Manufacturers of several POST corn herbicides recommend tank-mixing their herbicides with atrazine to improve performance; however, future regulatory changes may place greater restrictions on atrazine use and limit its availability to growers. Our research objectives were to quantify the effects of tank-mixing atrazine with tembotrione compared to tembotrione alone on (1) weed control, (2) variability in weed control, and (3) sweet corn yield components and yield variability. Field studies were conducted for 2 yr each in Illinois, Oregon, Washington, and Ontario, Canada. Tembotrione at 31 g ha−1 was applied alone and with atrazine at 370 g ha−1 POST at the four- to five-collar stage of corn. The predominant weed species observed in the experiment were common to corn production, including large crabgrass, wild-proso millet, common lambsquarters, and velvetleaf. For nearly every weed species and species group, the addition of atrazine improved tembotrione performance by increasing mean levels of weed control 3 to 45% at 2 wk after treatment. Adding atrazine reduced variation (i.e., standard deviation) in control of the weed community by 45%. Sweet corn ear number and ear mass were 9 and 13% higher, respectively, and less variable when atrazine was applied with tembotrione, compared to tembotrione alone. Additional restrictions or the complete loss of atrazine for use in corn will necessitate major changes in sweet corn weed management systems.

Field, greenhouse, and laboratory studies were conducted to determine the effect of MCPA ester, fertilizer type, and fertilizer rate on feral rye control with imazamox. In field studies near Sidney, NE, increasing the concentration of liquid ammonium phosphate (10–34–0) from 2.5 to 50% of the spray solution decreased feral rye control with imazamox by as much as 73%. Conversely, adding MCPA ester to imazamox significantly increased feral rye control in field studies by up to 77%. Initial greenhouse studies confirmed the liquid ammonium phosphate antagonism effect, but subsequent greenhouse studies were inconsistent with regard to the interaction between fertilizer and imazamox. At least one source of liquid ammonium phosphate was shown not to be antagonistic, and therefore fertilizer source or contaminants may be responsible for initial field observations. Greenhouse studies confirmed the synergistic interaction between MCPA and imazamox. MCPA ester applied at 560 g ai ha−1 decreased the rate of imazamox required to cause 50% reduction in feral rye dry weight (GR50) to 13 g ha−1 compared to 35 g ha−1 for imazamox alone. Although addition of MCPA ester increased 14C-imazamox absorption by 8% in laboratory studies, less 14C translocated out of the treated leaf; therefore the mechanism of synergism does not appear to be related to imazamox absorption or translocation.

Glyphosate-resistant (GR) horseweed management continues to be a challenge in no-till cotton systems in Tennessee and Mississippi. Field studies were conducted in 2009 and 2010 to evaluate saflufenacil in tank mixtures with glyphosate, glufosinate, or paraquat on GR horseweed prior to planting cotton. Saflufenacil and saflufenacil tank mixtures were applied 7 d before planting (DBP). Three broad spectrum herbicides were tank-mixed with saflufenacil at rates of 0, 6.3, 12.5, 25, and 50 g ai ha−1. Saflufenacil at 25 and 50 g ai ha−1 in tank mixture with all three broad-spectrum herbicides provided similar GR horseweed control when compared to the current standard of glyphosate + dicamba. Across all saflufenacil rates, lint cotton yield among the glyphosate, glufosinate, and paraquat tank mixture treatments did not differ from each other. Control of horseweed with 25 or 50 g ha−1 of saflufenacil across all tank mixtures also was not different from the standard of glyphosate + dicamba. Moreover, saflufenacil, on silt loam soil evaluated in this study, showed no more cotton injury than glyphosate applied 7 d or more before planting. Saflufenacil applied alone at 25 g ha−1 provided lower control of GR horseweed than the standard, which translated to lower lint yield compared to the glyphosate + dicamba treatment or saflufenacil with each tank mixture partner. The 12.5 g ha−1 rate of saflufenacil tank mixed with either paraquat or glufosinate provided less horseweed control (< 85%) than if higher rates of saflufenacil were used (> 95%). However, lint cotton yield was not different between these treatments. This research suggests that saflufenacil at 25 g ha−1 is the most optimal rate for tank mixtures with glyphosate, glufosinate, or paraquat. It also reaffirms earlier research that the 25 g ha−1 saflufenacil rate safely can be applied inside the currently labeled 42-d waiting period between a saflufenacil application and cotton planting.

Field studies at six locations over 3 yr in Kansas compared pyroxsulam at two application timings to competitive standards for winter annual weed control in winter wheat. Pyroxsulam applied fall-POST (FP) controlled downy brome 84 to 99% and was similar to or greater than sulfosulfuron, propoxycarbazone, or propoxycarbazone plus mesosulfuron. Downy brome control was lower when application timing was delayed until spring (SP), such that no herbicide provided more than 90% downy brome control. Cheat control was 97% or more with almost all herbicides applied FP, and greater than 90% in most locations when herbicides were applied SP. Sulfosulfuron was the exception with only 30 to 81% cheat control. All FP-applied herbicides, except sulfosulfuron at Manhattan, KS, controlled blue mustard 95% or more. Pyroxsulam and propoxycarbazone plus mesosulfuron FP completely controlled henbit at Hesston, KS, in 2009, but no herbicide treatment provided more than 60% control when applied SP. Averaged over application timings, pyroxsulam provided the greatest henbit control (76 and 78%) at Manhattan and Hays, respectively, in 2009, and FP treatments were 33 and 28 percentage points more effective than SP treatments at those locations. Averaged over application timing, wheat yields did not differ between herbicide treatments in five of six locations. Averaged over herbicide treatment, FP-treated wheat yielded more grain than SP-treated wheat at three of the six locations.

Weeds are a major constraint to rice production in labor-limited, upland rice-based systems in West Africa. The effects of weeding regimes and rice cultivars on weed growth and rice yield were investigated at two upland locations (Abomey-Calavi and Niaouli) in the degraded coastal savanna zone of Benin in 2005 and 2006 with below-average rainfall. Four weeding regimes (hoe weeding at 21 d after sowing [DAS], delayed hoe weeding at 31 DAS, hoe weeding at 21 and 42 DAS, and a no weeding control) were the main plot treatments. Cultivars comprising three interspecific upland rice cultivars (NERICA 1, NERICA 2, and NERICA 7) and the parents (Oryza sativa WAB56-104 and O. glaberrima CG14) were tested in subplots. The most dominant weed species identified were Jamaican crabgrass, Mariscus, and silver spinach. Rice yield was generally low because of drought stress; none of the experiments had a higher mean yield than 1,400 kg ha−1 across cultivars. Across cultivars, the best weeding regimes in terms of weed control and rice yields were single weeding at 31 DAS (W31) and double weeding at 21 and 42 DAS (W21+42). Under these weeding regimes, WAB56-104 out-yielded the three NERICA cultivars. CG14 showed the strongest weed suppressive ability (WSA) in Abomey-Calavi but did not have strong WSA in Niaouli because of lower biomass accumulation. WSA of WAB56-104 was similar to that of the three NERICA cultivars. Single weeding at 31 DAS, together with the use of cultivars with good adaptation to unfavorable rice growing conditions, would increase land and labor productivity of upland rice-based systems in West Africa.

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.

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.

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.

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.

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.

Indaziflam is an alkylazine herbicide that controls annual grasses by inhibiting cellulose biosynthesis. Compared with other PRE herbicides like prodiamine, indaziflam has a longer half-life in soil (> 150 d), which may allow for greater flexibility with application timing. Research was conducted in 2010 in Tennessee and Georgia evaluating smooth crabgrass control efficacy with indaziflam applied at early PRE, PRE, and early POST timings on the basis of soil temperature. Regardless of application timing, all rates of indaziflam (35, 52.5, and 70 g ai ha−1) controlled smooth crabgrass 89 to 100%. Prodiamine at 840 g ai ha−1 applied PRE provided ≥ 99% smooth crabgrass control on all rating dates. Smooth crabgrass plant counts were significantly correlated (r = −0.961; p < 0.0001) with visual ratings of smooth crabgrass control at the end of the study. Application flexibility with indaziflam may benefit turf managers in scheduling herbicide applications for smooth crabgrass control in Tennessee and Georgia.

Synthetic auxin herbicides are widely used because of their effective control of broadleaf weeds and safety in many turfgrass species. However, two synthetic auxin herbicides, triclopyr and aminocyclopyrachlor (AMCP; DPX-KJM44), are known to injure warm-season turfgrasses. Our objective was to quantify this injury through evaluations of turfgrass quality and turfgrass green cover in response to herbicide treatment. The results of this study indicate that relative to the labeled use rates of triclopyr (0.56 to 1.12 kg ae ha−1) and AMCP (0.053 kg ai ha−1), zoysiagrass is the only turfgrass tested with sufficient tolerance to the respective compounds for their use as weed-control agents. Bermudagrass and centipedegrass may be injured by triclopyr and AMCP at labeled rates, characterized by a reduction in turfgrass quality and green cover. St. Augustinegrass is not tolerant of either triclopyr or AMCP at labeled rates.

The effectiveness of cover crops as an alternative weed control strategy should be assessed as the demand for food and fiber grown under sustainable agricultural practices increases. This study assessed the effect of fall cover crops on weed populations in the fall and spring prior to sweet corn planting and during sweet corn growth. The experiment was a split-plot design in a pea cover–cover crop–sweet corn rotation with fall cover crop type as the main plot factor and presence or absence of weeds in the sweet corn as the split-plot factor. The cover crop treatments were a control with no cover crop (no-cover), oat, cereal rye (rye), oilseed radish (OSR), and oilseed radish with rye (OSR+rye). In the fall, at Ridgetown, weed biomass in the OSR treatments was 29 and 59 g m−2 lower than in the no-cover and the cereal treatments, respectively. In the spring, OSR+rye and rye reduced weed biomass, density, and richness below the levels observed in the control at Bothwell. At Ridgetown in the spring, cover crops had no effect on weed populations. During the sweet corn season, weed populations and sweet corn yields were generally unaffected by the cover crops, provided OSR did not set viable seed. All cover crop treatments were as profitable as or more profitable than the no-cover treatment. At Bothwell profit margins were highest for oat at almost Can$600 ha−1 higher than the no-cover treatment. At Ridgetown, compared with the no-cover treatment, OSR and OSR+rye profit margins were between Can$1,250 and Can$1,350 ha−1 and between Can$682 and Can$835 ha−1, respectively. Therefore, provided that OSR does not set viable seed, the cover crops tested are feasible and profitable options to include in sweet corn production and provide weed-suppression benefits.

Field and laboratory experiments were conducted in New Jersey to investigate the influence of nitrogen on annual bluegrass and creeping bentgrass metabolism and responses to bispyribac-sodium. In field experiments, withholding nitrogen during the test period increased sensitivity of both grasses to bispyribac-sodium, and grasses fertilized biweekly had darker color on most rating dates. Nitrogen generally increased annual bluegrass tolerance to bispyribac-sodium at 74 g ha−1 but not at 148 g ha−1. Creeping bentgrass was influenced by nitrogen at both herbicide rates. In laboratory experiments, weekly nitrogen treatments increased 14C-bispyribac-sodium metabolism in both grasses compared to unfertilized plants. Annual bluegrass metabolized approximately 50% less herbicide regardless of nitrogen regime compared to creeping bentgrass. Overall, routine nitrogen fertilization appears to improve annual bluegrass and creeping bentgrass tolerance to bispyribac-sodium, which may be attributed to higher metabolism.

Field experiments were conducted near Hays, KS in 2007 and 2008 to evaluate the effects of single and sequential postemergent applications of tribenuron on broadleaf weed control and crop response in tribenuron-resistant sunflower. Weeds were acetolactate-synthase–susceptible biotypes of kochia, puncturevine, Russian thistle, and tumble pigweed in 2007 and puncturevine, redroot pigweed, and tumble pigweed in 2008. Tribenuron at 18 g ai ha−1 applied early POST with methylated seed oil (MSO) provided > 96% control of all species in 2007 and 92 and 99% control of redroot pigweed and puncturevine, respectively, but only 69% control of tumble pigweed in 2008. Early-POST tribenuron at 9 g ha−1 and late-POST tribenuron at 18 g ha−1 generally provided less weed control compared to early-POST tribenuron at 18 g ha−1. Sequential applications slightly improved redroot pigweed and tumble pigweed control in 2008 compared to single applications of tribenuron. Some tribenuron treatments caused transitory crop injury, but imazamox at 35 g ha−1 caused 24 to 44% crop injury at 7 d after treatment and permanent crop stunting in 2007. Significant yield losses occurred with imazamox and single treatments of tribenuron in 2008. Collectively, tribenuron at 18 g ha−1 alone can provide satisfactory control of the evaluated broadleaf weed species when applied to appropriate weed sizes, and this rate does not cause significant injury to tribenuron-resistant sunflower, regardless of the crop size.

Studies were conducted under greenhouse conditions at Michigan State University and Texas Tech University to investigate the tolerance of Miscanthus × giganteus and Miscanthus sinensis to POST herbicides. Miscanthus sinensis and M. × giganteus were treated with 10 and 18 POST herbicide treatments, respectively. Plants were evaluated for injury as well as dry aboveground and belowground biomass production 28 days after treatment. Imazethapyr at 0.069 kg ai ha−1 caused 5% injury to M. sinensis, which was greater than the nontreated check. Imazethapyr, imazamox at 0.044 kg ai ha−1, and rimsulfuron at 0.017 kg ai ha−1 reduced aboveground biomass of M. sinensis compared with the nontreated check. Dicamba at 0.56 kg ai ha−1 and halosulfuron at 0.035 kg ai ha−1 resulted in M. sinensis aboveground biomass similar to the nontreated check. Injury exhibited by M. × giganteus was greater than the nontreated check with glyphosate at 0.84 kg ae ha−1 (54%), foramsulfuron at 0.037 kg ai ha−1 (32%), nicosulfuron at 0.035 kg ai ha−1 (28%), and imazamox at 0.044 kg ai ha−1 (10%). These treatments also yielded the lowest aboveground biomass values. The results of this study demonstrate that M. sinensis is more tolerant of the POST herbicides tested here than M.×x. giganteus. Several herbicide options may be available for weed management in M. sinensis and M. × giganteus stands following additional field trials to validate initial findings.

Options for suppressing bermudagrass seedheads in managed turfgrass systems are limited. Experiments were conducted in 2009 and 2010 evaluating the use of fenoxaprop (25, 50, 75, and 100 g ha−1) for ‘Riviera’ bermudagrass seedhead suppression and growth regulation compared to imazapic (52 g ha−1), trinexapac-ethyl (91 g ha−1) and mefluidide (561 g ha−1). In field experiments, seedhead suppression ranged from 77 to 100% for fenoxaprop and imazapic at 35 d after treatment (DAT). Comparatively, seedhead suppression was < 25% for either trinexapac-ethyl or mefluidide at 35 DAT. Seedhead suppression was > 90% from 7 to 35 DAT for fenoxaprop applied at ≥ 50 g ha−1. Injury, determined visually, from fenoxaprop and imazapic in both the field and greenhouse measured < 25% on all rating dates, with no significant injury present after 21 DAT. In greenhouse experiments, fenoxaprop and trinexapac-ethyl showed similar reductions of bermudagrass growth; no differences in aboveground biomass were detected between these treatments at 42 DAT. Results of the current study illustrate that fenoxaprop and imazapic can be applied for bermudagrass seedhead suppression and growth regulation if moderate (< 25%) injury can be tolerated up to 21 DAT. Additional research is needed to evaluate the use of fenoxaprop and imazapic for seedhead suppression on other common and hybrid bermudagrasses.

Growth of vegetation in curbside cracks causes deterioration of asphalt and curbs, reducing road longevity and safety capabilities. Road managers spend a considerable amount of time and money on roadside vegetation management every year. The vegetation in curbside cracks in these study regions is managed approximately once a year by mowing and road sweeping using street-sweeper trucks. Nevertheless, ideal management practices of roadside vegetation have not yet been established partly due to insufficient knowledge of the ecological strategies of plants invading roadsides, especially curbside cracks. Although establishment of plants in the cracks might be restricted due to severe anthropogenic road disturbances, the cracks could be habitats for species with specific ecological traits. The objective of this study was to clarify the floristic and functional characteristics of roadside weeds, particularly species invading curbside cracks, to provide information for effective road management. The species composition of plants invading the cracks was surveyed along Route 3 (southern Japan) and Route 4 (eastern Japan) in different climatic zones, based on 108 floristic inventories. We compared species occurrence and composition to characterize the dominant ecomorphological traits of the species. In total, 163 species occurred in curbside cracks along both routes. Species composition of vegetation in curbside cracks was more variable between the routes than between land-use types. Of the 54 species, more than 5% occurred in all plots, and only three had differences in occurrence among land-use types. Ecomorphological trait composition patterns of the species were similar across land-use types. From these results, we found that regardless of differences in species composition among regions, climatic conditions, and surrounding land-use type, there were some dominant ecomorphological traits of roadside vegetation with plants in curbside cracks, such as ephemeral monophytes that are barochorous or anemochorous. By contrast, rhizomatous perennials, which cause greater deterioration of asphalt than ephemeral monophytes, were rare along the cracks. Although vegetation composition and structure generally depend on land-use types and disturbance regimes, linear landscape elements such as curbsides might be habitats for plants adapted to road disturbances. Roadside vegetation management, such as mowing and road sweeping once a year, seems sufficient to restrict establishment of rhizomatous perennials around Japan.

Field experiments were established near Casselton and Fargo, ND, to evaluate the effect of aminopyralid soil residue on alfalfa, corn, soybean, and sunflower planted one or two growing seasons after treatment. At Fargo, ND, aminopyralid caused no injury or yield reduction to alfalfa, corn, and sunflower seeded 20 or 23 mo after treatment (MAT) in a silty clay soil. However, soybean yield was reduced when aminopyralid at 120 or 240 g ae ha−1 was fall- or spring-applied 20 or 23 mo before seeding. At Casselton, ND, aminopyralid injured alfalfa, soybean, and sunflower planted 8 and 11 MAT. Injury and yield reduction were less from treatments spring-applied than from those that were fall-applied. For example, aminopyralid at 120 g ha−1 applied in September caused 95, 94, and 100% injury to alfalfa, sunflower, and soybean, respectively, 8 MAT, whereas the same treatment applied in June caused 10, 8, and 44% injury 11 MAT. Aminopyralid at 120 g ha−1 continued to reduce soybean yield by an average of 45% at 20 MAT (fall-applied), but yield was similar to the control when aminopyralid was applied 23 mo before seeding (spring-applied). Warm soil with moderate moisture during the summer months appeared to be very important for degradation of aminopyralid. Corn was not affected by aminopyralid when seeded 8 or 11 MAT and appeared to be the best cropping option for land recently treated with aminopyralid. Aminopyralid applied at spot-treatment rates of 240 g ha−1 had long-term soil activity similar to picloram at 560 g ha−1.

Glyphosate-resistant giant ragweed in Arkansas was reported in 2005. A study was conducted to (1) confirm and characterize the glyphosate resistance in giant ragweed, (2) determine if reduced absorption or translocation is the mechanism of glyphosate resistance in giant ragweed, and (3) evaluate the efficacy of nine POST-applied soybean herbicides to control glyphosate-resistant and -susceptible giant ragweed. Based on the rate required to kill 50% of plants (LD50 values), resistant giant ragweed biotypes from Greene and Jefferson counties were 2.3- to 7.2-fold less sensitive to glyphosate compared to susceptible biotypes. Glyphosate absorption and translocation for glyphosate-resistant and -susceptible biotypes was similar at 24 and 72 h after treatment. Thus, differential absorption or translocation is not a mechanism of glyphosate resistance in this resistant giant ragweed biotype. Control of resistant giant ragweed biotypes with glyphosate at a labeled field application rate of 840 g ha−1 was only 60% or less compared to complete control of a susceptible giant ragweed biotype. However, bentazon, carfentrazone, cloransulam, and fomesafen controlled both biotypes more than 95%.