The organic grain sector is one of the fastest growing sectors of the organic market, but farmers in the mid-Atlantic cannot meet the organic grain demand, including the demand for organic soybean. Weed management is cited by farmers as the largest challenge to organic soybean production. Recent soybean population studies show that lower seeding rates for genetically modified organism soybean farmers provide maximum economic return due to high seed technology fees and inexpensive herbicides. Such economic analysis may not be appropriate for organic soybean producers due to the absence of seed technology fees, stronger weed pressures, and price premiums for organic soybean. Soybean seeding rates in North Carolina have traditionally been suggested at approximately 247,000 live seeds/ha, depending on planting conditions. Higher seeding rates may result in a more competitive soybean population and better economic returns for organic soybean producers. Experiments were conducted in 2006 and 2007 to investigate seeding rates of 185,000, 309,000, 432,000, and 556,000 live seeds/ha. All rates were planted on 76-cm row spacing in organic and conventional weed management systems. Increased soybean seeding rates reduced weed ratings at three of the five sites. Increased soybean seeding rates also resulted in higher yield at three of the four sites. Maximum economic returns for organic treatments were achieved with the highest seeding rate in all sites. Results suggest that seeding rates as high as 556,000 live seeds/ha may provide organic soybean producers with better weed control, higher yield, and increased profits.

Field experiments were conducted in Hale Co., TX, in 2005 and 2006 to determine the effects of 2,4-D amine and dicamba applied at varying rates and growth stages on cotton growth and yield, and to correlate cotton injury levels and lint yield reductions. Dicamba or 2,4-D amine was applied at four growth stages including cotyledon to two-leaf, four- to five-leaf, pinhead square, and early bloom. Dicamba and 2,4-D amine were applied at 1/2, 1/20, 1/200, and 1/2000 of the recommended use rate. Crop injury was recorded at 14 days after treatments and late-season, and cotton lint yields were determined. Across all growth stages, 2,4-D caused more crop injury and yield loss than dicamba. Cotton lint was reduced more by later applications (especially pinhead square) and injury underestimated yield loss with 2,4-D. Visual estimates of injury overestimated yield loss when 2,4-D or dicamba was applied early (cotyledon to two leaf) and was not a good predictor of yield loss.

Although glyphosate controls many plant species, certain broadleaf weeds in Nebraska's cropping systems exhibit various levels of tolerance to the labeled rates of this herbicide, including ivyleaf morningglory, Venice mallow, yellow sweetclover, common lambsquarters, velvetleaf, kochia, Russian thistle, and field bindweed. Therefore, two field studies were conducted in 2004 and 2005 at Concord and North Platte, NE, to evaluate performance of (1) seven preemergence (PRE) herbicides and (2) glyphosate tank mixes applied postemergence (POST) at three application times for control of eight weed species that are perceived as problem weeds in glyphosate-resistant soybean in Nebraska. The PRE herbicides, including sulfentrazone plus chlorimuron, pendimethalin plus imazethapyr, imazaquin, and pendimethalin plus imazethapyr plus imazaquin provided more than 85% control of most weed species tested in this study 28 d after treatment (DAT). However, sulfentrazone plus chlorimuron and pendimethalin plus imazethapyr plus imazaquin were the only PRE treatments that provided more than 80% control of most weed species 60 DAT. In the POST glyphosate tank-mix study, the level of weed control was significantly affected by the timing of herbicide application; control generally decreased as weed height increased. In general, glyphosate tank mixes applied at the first two application times (early or mid-POST) with half label rates of lactofen, imazamox, imazethapyr, fomesafen, imazaquin, or acifluorfen, provided more than 80% control of all species that were 20 to 30 cm tall except ivyleaf morningglory, Venice mallow, yellow sweetclover, and field bindweed. Glyphosate tank mixes applied late POST with lactofen, imazethapyr, or imazaquin provided more than 70% control of common lambsquarters, velvetleaf, kochia, and Russian thistle that were 30 to 50 cm tall. Overall, glyphosate tank mixes with half label rates of chlorimuron or acifluorfen were the best treatments; they provided more than 80% control of all the studied weed species when applied at early growth stages. Results of this study suggested that mixing glyphosate with other POST broadleaf herbicides, or utilizing soil-applied herbicides after crop planting helped effectively control most problematic weeds in glyphosate-resistant soybean in Nebraska.

POST combinations of photosystem II (PSII) and the 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors are effective for control of broadleaf weeds in corn. Field studies were conducted during 2007 and 2008 in Urbana and Dekalb, IL, to determine the nature of interactions between two PSII inhibitors, atrazine and bromoxynil, and the HPPD inhibitor mesotrione, based on control of common waterhemp, common lambsquarters, and giant ragweed. Two rates of each herbicide were evaluated, with the highest representing a typical recommended rate, and the lowest a fraction of that rate. Synergistic interactions occurred for common waterhemp control from 10 to 30 d after treatment (DAT) regardless of herbicide rates, rainfall accumulation, or plant height. Synergism between mesotrione and bromoxynil was not observed for common lambsquarters control at Urbana in 2008 at the lower herbicide rates, possibly due to taller weed heights at the time of herbicide application relative to 2007. Giant ragweed control indicated that a synergistic interaction occurred for all herbicides and rates in 2008. However, synergism between bromoxynil and mesotrione did not occur in 2007, likely due to limited rainfall before herbicide application. Reduced herbicide rates and adverse environmental conditions have the potential to regulate the expression of synergism between mesotrione and PSII inhibitors, and therefore may limit their effectiveness for weed management in corn.

Spray adjuvants may enhance bispyribac–sodium efficacy for annual bluegrass control but chelated iron may be needed to reduce potential turf discoloration. Field and laboratory experiments were conducted to investigate the influence of iron and adjuvants on bispyribac–sodium efficacy for annual bluegrass control in cool-season turf. In laboratory experiments, 14C–bispyribac–sodium foliar absorption increased in four grasses by approximately 50 and 100% when applied with a nonionic surfactant and methylated seed oil, respectively, compared to the herbicide alone. Chelated iron did not reduce 14C–bispyribac–sodium absorption. In field experiments, spray adjuvants enhanced annual bluegrass control from bispyribac–sodium at 37 g ai/ha but not at 74 g ai/ha. Iron did not reduce annual bluegrass control from bispyribac–sodium, with or without adjuvants, but mitigated creeping bentgrass discoloration for all treatments.

Progress in bioherbicide development has been hindered by the strict moisture and temperature requirements of the living active ingredient. Application of a jute fabric to areas treated with a Sclerotinia minor granular bioherbicide improved broadleaf weed control and broadened the effective application period to include the warm summer season. When turfgrass plots treated with the bioherbicide were covered with burlap fabric for 3 d, broadleaf weed (dandelion, white clover, broadleaf plantain, buckhorn plantain, ground ivy, and prostrate knotweed) control was greatly enhanced. The cover was made of natural jute fibers that retained water but had sufficient transparency to allow 33% light penetration for continued growth of the grass. Virulence of the bioherbicide was maintained under elevated temperatures that would otherwise reduce efficacy. The bioherbicide was ineffective in the summer unless covered, but dandelion density, broadleaf weed ground cover, and dandelion survival were all reduced by the bioherbicide when plots were covered, even if applications were made in July. The efficacy of the bioherbicide was also enhanced under favorable conditions, and covering permitted reduced application rates without loss of efficacy. When applied at a rate of 20 g/m2 and covered, S. minor granules exerted significantly greater biocontrol of dandelion than 40 g/m2 without covering. Covering for up to 5 d did not cause any adverse effects on the turfgrass. This approach may overcome one obstacle to the commercialization of the Sclerotinia minor bioherbicide, permitting its deployment under challenging environmental conditions.

Blackberry is a troublesome species across much of the southeastern United States. Control of blackberry with the pyridine herbicides is often variable among different locations. Experiments were conducted to determine whether application timing, either spring or fall, affected efficacy of the pyridine herbicides triclopyr, fluroxypyr and picloram, and metsulfuron. The pyridine herbicides provided greater control when applied in the fall. At 12 mo after treatment, fluroxypyr plus picloram and fluroxypyr plus triclopyr provided 83% control when applied in the fall and 65% when applied in the spring. Conversely, metsulfuron provided 85% control, and application timing was not significant. Although metsulfuron effectively controls blackberry, it is also highly injurious to bahiagrass. Therefore, chlorosulfuron was tested to determine whether it would provide blackberry control while not injuring bahiagrass. Blackberry control with chlorosulfuron was similar to metsulfuron. These data indicated blackberry control in bahiagrass pastures can be effectively accomplished with chlorosulfuron.

Five field trials were conducted over a 2-yr period (2007, 2008) at various locations in Ontario to evaluate the tolerance of black, cranberry, kidney, otebo, pink, pinto, small red Mexican (SRM), and white bean to halosulfuron applied PPI, PRE, and POST at 35 and 70 g ai/ha. There was minimal injury (3% or less) with halosulfuron applied PPI or PRE in dry bean. At Exeter and Ridgetown, halosulfuron applied POST at 35 and 70 g/ha caused 3 to 5% and 4 to 8% injury in dry bean, respectively at 1 wk after herbicide application (WAA). The injury was transient with no significant injury at 2 and 4 WAA. At Harrow, halosulfuron POST at 35 and 70 g/ha caused as much as 4% injury at 35 g/ha and 14% injury at 70 g/ha in dry bean. Halosulfuron applied PPI, PRE, and POST at 35 and 70 g/ha caused no decrease in plant height of dry bean except for kidney bean, which was reduced 6% at 70 g/ha, and white bean, which was reduced 3% at both 35 and 70 g/ha. Halosulfuron applied PPI, PRE, and POST at 35 and 70 g/ha caused no decrease in dry bean yield except for kidney bean, which was reduced 9% at 35 g/ha and 10% at 70 g/ha; otebo bean, which was reduced 3% at 70 g/ha; and white bean, which was reduced 7% at both 35 and 70 g/ha. On the basis of these results, there is an adequate margin of crop safety in dry bean to halosulfuron applied PPI or PRE at 35 and 70 g/ha. In addition, there is an adequate margin of crop safety in black, cranberry, pink, pinto, and SRM bean to halosulfuron applied POST at 35 and 70 g/ha. However, further research is required to ascertain the tolerance of kidney, otebo, and white bean to halosulfuron applied POST.

Military bases in the United States were mandated to reduce the amount of pesticide used to 50% of 1993 levels by 2000. Historically, 2,4-D was applied to control common sunflower, which establishes itself in disturbed soils and obstructs gunners' views of targets. A 25-ha lowland field in Camp Forsyth was selected to compare efficacy of alternative herbicides with that of 2,4-D low-volatile ester (LVE), with the goal of reducing the amount of herbicide applied by at least half. Site vegetation was mostly native tallgrass prairie dominated by warm-season C4 grasses (e.g., big bluestem, Indiangrass, little bluestem, and switchgrass) and including less abundant C3 species in the Asteraceae, Fabaceae, Brassicaceae, and other families. Initially, the site had a high population of common sunflower. All herbicide treatments from 3 yr of field trials were highly and equally effective at reducing common sunflower, decreasing stem density by 83 to100%. Treatments that offer substantial reductions in the amount of herbicide applied are chlorimuron (0.01 kg ae/ha), dicamba + 2,4-D amine (0.07 kg ae/ha + 0.20 kg ae/ha), clopyralid + 2,4-D amine (0.06 kg ae/ha + 0.28 kg ae/ha), 2,4-D LVE (0.56 kg ae/ha), and metsulfuron + 2,4-D amine (0.002 kg ai/ha + 0.28 kg ae/ha). Use of these herbicides at Ft. Riley would reduce total active ingredient applied by 73 to 99% and lower chemical costs for this particular use by as much as 88%.

Glyphosate-based, ready-to-use weed control products often contain pelargonic acid (PA) at a concentration equivalent to that of the glyphosate. It remains unclear what benefit, if any, this combination provides. Greenhouse experiments using large crabgrass, yellow nutsedge, longstalked phyllanthus, and prostrate spurge were conducted to determine whether the addition of PA improved weed control efficacy compared to glyphosate alone. Glyphosate was applied at a series of rates, ranging from 0.11 to 1.12 kg ae/ha, either alone or with an equal rate of PA. Addition of PA to glyphosate was synergistic only in longstalked phyllanthus and yellow nutsedge, and this synergism was manifested only as an increase in the amount of early (i.e., 5 to 7 d after treatment) visual injury. Conversely, longer-term control and control of regrowth was either not affected or reduced by the addition of PA. We conclude that the addition of PA to glyphosate in ready-to-use weed control products is neither warranted nor justified. However, we also note that the increase in early injury that was observed in only two of the four species evaluated could be an important attribute for the consumers for which these products are targeted.

Dodder is a serious parasitic weed in the crops in which it is a problem (particularly citrus). Alternaria destruens is the active ingredient in a registered bioherbicide for control of dodder species. In greenhouse studies, the treatments applied to citrus parasitized with field dodder were a nontreated control; oil at 7.5% v/v in water; ammonium sulfate at 0.125% w/v in water; glyphosate at 0.02 kg ae/L; A. destruens at 1.8 × 1010 spores/L; A. destruens (1.8 × 1010 spores/L) + oil at 7.5% v/v in water; and a mixture of A. destruens (1.8 × 1010 spores/L) + oil at 7.5% v/v in water + glyphosate at 0.02 kg ae/L + ammonium sulfate 0.125% w/v (the mixture treatment). The highest disease or damage severity rating out of all treatments, measured as the area under the disease or damage progress curve (AUDPC), was obtained for the mixture treatment. By 35 d after treatment, all field dodder plants that received the mixture treatment were dead but the host plant, citrus, was not. These results indicate the feasibility of integrating glyphosate, ammonium sulfate, and A. destruens to manage dodder.

Canada thistle is a perennial spreading weed that is difficult to control in farming systems with reduced reliance upon herbicides for weed management. Experiments were conducted from 2006 to 2008 at Champaign, IL, to evaluate the combined effects of summer annual cover crops and mowing on Canada thistle growth and survival. Whole plot treatments were fallow, buckwheat, sudangrass–cowpea mixture (MIX), and sudangrass. The subplot treatments were mowing frequencies (0 to 2 times). Cover crop and mowing did not interact to suppress Canada thistle. MIX and sudangrass produced more standing biomass, greater regrowth, and more surface mulch following mowing than the buckwheat. A single season with sudangrass or MIX reduced Canada thistle shoot density and mass to less than 20% of the initial values through two growing seasons. Mowing alone only suppressed Canada thistle shoot density and mass on the site with greater initial density. A sudangrass or MIX cover crop alone or combined with mowing suppresses Canada thistle, but intensive management must continue for several years to eliminate patches.

Improving crop vigor can suppress growth of weeds present in the crop. This study examined the impact of preceding crop and cultural practices on rye growth in winter wheat. Preceding crops were soybean, spring wheat, and an oat/dry pea mixture. Two cultural treatments in winter wheat were also compared, referred to as conventional and competitive canopies. The competitive canopy differed from the conventional in that the seeding rate was 67% higher and starter fertilizer was banded with the seed. The study was conducted at Brookings, SD. Rye seed and biomass production differed fourfold among treatments, with winter wheat following oat/pea being most suppressive of rye growth. Rye produced 63 seeds/plant in winter wheat with a competitive canopy that followed oat/pea, contrasting with 273 seeds/plant in conventional winter wheat following spring wheat. Yield loss in winter wheat due to rye interference increased with rye biomass, but winter wheat was more tolerant of rye interference following oat/pea compared with the other preceding crops. Regression analysis indicated that winter wheat yield loss at the same rye biomass was threefold higher following spring wheat or soybean compared with oat/pea as a preceding crop. Winter wheat competitiveness and tolerance to rye can be improved by increasing the seeding rate, using a starter fertilizer, and growing winter wheat after an oat/pea mixture.

Field experiments were carried out during two growing seasons at Adnan Menderes University, Faculty of Agriculture, in Aydın-Turkey to evaluate the weed control efficacy of olive processing waste (OPW) in okra, faba bean, and onion. OPW was incorporated into the soil prior to seeding at 10, 20, 30, and 40 tons (t)/ha. Non-treated plots and plots treated with trifluralin in okra and pendimethalin in faba bean and onion were used for comparison. OPW suppressed common purslane, redroot pigweed, and junglerice in okra; littleseed canarygrass, annual bluegrass, wild chamomile, and shepherd's-purse in faba bean and onion. OPW was in most cases equally as effective as soil herbicides; however, 10 t/ha provided sometimes lower efficacy than herbicides. OPW had no negative effects on okra and faba bean, while onion was negatively affected by doses over 30 t/ha. Overall, OPW can be applied at 10 to 20 t/ha doses for weed control with adequate crop safety.

Cotton gins in Arkansas, western Tennessee, and western Mississippi were sampled for weed seed contamination of gin trash in fall 2007. A total of 473 samples were collected, with 453 samples from Arkansas. The objectives of this research were to determine the weed species most frequently found in gin trash and determine what means gin operators are using to dispose of gin trash. There were 25 weed species found in the gin trash samples—11 grass and 14 broadleaf weeds. Grass and broadleaf weeds were present in 41.4 and 8.5% of the samples, respectively. The most frequently found species were large crabgrass (19.0%), barnyardgrass (14.0%), goosegrass (12.9%), red sprangletop (8.2%) and Palmer amaranth (4.2%). Viable seeds of barnyardgrass, large crabgrass, Palmer amaranth, and prickly sida were present in the surface layer (0- to 25-cm depth) of gin trash piles after 1 yr of composting. Viable Palmer amaranth seeds were present in the surface layer of gin trash piles after 2 yr of composting, but no germinable seeds were found deeper than 25 cm following 1 yr of composting. Gin trash disposal involved application of the material to crop fields during the fall or winter months; composting followed by application of the compost as mulch or a soil amendment to gardens, flower beds, or crop fields; use as cattle feed; and coverage for landfills to reduce erosion and encourage growth of vegetation. Because of the demonstrated potential for weed seed dispersal via gin trash, including composted material, development of technologies to ensure disposal of a gin-trash product free of viable weed seed are justified.

Field studies were conducted in Powell, WY, in 2006 and 2007 to determine the influence of season-long interference of various Venice mallow densities and duration of interference on sugarbeet. Sucrose concentration was not affected by Venice mallow interference. The effect of Venice mallow density on sugarbeet root and sucrose yield loss was described by the rectangular hyperbola model. Root and sucrose yield loss increased as Venice mallow density increased. The estimated asymptote, A (percent yield loss as density approaches infinity) was 61% for both root and sucrose yield loss, and the estimated parameter, I (percent yield loss per unit weed density as density approaches zero) was 6% for both root and sucrose yield loss. Sugarbeet root yield decreased as the duration of Venice mallow interference increased. The critical timing of weed removal to avoid 5 and 10% root yield loss was 30 and 43 d after sugarbeet emergence, respectively. Results show that Venice mallow is competitive with sugarbeet implying that it should be managed appropriately to reduce negative effects on yield and prevent seed bank replenishment and re-infestation in subsequent years.

Dose-response experiments were conducted on a biotype of prickly lettuce collected from Whitman County, WA, to determine the level of resistance to 2,4-D. Initially, progeny of prickly lettuce that survived two applications of glyphosate and 2,4-D in mixture were collected to determine if antagonism of the 2,4-D or glyphosate was occurring. Prickly lettuce survival was determined to not be due to antagonism of 2,4-D or glyphosate when the two herbicides were applied in mixture. The doses required to reduce growth 50% (GR50 ) for resistant and susceptible field-collected prickly lettuce were 150 and 6 g ae/ha 2,4-D, respectively, indicating the resistant biotype was 25 times more resistant to 2,4-D than the susceptible biotype. The resistant biotype expressed injury but produced regrowth following application. A dose of 2,4-D at 220 g/ha was required to reduce regrowth frequency 50% (FR50 ) for resistant field-collected prickly lettuce. Regrowth was also observed with the susceptible biotype, although the FR50 was much lower (10 g/ha), resulting in an R/S ratio of 22 based on the respective FR50 values. A rate of 4,300 g/ha 2,4-D (10 times the maximum labeled rate in wheat) was required to reduce the regrowth frequency in the resistant biotype to zero.

Two greenhouse studies were conducted to investigate the variability in tolerance to a sublethal dose of glyphosate among accessions of pitted morningglory, hybrid morningglory (a fertile hybrid between pitted and sharppod morningglory), and sharppod morningglory, collected from several states in the southern United States. The first study was conducted to evaluate the variability in tolerance to glyphosate among accessions. Glyphosate at 420 g ae/ha was applied to plants at the four- to five-leaf stage, and control (percent shoot fresh weight reduction) was determined 2 wk after treatment (WAT). Pitted morningglory response ranged from −9% (indicating no response to glyphosate) to 39% control. A similar trend was observed in hybrid morningglory. Control of two related species, cypressvine morningglory and red morningglory, averaged 40 and 29%, respectively, and was similar to control of the most susceptible pitted morningglory and hybrid morningglory accessions. Ivyleaf morningglory control was 9%. Sharppod morningglory control was highest (48%) among the morningglories studied. A second study was conducted to determine levels of tolerance to glyphosate based on GR50 (dose required to cause a 50% reduction in plant growth) in 10 accessions that were least to most sensitive to glyphosate (7 pitted, 2 hybrid, and 1 sharppod morningglory). Glyphosate GR50 doses ranged from 0.65 to 1.23 kg/ha, a two-fold variability in tolerance to glyphosate among the 7 pitted morningglory accessions. Increasing levels of tolerance were associated with the absence of a leaf notch. These results indicate the existence of variable tolerance to a sublethal dose of glyphosate among accessions of pitted morningglory.

Herbicide-resistant weeds have impacted crop production throughout the United States, but the effect they have on extension programming has not been evaluated. In June 2007, 38 extension weed specialists throughout the United States, responded to a survey on herbicide-resistant (HR) weeds and the impact they are having on extension education programming. Survey results revealed that HR weeds have had a significant impact on extension programming particularly for agronomic crops. In the last 10 yr, agronomic weed specialists' extension programming was almost twice as likely to be impacted by the presence of HR weeds as compared to horticultural programming. In the next 5 yr, agronomic extension programming is twice as likely to be altered. Of 37 weed species reported, seven genera or species of weeds represented 80% of the major HR biotypes reported. These include Amaranthus species, horseweed, Setaria species, common lambsquarters, kochia, giant ragweed, and Lolium species. Five weed species (common ragweed, common lambsquarters, horseweed, kochia, and three foxtail species) exhibited weed by mode of action (MOA) interactions when evaluated as major or minor problems. Herbicide resistance problem severity differed for weed species, herbicide MOA, and crops. The results of this survey of university extension personnel confirm that HR weeds have impacted extension programming and will continue to impact programming in the future.

New Zealand bittercress is reported as new to the United States. While collecting specimens to determine what Cardamine species occur in the nursery trade, New Zealand bittercress was discovered in a container nursery in Wilkes County, North Carolina. The nursery tracked the shipment of contaminated plants to a wholesale nursery in Washington County, Oregon. It was subsequently confirmed that New Zealand bittercress also occurs in a nursery in Clackamas County, Oregon, and has likely been distributed throughout the United States as a contaminant in container grown ornamental plants. Thus far there have been no reports of naturalized populations outside of container nursery crop production facilities.