Following six consecutive annual applications of imazaquin in combination with trifluralin or pendimethalin to several soybean fields on the Delmarva Peninsula, unacceptable smooth pigweed control was observed. Field and greenhouse studies were conducted to determine if this population of smooth pigweed was resistant to imazaquin and other herbicides. In field research, imazaquin and imazethapyr gave complete control of the susceptible (S) population while providing no control of the resistant (R) population; pyrithiobac controlled 99 and 90% of the R and S populations, respectively. Pendimethalin, metribuzin, MON-12000, and flumiclorac gave less than 75% control of both S and R populations. Chlorimuron, primisulfuron, CGA-152005, and lactofen gave above 75% control, and thifensulfuron and nicosulfuron gave above 90% control of both S and R populations. Seeds were collected from the R and S smooth pigweed populations for research in the greenhouse. Greenhouse studies confirmed high levels of resistance to imazaquin and imazethapyr and low levels of cross-resistance to rimsulfuron and chlorimuron in the R population. Susceptibility of the R population to nicosulfuron, thifensulfuron, pyrithiobac, and pendimethalin was comparable to that of the S population.

Common St. Johnswort is widespread in eastern Canada but it seldom constitutes a serious weed problem. A demographic study conducted in 1993 and 1994 at four typical undisturbed sites indicated that 36 to 96% of established St. Johnswort shoots died during the growing season. Mortality was always associated with infection by a host-specific Colletotrichum gloeosporioides. The leaf-feeding beetle Chrysolina hyperici occurred at all sites and caused maximum midsummer defoliation of 27% in 1993 and 51% in 1994. Healthy plants readily recovered from defoliation during pupation of the fourth instar of the insect in June and following adult estivation in August. Although widespread, C. hyperici populations appear transient and alone do not cause sustained feeding pressure resulting in weed control. When Chrysolina larvae and adults were collected at six field sites and placed on healthy seedlings under controlled conditions, up to 36% of the plants became infected with C. gloeosporioides. Scanning electron micrographs commonly showed Colletotrichum conidia among the setae on legs, tarsal pads, and antennae of adults and larvae. In a series of three experiments conducted under controlled conditions in which Chrysolina larvae and adults were placed on healthy plants after feeding on diseased ones, the incidence of infection ranged from 63 to 100%. Hence, under favorable conditions Chrysolina adults may selectively transmit the pathogen in the field. This study demonstrated the potential of enhancing biological control of weeds by insects with the integration of an effective, host-specific pathogen.

Field studies were conducted in 2008 in Ontario, OR and Paterson, WA to determine the effect of simulated glyphosate drift on ‘Ranger Russet’ potato, including visual injury, shikimic acid accumulation, and tuber yield. Glyphosate was applied at 8.5, 54, 107, 215, and 423 g ae ha−1; which corresponds to 0.01, 0.064, 0.126, 0.254, and 0.5 of the lowest recommended (846 g ha−1) single application dose for glyphosate-resistant corn and sugar beet. Glyphosate was applied when potato plants were at 10-cm height, stolon hooking, tuber initiation, or bulking stage. The greatest visual foliar injury was observed when glyphosate was applied at a dose of 54 g ha−1 or greater and potato plants were at the hooking stage. The lowest foliar injury was observed when glyphosate was applied to potato plants at the bulking stage. The I50 glyphosate dose at 42 d after treatment (DAT) was estimated to be 167 g ha−1 for potatoes sprayed at the hooking stage. The corresponding glyphosate dose to result in 50% injury for potatoes sprayed at tuber initiation, 10-cm height, and bulking stages were 129%, 338%, and 438%, respectively, greater than hooking stage. The U.S. No.1 potato yield was inversely related to vine injury and shikimic acid accumulation. Shikimic acid accumulation increased when glyphosate was applied at 107 g ha−1 or greater. U.S. No.1 potato yield was reduced by 46% and 84% relative to the untreated control (55 and 76 T/ha) when glyphosate was applied at 107 g ha−1 to plants in the hooking stage at Ontario and Paterson, respectively. Tuber yields at both sites were lowest when glyphosate was applied at hooking and tuber initiation stages.

Few herbicides are available that will selectively control annual bluegrass in a polyculture of bermudagrass overseeded with perennial ryegrass. Research was conducted to evaluate multifaceted annual bluegrass control programs in overseeded bermudagrass. Treatments included various combinations of four herbicides: foramsulfuron applied 2 wk prior to overseeding (WPO); ethofumesate and bispyribac-sodium (bispyribac) applied 12 or 12 followed by (fb) 15 wk after overseeding (WAO); and prodiamine applied 15 WAO to provide continued PRE annual bluegrass control. Foramsulfuron at 0.03 kg ha−1 applied 2 WPO controlled annual bluegrass 63% 28 WAO. Foramsulfuron fb ethofumesate or bispyribac applied 12 or 12 fb 15 WAO improved control to 91% or greater. Ethofumesate or bispyribac applied with prodiamine at 1.1 kg ha−1 at 15 WAO did not improve annual bluegrass control compared with ethofumesate or bispyribac treatments alone according to pairwise contrasts. Only bispyribac-containing treatments induced unacceptable perennial ryegrass injury. Bispyribac applied with prodiamine reduced perennial ryegrass cover greater than bispyribac alone according to pairwise contrast. These data indicate that ethofumesate or bispyribac applied sequentially 12 and 15 WPO can effectively control annual bluegrass in bermudagrass turf overseeded with perennial ryegrass.

Putatively resistant (PR) and putatively susceptible (PS) common waterhemp populations were grown in the greenhouse and sprayed at the three- to four-leaf stage with glyphosate (0.63 kg ae/ha). Surviving plants from PR populations and randomly selected plants from PS populations were clonally propagated and the clones were sprayed with 0.1 to 10.0 kg/ha glyphosate. The glyphosate rates required to reduce growth by 50% (GR50) among the clones were relatively similar, but the concentration required to reduce growth by 90% (GR90) ranged from 1.5 to 16.3 kg/ha. The concentration of glyphosate required to kill 90% of plants (LD90) ranged from 2.3 kg/ha to over 10.0 kg/ ha. This range of responses to glyphosate in common waterhemp clones from different parts of the Midwestern United States indicates a risk of evolution of resistance in common waterhemp populations that are repeatedly selected by applications of glyphosate in the field.

The effects of the imidazolinone herbicide AC 263,222 on crops that may be rotated with soybean were studied in the greenhouse and field. Corn, grain sorghum, cotton, rice, wheat, soybean, and Italian ryegrass were planted directly into treated soil after AC 263,222 was incorporated. In the field, AC 263,222 at rates as high as 35 g ai ha–1 did not reduce shoot weight or emergence of these species. Some visual injury occurred, but was 20% or less 28 d after planting for all crops. Crops were more sensitive in the greenhouse, where reductions in both shoot and root dry weight were observed. AC 263,222 at 13, 13, and 7 g ha–1 reduced corn, grain sorghum, and Italian ryegrass shoot weights, respectively. Cotton, rice, and wheat tolerated rates from 27 to 55 g ha–1. Biomass reduction occurred with all crops at 55 g ha–1 and above. Because AC 263,222 will be registered for use in soybean at 9 to 18 g ha–1, and potential rotational crops were planted immediately after soil treatment, problems with injury to these crops are not expected.

Water and paraffinic oils were used to determine variables that affect positive displacement pumps used to meter liquid flow in an air-assist spray system. Components needed to construct an air-assist spray system were described for use in spray chambers and on a tractor for field application of herbicides in ultralow volume (ULV). The flow rate of the pump varied less than 2% for the water and oil as the pump setting was varied from 0 to 100% of the pump's capacity. Linear relationships were shown for both liquids as the voltage input varied from 4 to 13 volts. Air pressures to 100 kPa and liquid temperatures to 35 C did not affect flow rate of the pump. Volume median diameter of the spray droplets decreased from 295 μm to 77 μm and 159 μm to 85 μm for water and paraffinic oil, respectively, as the air pressure was adjusted from 14 to 110 kPa.

A study was conducted from 1994 through 1996 to evaluate barnyardgrass control, rice yield, and estimated economic returns with delayed preemergence (DPRE) applications of pendimethalin, quinclorac, thiobencarb, pendimethalin plus quinclorac, and pendimethalin plus thiobencarb applied alone or followed by postemergence (POST) applications of propanil or propanil plus molinate. Late-season barnyardgrass control with pendimethalin plus quinclorac was greater than with pendimethalin, quinclorac, thiobencarb, or pendimethalin plus thiobencarb. Control with pendimethalin, quinclorac, and pendimethalin plus thiobencarb exceeded that with thiobencarb. Propanil plus molinate controlled barnyardgrass more effectively than propanil. Quinclorac and pendimethalin plus quinclorac were the only DPRE herbicide treatments where yield and estimated economic returns were not improved by POST herbicides.

Research was conducted at Marianna, AR, for 2 yr to determine whether hairy vetch and Austrian winter pea cover crops would aid weed management programs in conservation-tilled, enhanced glyphosate-resistant cotton. Both cover crops were easily established and produced rapid growth in early spring, with biomass production of 435 to 491 g m−2 by Austrian winter pea and 415 to 438 g m−2 by hairy vetch. The effect of cover crops on weed control was short-lived in both years, with herbicide programs being the major determinant of weed control and seed-cotton yield. Averaged over cover crops, seed-cotton yields when the initial in-crop glyphosate application was delayed to the four-node cotton stage were up to 710 kg ha−1 less than in a PRE herbicide program. In 1 of 2 yr, seed-cotton yields were greater in PRE-treated plots compared with a program where initial weed management was delayed to the one-leaf stage of cotton. As a result of rapid decay of hairy vetch and Austrian winter pea biomass following cotton planting and the lack of adequate Palmer amaranth, pitted morningglory, and goosegrass control in the absence of herbicides, it appears there may be minimal weed management benefits from the use of hairy vetch and Austrian winter pea in Midsouth cotton production.

Field experiments were conducted in Michigan, Minnesota, and Wisconsin in 1990 to explore interactions between nicosulfuron applied POST and terbufos insecticide at 0.06 or 0.11 g ai/m of row applied in-furrow on ‘Pioneer 3751’ field corn and ‘Jubilee’ sweet corn. Nicosulfuron at 0, 70, and 140 g ai/ha plus nonionic surfactant and 28% nitrogen fertilizer was applied to both corn types. Field corn response to nicosulfuron and terbufos was similar at all locations, whereas sweet corn injury varied with location. Nicosulfuron injured field corn more when applied at the four-leaf than the three-leaf stage. Injury to both corn types increased as nicosulfuron rate increased or when applied following terbufos. Nicosulfuron at 140 g/ha without terbufos did not reduce yield of either corn type; however, corn previously treated in-furrow with terbufos reduced yield.

Field studies were conducted in northern Tamaulipas, Mexico in 1991 and 1992 to determine the effectiveness of POST applications of primisulfuron and nicosulfuron on shattercane control in corn. Primisulfuron at 15 g/ha and nicosulfuron at 30 g/ha were the minimal doses required for shattercane control. Both herbicides applied on 5- to 9-leaf shattercane provided excellent (> 90%) control 35 d after application. No corn injury was evident with any herbicide and rates evaluated. Untreated check plots sustained 30 to 56% of yield reduction as compared to herbicide treatment plots.

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.

Field observations in Ohio suggested a possible interaction between soybean cyst nematode (SCN) and glyphosate in Countrymark 316 soybean, a transgenic glyphosate-resistant (R) variety that also expresses SCN resistance derived from ‘PI88788’ soybean. To investigate this possible interaction under controlled conditions, greenhouse experiments were conducted in which Countrymark 316 (R) and Corsoy 79 (S = susceptible to glyphosate and SCN) soybean root and shoot weights were measured in response to three concentrations of race 3 SCN inoculum and five glyphosate doses. Experiments were conducted in which glyphosate was applied 9, 18, or 27 d after inoculation (DAI). Data from SCN treatments in the (S) soybean experiments were regressed on glyphosate dosage and fit to a log-logistic dose–response model. SCN–glyphosate interaction in the (S) soybean variety reduced root dry weight of SCN-inoculated plants 6% compared with noninoculated plants when 1.0 kg/ha glyphosate was applied 18 DAI. The most significant interaction of SCN and glyphosate in the (S) variety occurred when glyphosate was applied 27 DAI; the glyphosate dose required to reduce shoot fresh weight 25% was 0.55 kg/ha in the noninoculated control compared with 0.32 kg/ha in plants inoculated with 1,000 SCN eggs/200 cm3 soil. Glyphosate rates of 0.84 and 1.69 kg/ha reduced root dry weights of Countrymark 316 (R) soybean 10 to 13% at the 18 DAI application timing only. Inoculation with SCN reduced shoot fresh weight of (R) soybean 8 to 29% across all experiments, but there was no interaction of glyphosate and SCN in (R) soybean.

Sulfentrazone is commonly used for weed control in soybeans and tobacco, and vegetable crops and cotton are often rotated with soybeans and tobacco. Studies were conducted to evaluate the potential for sulfentrazone to carryover and injure several vegetable crops and cotton. Sulfentrazone was applied PRE to soybean at 0, 210, 420, and 840 g ai/ha before planting bell pepper, cabbage, cotton, cucumber, onion, snap bean, squash, sweet potato, tomato, and watermelon. Cotton, known to be susceptible to sulfentrazone carryover, was included as an indicator species. Cotton injury ranged from 14 to 18% with a 32% loss of yield in 1 of 2 yr when the labeled use rate of sulfentrazone (210 g/ha) was applied to the preceding crop. High use rates of sulfentrazone caused at least 50% injury with yield loss ranging from 36 to 100%. Bell pepper, snap bean, onion, tomato, and watermelon were injured < 18% by sulfentrazone at 840 g/ha. Squash was injured < 3% and < 36% by sulfentrazone at 210 and 840 g/ha, respectively. Yield of these crops was not affected regardless of sulfentrazone rate. Cabbage and cucumber were injured < 13% by sulfentrazone at 210 and 420 g/ha, and yields were not affected. Sulfentrazone at 840 g/ha injured cabbage up to 46% and reduced yield in 1 of 2 yr. Sulfentrazone injured cucumber up to 63% and reduced yield of No. 2 grade fruits. Sulfentrazone at 210 and 420 g/ha injured sweet potato < 6% and did not affect yield. Sulfentrazone at 840 g/ha injured sweet potato 14% and reduced total yield 26%. Our results suggest little to no adverse effect on bell pepper, cabbage, cucumber, onion, snap bean, squash, sweet potato, tomato, or watermelon from sulfentrazone applied at registered use rates during the preceding year.

Field studies were conducted in 2001 in Lewiston, NC, and in 2002 at Clayton and Lewiston, NC, to investigate the response of nontransgenic cotton to simulated glyphosate drift in a weed-free environment. Nontransgenic cotton variety ‘Fibermax 989’ was planted in a conventional seedbed at all locations. Glyphosate treatments were applied early postemergence (EPOST) at the four-leaf growth stage of cotton at 0, 8.7, 17.5, 35, 70, 140, 280, 560, and 1,120 g ai/ha and represent 0, 0.78, 1.55, 3.13, 6.25, 12.5, 25, 50, and 100% of the commercial use rate, respectively. Rates as low as 140 g/ha caused lint yield reductions depending on year and location. When averaged over all locations, lint yield reductions of 4, 49, 72, and 87% compared with nontreated cotton were observed with glyphosate rates of 140, 280, 560, and 1,120 g/ha, respectively. Visual injury and shikimic acid accumulation were evident at glyphosate rates greater or equal to 70 g/ha. Collectively, visual injury and shikimic acid accumulation at 7 d after EPOST treatment might be used as a diagnostic indicator to predict potential yield reductions from simulated glyphosate drift.

Crop Protection Online (CPO) is a decision support system, which integrates decision algorithms quantifying the requirement for weed control and a herbicide dose model. CPO was designed to be used by advisors and farmers to optimize the choice of herbicide and dose. The recommendations from CPO for herbicide application in spring barley in Denmark were validated through field experiments targeting three levels of weed control requirement. Satisfactory weed control levels at harvest were achieved by a medium control level requirement generating substantial herbicide reductions (∼ 60% measured as the Treatment Frequency Index (TFI)) compared to a high level of required weed control. The observations indicated that the current level of weed control required is robust for a range of weed scenarios. Weed plant numbers 3 wk after spraying indicated that the growth of the weed species were inhibited by the applied doses, but not necessarily killed, and that an adequate level of control was reached later in the season through crop competition.

Six trials were conducted during 2014/15 and 2015/16 growing seasons in Brazil to determine the effect of 2,4-D formulations and spray nozzles on 2,4-D spray drift under conventional field conditions. An experimental 2,4-D choline formulation with Colex-D® Technology (GF-3073) and a 2,4-D dimethylamine (DMA) formulation were applied with either XR and AIXR flat-fan spray nozzles. Each plot was 30 m wide by 24 m long (720 m2) with 60 glyphosate-resistant soybean rows spaced 50 cm apart and also 35 potted tomato plants distributed on a grid across the plot 5-m apart. Applications were performed one meter away from the plot edge perpendicular to the soybean rows when wind direction was parallel to the rows with less than 30 degrees of angle deviation. Spray drift treatments were applied in 100 L ha−1 with tractor sprayers at 276 kPa equipped with a 7-m wide boom at 50 cm above the canopy of the soybean plant, operating at 6.8 km h−1. The distance from the plot edge to the farthest plant with 2,4-D symptoms was assessed for every four soybean rows at 10 and 20 days after treatment (DAT) and potted tomatoes at 10 DAT. GF-3073 reduced the distance of the farthest injured plant with 2,4-D symptoms compared to the 2,4-D DMA formulation regardless of the spray nozzle, assessment date and sensitive species. GF-3073 applied through the AIXR nozzle reduced the relative drift affected area to the standard by 68% at 10 DAT and 67% at 20 DAT for soybean and 60% at 10 DAT for potted tomatoes.

A survey of soybean fields containing waterhemp infestations was conducted just prior to harvest in 2008 and 2009 to determine the frequency and distribution of glyphosate-resistant waterhemp in Missouri, and to determine if there are any in-field parameters that may serve as indicators of glyphosate resistance in this species in future crop production systems. Glyphosate resistance was confirmed in 99 out of 144, or 69%, of the total waterhemp populations sampled, which occurred in 41 counties of Missouri. Populations of glyphosate-resistant waterhemp were more likely to occur in fields with no other weed species present at the end of the season, continuous cropping of soybean, exclusive use of glyphosate for several consecutive seasons, and waterhemp plants showing obvious signs of surviving herbicide treatment compared to fields characterized with glyphosate-susceptible waterhemp. Therefore, it is suggested that these four site parameters, and certain combinations of these parameters, serve as predictors of glyphosate resistance in future waterhemp populations.

The chlorophyll-lacking holoparasite Egyptian broomrape is a major threat for many field crops in Israel. In carrot, a high-value crop that is grown year round in Israel, heavy infestation with broomrape can cause severe damage and even total yield loss. The objective of this study was to determine, under field conditions, selective herbicides that would effectively control Egyptian broomrape without damaging the carrots. Ten field experiments were performed between the years 2010 and 2013. The acetolactate synthase inhibitor herbicides imazapic and imazamox caused deformation of carrot taproots at low doses, and significantly reduced yield amount and quality. Glyphosate was found to be the safest herbicide for broomrape management in carrot. Carrot selectivity and broomrape control efficacy were examined with three sequential applications of nine glyphosate doses. A nonlinear log-logistic curve described the response of noninfested carrot taproot biomass to glyphosate. No significant reduction in taproot biomass was observed when glyphosate was applied at up to 149 g ae ha−1. When glyphosate was applied in an Egyptian broomrape-infested carrot field, a hormetic effect was observed, perhaps due to Egyptian broomrape control. A two-parameter exponential decay curve described the broomrape response to glyphosate. Three sequential foliar applications of glyphosate, at 108 g ha−1, completely controlled Egyptian broomrape. Our results demonstrate that glyphosate applied sequentially at a low dose on Egyptian broomrape-infested carrot can control this parasitic weed.

Irrigated field experiments were conducted near Torrington, WY, during the 2001 to 2002 (year 1) and 2002 to 2003 (year 2) winter wheat growing seasons to evaluate cultivar response to different imazamox rates, adjuvants, and application timings. Five cultivars were treated postemergence in the early fall (EF), late fall (LF), or early spring (ES) with imazamox at 54 or 108 g ai/ha, including either nonionic surfactant (NIS) at 0.25% or methylated seed oil (MSO) at 1% (v/v) as adjuvants. A 28% urea ammonium nitrate solution at 1% (v/v) was included with all treatments. Spring injury was more severe in year 1 than year 2. Severe spring injury on ‘AP502 CL’, ‘Above’, ‘IMI-Fidel’, ‘IMI-Jagger’, and ‘IMI-Madsen’ was linked to fall application of 108 g/ha imazamox with MSO. Imazamox applied at 108 g/ha plus MSO applied in the fall consistently injured all cultivars more than the same rate with NIS and 54 g/ha imazamox regardless of adjuvant and timing, although severity of injury in the experiments differed between EF and LF timings in years 1 and 2, respectively. Correlation analysis supports injury reduced reproductive tillers per meter of row and wheat yields and increased the number of seeds per spike in year 1. The reduction of reproductive tillers per meter of row in year 1 was likely the result of severe injury caused by 108 g/ha imazamox applied in the EF coupled with little snow cover to protect against cold winter temperatures. Wheat yield in year 1 was reduced by 108 g/ha imazamox applied in the early fall; however, imazamox applied at 54 g/ha with either adjuvant in EF, LF, or ES were safe. Yield parameters and wheat yields in year 2 were not affected by imazamox rate, adjuvant, timing, or interactions of these factors.