A new, prepackaged mixture of saflufenacil + dimethenamid-P has been introduced for PRE control of grass and broadleaf weeds in corn. Field experiments with this new herbicide combination were conducted in 2008 and 2009 at four locations in southern Ontario, Canada. The objective of this study was to determine the dose of saflufenacil + dimethenamid-P required for overall weed control and species-specific weed control, as well as the dose required for early season weed control when followed with glyphosate at the six- to eight-leaf corn stage. Based on weed dry weight, the GR95 across locations ranged from 126 to 675 g ha−1. The 95% growth reduction (GR95) for common ragweed, common lambsquarters, pigweed, and wild mustard were 933, 325, 186, and 115 g ha−1, respectively. Highest corn yields were achieved with saflufenacil + dimethenamid-P applied alone at doses ranging from 368 to 1470 g ha−1. When followed by glyphosate, the dose range of saflufenacil + dimethenamid-P required to achieve the greatest corn yields was 46 to 1,470 g ha−1. A minimum dose of 184 g ha−1 of saflufenacil + dimethenamid-P followed by glyphosate was required for the yield to exceed that of the single treatment of glyphosate applied alone.

Whether season-long weed control can be achieved in a furrow-irrigated rice system with similar herbicide inputs to that of a flooded system is not known. Field experiments were conducted in 2007 and 2008 at Pine Tree, AR to evaluate different herbicide programs on the weed control efficacy and rice grain yield in furrow-irrigated and flooded rice production systems. Six herbicide programs were evaluated with and without additional late-season “as-needed” herbicide treatments. Minor injury to rice was noted for quinclorac plus propanil. However, the injury was transient and the plants fully recovered. Overall weed control was greater in the flooded system compared with the furrow-irrigated system (up to 20% greater), because flooding effectively prevented the emergence of most terrestrial weeds. In addition, rice grain yields were 13 to 14% greater in flooded compared with furrow-irrigated plots. Irrespective of the irrigation system, herbicide programs that contained a PRE-applied herbicide provided greater weed control and resulted in greater yield compared with those that did not contain PRE-applied herbicide, indicative of the importance of early-season weed control in achieving higher grain yields. On the basis of weed control, yield, and weed treatment cost, the herbicide program with clomazone PRE followed by propanil at four- to five-leaf rice was more efficient than other programs evaluated in both irrigation systems. However, furrow-irrigated plots required as-needed herbicide applications, which were applied after the four- to five-leaf rice stage when two or more plots within a program exhibited ≤ 80% control for any of the weed species. This suggests that furrow-irrigated rice production demands additional weed management efforts and thereby increases production costs. There is also a possibility for substantial yield reduction in the furrow-irrigated system compared with the flooded system. Nevertheless, furrow-irrigated rice production can still be a viable option under water-limiting situations and under certain topographic conditions.

Field studies were conducted in 2008 and 2009 near Crowley, Louisiana, to evaluate the addition of different propanil formulations in mixture with a standard imazethapyr program of 70 g ai ha−1 early postemergence followed by (fb) 70 g ha−1 late postemergence. Weeds evaluated included red rice, barnyardgrass, Texasweed, and alligatorweed. Control of all species with treatments, including a propanil formulation applied at 3,400 g ai ha−1, was equivalent to, or greater than, the standard imazethapyr program. Rough rice yield and economic returns were maximized when the propanil formulations of Propanil 1 or Propanil 3 were mixed with imazethapyr in the early postemergence applications. The addition of propanil to imazethapyr increased rough rice yield and economic returns because of improved weed control.

The widespread evolution of resistance in rigid ryegrass populations to the highly effective, in-crop, selective herbicides used within southern Australian grain-crop production systems has severely diminished the available herbicide resource. A new PRE grass-selective herbicide, pyroxasulfone, may offer Australian grain producers a new option for rigid ryegrass control in wheat crops. The efficacy and level of selectivity of rigid ryegrass control with pyroxasulfone was investigated for a range of annual crop species in potted-plant, dose–response studies. In comparison with other currently available PRE herbicides, pyroxasulfone provided effective control of both resistant and susceptible rigid ryegrass populations. Additionally, control of these populations was achieved at rates that had little or no effect on the growth and survival of wheat. This crop was also the most tolerant of cereal species, with triticale, barley, and oat being more injured at higher pyroxasulfone rates than wheat was. In general though, pulse-crop species were found to be more tolerant of high pyroxasulfone rates than cereal-crop species. There were subtle effects of soil type on the efficacy of pyroxasulfone, where higher rates were required to achieve effective control on soils with higher clay or organic matter contents. The ability of pyroxasulfone to selectively control resistant and susceptible rigid ryegrass populations as identified in these studies clearly indicate the potential for widespread use and success of this herbicide in Australian cropping systems.

A simulation model is used to explore management options to mitigate risks of glyphosate resistance evolution in Palmer amaranth in glyphosate-resistant cotton in the southern United States. Our first analysis compares risks of glyphosate resistance evolution for seven weed-management strategies in continuous glyphosate-resistant cotton monoculture. In the “worst-case scenario” with five applications of glyphosate each year and no other herbicides applied, evolution of glyphosate resistance was predicted in 74% of simulated populations. In other strategies, glyphosate was applied with various combinations of preplant, PRE, and POST residual herbicides. The most effective strategy included four glyphosate applications with a preplant fomesafen application, and POST tank mixtures of glyphosate plus S-metolachlor followed by glyphosate plus flumioxazin. This strategy reduced the resistance risk to 12% of populations. A second series of simulations compared strategies where glyphosate-resistant cotton was grown in one-to-one rotations with corn or cotton with other herbicide resistance traits. In general, crop rotation reduced risks of resistance by approximately 50% and delayed the evolution of resistance by 2 to 3 yr. These analyses demonstrate that risks of glyphosate resistance evolution in Palmer amaranth can be reduced by reducing glyphosate use within and among years, controlling populations with diverse herbicide modes of action, and ensuring that population size is kept low. However, no strategy completely eliminated the risk of glyphosate resistance.

Torpedograss infestation in centipedegrass has reduced centipedegrass quality in home lawns along the Gulf Coast. This study was conducted to evaluate three selective postemergence herbicides. Field trials were conducted at two sites in Louisiana to evaluate quinclorac, sethoxydim, and clethodim applied once or sequentially every 4 wk for selective torpedograss control in centipedegrass turf. Herbicides were applied to mixed stands of torpedograss/centipedegrass at two locations in Louisiana and evaluated for changes in torpedograss coverage and centipedegrass injury every 2 wk for 16 wk. All herbicides controlled torpedograss more with each sequential application. Sethoxydim and clethodim applied three times reduced torpedograss cover 84 and 87%, respectively, and more than quinclorac 12 wk after initial treatment (WAIT). Increasing clethodim or sethoxydim rates did not improve torpedograss control. Torpedograss regrowth occurred within weeks after final herbicide applications regardless of herbicide. Only multiple clethodim applied at twice manufacturer's labeled rate or quinclorac applications resulted in commercially unacceptable (> 25%) injury to centipedegrass. Multiple sethoxydim or clethodim applications at 0.32 kg ha−1 or 0.30 kg ha−1 every 4 wk reduced torpedograss competitiveness in centipedegrass; however, multiple applications for more than 1 yr might be necessary to achieve torpedograss control.

Field studies were conducted to compare the effectiveness of PRE and POST applications of a prepackaged mixture of flufenacet plus metribuzin with that of diclofop for winter wheat tolerance and control of Italian ryegrass. Additional studies investigated the effectiveness of reduced rates of flufenacet plus metribuzin applied POST to Italian ryegrass when wheat was in the spike stage. All PRE and POST applications of flufenacet plus metribuzin produced similar or greater injury to wheat and more consistent control of Italian ryegrass than PRE or POST applications of diclofop. PRE applications of flufenacet plus metribuzin controlled Italian ryegrass 73 to 77%, whereas POST applications controlled Italian ryegrass 77 to 99%. PRE applications of diclofop controlled Italian ryegrass 57%; POST application controlled Italian ryegrass 78%. Wheat injury from flufenacet plus metribuzin applications varied with application rate, cultivar, and year of application.

Research was conducted over 2 yr to evaluate soybean response to harvest aid herbicide treatments paraquat at 0.28 kg ai ha−1, paraquat with carfentrazone at 0.014 kg ai ha−1, and sodium chlorate at 6.72 kg ai ha−1. Indeterminate and determinate soybean cultivars were treated when moisture of seed collected from the uppermost four nodes of plants averaged 60, 50, 40, 30, and 20% (± 2%). For each soybean cultivar, the harvest aid treatment by application timing interaction was not significant, and data for harvest aid treatments were averaged. Application of harvest aid at 60% average seed moisture reduced yield for the maturity group (MG) IV indeterminate cultivar 15.4% compared with the nontreated; 100-seed weight was reduced 12.4%. Yield and seed weight were not negatively affected when harvest aid was applied at 50% average seed moisture and soybean was harvested 14 and 15 d before the nontreated control. Although planting date in the 2 yr for the indeterminate cultivar differed by 26 d, number of days from planting to harvest aid application at 50% average seed moisture was 112 and 116 d. For MG V and MG VI determinate cultivars, application of harvest aid at 60% average seed moisture reduced yield compared with the nontreated control 22 and 18.1%, respectively, and at 50% average seed moisture 15.6 and 4%, respectively; seed weight reductions of 8.9 to 33.3% accompanied the yield reductions of the two cultivars. Reduction in soybean yield and seed weight was not observed when harvest aid was applied at 40% average seed moisture, and harvest for the 2 yr was 8 and 9 d earlier for the MG V cultivar and 10 and 14 d earlier for the MG VI cultivar.

Pyroxasulfone (KIH-485) is a seedling growth-inhibiting herbicide developed by Kumiai America that has the potential to control weeds in sunflower. However, little is known about how this herbicide will interact with various soil types and environments when combined with sulfentrazone. The objective of this research was to evaluate sunflower injury and weed control with pyroxasulfone applied with and without sulfentrazone across the Great Plains sunflower production area. A multisite study was initiated in spring 2007 to evaluate sunflower response to pyroxasulfone applied PRE at 0, 167, 208, or 333 g ai ha−1. In 2008, pyroxasulfone was applied alone and in tank mixture with sulfentrazone. In 2007, no sunflower injury was observed with any rate of pyroxasulfone at any location except Highmore, SD, where sunflower injury was 17%, 4 wk after treatment (WAT) with 333 g ha−1. In 2008, sunflower injury ranged from 0 to 4% for all treatments. Adding sulfentrazone did not increase injury. Sunflower yield was only reduced in treatments in which weeds were not effectively controlled. These treatments included the untreated control and pyroxasulfone at 167 g ha−1. Sunflower yield did not differ among the other treatments of pyroxasulfone or sulfentrazone applied alone or in combination. The addition of sulfentrazone to pyroxasulfone improved control of foxtail barley, prostrate pigweed, wild buckwheat, Palmer amaranth, and marshelder, but not large crabgrass or green foxtail. The combination of pyroxasulfone and sulfentrazone did not reduce control of any of the weeds evaluated.

We conducted a series of field experiments to determine the role of several factors that might contribute to the inconsistent control of common lambsquarters with glyphosate. Experiments in 2006 and 2007 determined common lambsquarters response to glyphosate under a wide range of measured environmental conditions. Glyphosate was applied at 0.84 kg ae ha−1 plus 3.8 kg ha−1 ammonium sulfate (AMS) to 10-cm-tall plants on 18 dates in each year and to 20-cm-tall plants on 18 dates in 2007. Control was less for six application dates relative to control for 48 other dates. Poor control was attributed to rainfall on one of these six dates, but for the other five dates, regression analysis did not identify any significant relationships between environmental conditions (relative humidity, temperature at time of treatment, or minimum and maximum temperature pre- and posttreatment) and control, even though a wide range of conditions occurred. To determine the effects of plant growth stage on control, glyphosate was applied at 0.1 to 3.2 kg ha−1 plus 3.8 kg ha−1 AMS to 10- and 20-cm-tall plants at four sites. The glyphosate ED50 value (the effective dose that reduced shoot mass by 50% relative to nontreated plants) was 1.9 to 3.0 times greater for 20- than 10-cm-tall plants in three site-years, but was not affected by plant height in one site-year. We also conducted experiments to determine the effect of rainfall on glyphosate efficacy. Across years, common lambsquarters control increased from 44 to 75% as the interval between glyphosate application (0.84 kg ha−1 + 3.8 kg ha−1 AMS) and simulated rainfall increased from 0.5 to 4.0 h, respectively. Our results did not identify environmental conditions that explained reduced glyphosate efficacy in all cases, but they suggest that rainfall after application and plant height can be important factors contributing to the inconsistent control of common lambsquarters.

Experiments were conducted in North Carolina during 2005, 2006, and 2007 to determine peanut and weed response when peanut was planted in strip tillage after desiccation of cereal rye, Italian ryegrass, oats, triticale, wheat, and native vegetation by glyphosate and paraquat before planting with three in-season herbicide programs. Control of common ragweed and yellow nutsedge did not differ among cover crop treatments when compared within a specific herbicide program. Applying dimethenamid or S-metolachlor plus diclosulam PRE followed by imazapic POST was more effective than a chloroacetamide herbicide PRE followed by acifluorfen, bentazon, and paraquat POST. Incidence of spotted wilt in peanut (caused by a Tospovirus) did not differ when comparing cover crop treatments, regardless of herbicide program. Peanut yield increased in all 3 yr when herbicides were applied POST, compared with clethodim only. Peanut yield was not affected by cover crop treatment. Response to cover crop treatments was comparable, suggesting that growers can select cereal rye, Italian ryegrass, oats, or triticale as an alternative to wheat as a cover crop in peanut systems without experiencing differences associated with in-season weed management.

Winter wheat is the predominant crop in Oklahoma, but winter annual grasses are becoming increasingly difficult to control. Summer crop rotations have not been generally adopted; it was decided, therefore, to use winter canola in a crop rotation. However, very little is known about how well herbicides used in canola production will control the winter annual grasses found in Oklahoma wheat fields. Thus, an experiment was conducted at three sites, and repeated the following year, to determine the efficacy of trifluralin, quizalofop, clethodim, and glyphosate in canola production. The weeds evaluated in the experiment were Italian ryegrass, feral cereal rye, and volunteer wheat, along with two varieties of canola: a glyphosate-resistant variety and a conventional variety. All herbicides effectively controlled volunteer wheat. Feral cereal rye and Italian ryegrass varied in response to the herbicide treatments. Trifluralin followed by (fb) quizalofop and glyphosate fb glyphosate were effective on all target species across locations. Effective control of grass weeds was obtained in both conventional and glyphosate-resistant winter canola. Most herbicide treatments improved canola yield over the nontreated check. This experiment demonstrates that Oklahoma wheat producers can effectively rotate to canola to use other herbicides for control of problematic grassy weeds.

Aminocyclopyrachlor (AMCP) is labeled for use on zoysiagrass, but some injury has been observed. Differential zoysiagrass cultivar response to herbicide treatment has been previously reported. This greenhouse study evaluated the response of ‘BK-7’, ‘Cavalier’, ‘Emerald’, ‘Empire’, ‘Meyer’, and ‘Zorro’ zoysiagrass to 0, 0.005, 0.02, 0.11, 0.52, and 2.4, 11 kg ai ha−1, AMCP. Visual estimation of percent necrosis and normalized difference vegetative index (NDVI) analysis were conducted. Based on rating dates and data types three tolerance groups were established: Cavalier, Meyer, and Zorro are the most tolerant; Emerald and Empire are intermediate; and BK-7 is the least tolerant to AMCP. All zoysiagrass cultivars had sufficient tolerance at the labeled rate. Visual and NDVI analyses were highly correlated; however, NDVI data were subject to greater standard error and pseudo R2 values.

Separate field trials were conducted in 2007 and 2008 to investigate the effects of increasing densities of common ragweed or common cocklebur on total yield and forage nutritive values in tall fescue pastures. Common ragweed densities ranged from 0 to 188 plants m−2, and common cocklebur densities ranged from 0 to 134 plants m−2. Total biomass yields (weeds plus tall fescue) were determined in response to each weed density and species; pure samples of tall fescue, common ragweed, or common cocklebur were also hand collected from each plot at the time of the total biomass harvest. Near-infrared spectroscopy was used to predict crude protein (CP) concentration and in vitro true digestibility (IVTD) of the total harvested biomass, pure tall fescue, and pure weed species in each plot. Results indicate that biomass yields may increase by as much as 5 kg ha−1 with each additional common ragweed plant m−2 within a tall fescue stand. Additionally, CP concentration of the total harvested biomass, pure weed species, and tall fescue decreased by 0.2 to 0.4 g kg−1 with each additional increase in common ragweed or common cocklebur plant per m−2. As weed densities increased, IVTD of pure tall fescue samples increased only minimally (0.04%), regardless of the weed species. An increase in common ragweed density also resulted in the CP concentration of pure samples of common ragweed to decrease by 0.2 g kg−1 for each additional plant per m2 and by 0.4 g kg−1 for each additional common cocklebur per m2. Overall, results from these experiments indicate that plant biomass yield and nutritive values of the total harvested biomass are only marginally influenced by increasing common ragweed or common cocklebur densities.

Field trials were conducted from 2006 through 2008 to determine the influence of ethofumesate applied at planting followed by dimethenamid-p or s-metolachlor applied to emerged sugarbeet for late-season weed control in glyphosate-resistant sugarbeet. The entire plot area was kept weed-free until mid-June by applying glyphosate at the four- and eight-true-leaf sugarbeet growth stages. Glyphosate was not applied from mid-June until late-July to allow weed growth as a measure of the residual benefit from ethofumesate, dimethenamid-p, and s-metolachlor applied earlier in the growing season. Dimethenamid-p was not as effective as s-metolachlor in reducing weed density in mid-July. Late-season weed suppression from both s-metolachlor and dimethenamid-p benefitted from ethofumesate applied at planting. Dimethenamid-p applied when sugarbeet reached the six-true-leaf growth stage reduced weed density and sugarbeet injury more than earlier applications. The lowest weed density in mid-July was achieved when s-metolachlor was applied at the six- to eight-true-leaf sugarbeet growth stage compared to earlier growth stages. A planting time application of ethofumesate followed by two glyphosate applications plus s-metolachlor at the eight-true-leaf sugarbeet growth stage provided 89% more weed control in mid-July than glyphosate alone. Suppressing late-season weed development increased sugarbeet root yield 15% compared with areas not receiving ethofumesate and s-metolachlor.

Field studies were conducted in Wyoming and Nebraska in 2007 through 2009 to evaluate winter wheat response to aminocyclopyrachlor. Aminocyclopyrachlor was applied at rates between 15 and 120 g ai ha−1 6, 4, and 2 mo before winter wheat planting (MBP). Redroot pigweed control was 90% with aminocyclopyrachlor rates of 111 and 50 g ha−1 when applied 4 or 2 MBP. Aminocyclopyrachlor at 37 g ha−1 controlled Russian thistle 90% when applied 6 MBP. At Sidney, NE, winter wheat yield loss was > 10% at all aminocyclopyrachlor rates when applied 2 or 4 MBP, and at all rates > 15 g ha−1 when applied 6 MBP. At Lingle, WY, > 40% winter wheat yield loss was observed at all rates when averaged over application timings. Although the maturing wheat plants looked normal, few seed were produced in the aminocyclopyrachlor treatments, and therefore preharvest wheat injury ratings of only 5% corresponded to yield losses ranging from 23 to 90%, depending on location. The high potential for winter wheat crop injury will almost certainly preclude the use of aminocyclopyrachlor in the fallow period immediately preceding winter wheat.

Quinoclamine is used in Europe, and was under evaluation in the Unites States for the control of liverwort in nursery crops. Liverwort is a nonvascular, chlorophyll-containing plant that can be problematic in greenhouse and nursery crops. POST-applied quinoclamine controls liverwort. However, liverwort structures vary in their sensitivity to POST-applied quinoclamine. Specifically, archegonial receptacles (female) are much more tolerant of quinoclamine than either antheridial receptacles (male) or thalli (leaflike structures). A series of studies were conducted to, first, document the degree of differential sensitivity between tissues to quinoclamine, and second, to determine the basis of this differential sensitivity. The dose that results in 50% of the population being controlled (I50) of antheridial receptacles and juvenile thalli were estimated to be 1.60 and 1.27 kg·ha−1, respectively. The I50 of archegonial receptacles could not be estimated, but exceeded 10.45 kg·ha−1. Chlorophyll content varied between liverwort tissues, but the content did not correlate to quinoclamine sensitivity. Absorption of 14C after application of radiolabeled quinoclamine was less in archegonial receptacles than in either antheridial receptacles or thalli. Scanning electron microscopy of the surface of the liverwort tissues revealed that archegonial receptacles had smaller pores (equivalent to stomata in higher plants) than either antheridial receptacles or thalli. The tolerance of archegonial receptacles to quinoclamine can be partially, but not exclusively, attributed to reduced absorption. This reduced absorption may be attributed to the limited pore size and less total pore area of the archegonial receptacles.

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.

Miscanthus is a perennial, rhizomatous C4 grass grown in the European Union and studied in the United States as a bioenergy feedstock. U.S. farmers might be more willing to grow this perennial species if methods for its control were established. Experiments were conducted from 2007 to 2009 to evaluate methods to control miscanthus. As glyphosate rate increased from 0 to 3.6 kg ae ha−1 in a greenhouse trial, miscanthus dry weight decreased. Aboveground biomass in the summer following treatments decreased 82, 77, and 95% with fall, spring, and fall followed by spring applications of glyphosate (1.7 kg ae ha−1), respectively, compared with nontreated plots in field experiments. Summer shoot count was reduced by 41% compared with the nontreated control with fall followed by spring glyphosate applications. A second field experiment demonstrated that spring tillage with one or two spring glyphosate applications (2.5 kg ae ha−1 application−1) reduced aboveground dry biomass by 94 and 95%, respectively, and reduced miscanthus shoot number by 38 and 67%, respectively, in the same growing season. These experiments suggest that although glyphosate and tillage can reduce miscanthus biomass, complete control of a mature stand likely will require more than one growing season.

The introduction of glyphosate-resistant (GR) alfalfa offers a new weed management system for alfalfa establishment; however, alfalfa seeding rates are based on conventional cultivars. Determining optimum seeding rates allows forage producers to maximize yield, quality, and profitability with GR alfalfa. Field experiments were established in 2005 and 2006 to determine the effect of seeding rate and weed control on GR alfalfa yield, forage quality, and persistence up to 3 yr after establishment. Seeding rates of 4.5, 9.0, and 18 kg ha−1 were evaluated. Weed control methods during the seeding year included no herbicide, glyphosate applied once before the first harvest, and glyphosate applied once before the first harvest and then 7 to 10 d following subsequent harvests. Alfalfa yield was greater at higher seeding rates and when weeds were removed with glyphosate. Season forage yields were the greatest with the 18 kg ha−1 seeding rate and where no herbicide was applied. Weed biomass often was lower at the higher seeding rates and was 91 to 98% lower in the glyphosate treatments compared to the nontreated. Forage quality was not affected by seeding rate but varied by herbicide treatment depending on establishment year. Plant density increased with seeding rate and treatment effects persisted for three growing seasons. Herbicide treatment did not affect stand density as greatly as seeding rate and did not influence stand longevity.