, johnsongrass, pitted morningglory, and sicklepod were grown in outdoor containers and treated with glyphosate at 1,120 g ai/ha alone or in coapplication at the three-to four- or seven-to eight-leaf growth stage. Glyphosate efficacy, based on visual control ratings at 7, 14, and 28 d after treatment (DAT) and fresh weight reduction of weed biomass at 28 DAT, was unaffected by chemical coapplication or application timing. Averaged across application timing and visual rating interval, glyphosate alone controlled barnyardgrass 97%, hemp sesbania 68%, johnsongrass 98%, pitted morningglory 68%, and sicklepod 89%. These results indicate that glyphosate coapplications evaluated offer producers the ability to combine pest and crop management strategies and reduce application costs without sacrificing control of weeds evaluated.

Winter cover crops are increasingly common on organic and conventional vegetable farms on the central coast of California between periods of intensive vegetable production. A 2-yr study was conducted in Salinas, California, to quantify (1) cover crop and weed biomass production during cover cropping, (2) early-season canopy development of cover crops, (3) weed seed production by burning nettle during cover cropping, and (4) weed emergence following cover crop incorporation. The cover crops included oats, a mustard mix, and a legume/oats mix that were planted in October and soil-incorporated in February. Weed and cover crop densities, early-season cover crop canopy development, above-ground weed and cover crop biomass production, seed production by the burning nettle, and postincorporation weed emergence was evaluated. Mustard produced more early-season biomass than oats and the legume/oats mix. There were no differences in above ground biomass production by the cover crops at the end of their growth period. Suppression of weed biomass and seed production of burning nettle was greatest in mustard, and least in oats and the legume/oats mix. The weed suppressive ability of each cover crop was affected by early-season canopy development and was highly correlated with cover crop plant density. Weed emergence following cover crop incorporation was in order of legume/oats mix > oats > mustard in yr 1, but was not different in yr 2. This study provides initial information on cover crop effects on weed management in irrigated and tilled vegetable production systems in the central coast of California. The results suggest that the legume/oats mix could exacerbate weed problems in subsequent vegetable crops.

Canada thistle is one of the most troublesome and difficult weed species to control in established alfalfa grown for seed production. Current tools available for control are limited because of cultural management strategies associated with seed production. Alfalfa seed losses due to Canada thistle interference include both reduced yields from competition and increased seed loss during seed cleaning operations. Additional tools are needed to alleviate these losses. Field experiments were conducted in 1998, 1999, and 2000 at two locations in Park County, WY, to evaluate Canada thistle control and alfalfa tolerance to several postemergence herbicides. Bentazon, imazamox, imazethapyr, and MCPB were applied, alone or in combination, at different Canada thistle growth stages. Methylated seed oil (MSO) was added at 1.5% v/v to the treatments containing imazamox or imazethapyr. MCPB applied alone when Canada thistle was 7.5- or 15-cm tall caused severe alfalfa injury (28 to 40%) and resulted in less Canada thistle control (23 to 27%). Imazamox or imazethapyr applied alone when Canada thistle was 15-cm tall did not cause any significant alfalfa injury but resulted in unsatisfactory Canada thistle control (29 to 35%). Bentazon was the only treatment containing a single herbicide that provided more than 50% Canada thistle control. The treatments providing the best balance between Canada thistle control (>80%) and alfalfa injury (<13%) were a single application of bentazon combined with either imazamox or imazethapyr. These two treatments also yielded the highest, more than 800 kg/ha. Split applications of bentazon combined with imazamox or imazethapyr were similar to single applications.