Pollen-mediated gene flow (PMGF) refers to the transfer of genetic information (alleles) from one plant to another compatible plant. With the evolution of herbicide-resistant (HR) weeds, PMGF plays an important role in the transfer of resistance alleles from HR to susceptible weeds; however, little attention is given to this topic. The objective of this work was to review reproductive biology, PMGF studies, and interspecific hybridization, as well as potential for herbicide resistance alleles to transfer in the economically important broadleaf weeds including common lambsquarters, giant ragweed, horseweed, kochia, Palmer amaranth, and waterhemp. The PMGF studies involving these species reveal that transfer of herbicide resistance alleles routinely occurs under field conditions and is influenced by several factors, such as reproductive biology, environment, and production practices. Interspecific hybridization studies within Amaranthus and Ambrosia spp. show that herbicide resistance allele transfer is possible between species of the same genus but at relatively low levels. The widespread occurrence of HR weed populations and high genetic diversity is at least partly due to PMGF, particularly in dioecious species such as Palmer amaranth and waterhemp compared with monoecious species such as common lambsquarters and horseweed. Prolific pollen production in giant ragweed contributes to PMGF. Kochia, a wind-pollinated species can efficiently disseminate herbicide resistance alleles via both PMGF and tumbleweed seed dispersal, resulting in widespread occurrence of multiple HR kochia populations. The findings from this review verify that intra- and interspecific gene flow can occur and, even at a low rate, could contribute to the rapid spread of herbicide resistance alleles. More research is needed to determine the role of PMGF in transferring multiple herbicide resistance alleles at the landscape level.

Nomenclature: Common lambsquarters, Chenopodium album L.; giant ragweed, Ambrosia trifida L.; horseweed, Erigeron canadensis (L.) Cronquist; kochia, Bassia scoparia (L.) A.J. Scott; Palmer amaranth, Amaranthus palmeri S. Watson.; waterhemp, Amaranthus tuberculatus (Moq.) Sauer

The evolution and widespread distribution of glyphosate-resistant broadleaf weed species catalyzed the introduction of dicamba-resistant crops that allow this herbicide to be applied POST to soybean and cotton. Applications of dicamba that are most cited for off-target movement have occurred in June and July in many states when weeds are often in high densities and at least 10 cm or taller at the time of application. For registration purposes, most field studies examining pesticide emissions are conducted using bare ground or very small plants. Research was conducted in Knoxville, TN, in the summer of 2017, 2018, and 2019 to examine the effect of application surface (tilled soil, dead plants, green plants) on dicamba emissions under field conditions. Dicamba emissions after application were affected by the treated surface in all years, with the order from least to most emissions being dead plants < tilled soil < green plant material. In fact, dicamba emissions were >300% when applied to green plants compared to other surfaces. These findings suggest that dicamba applications made to bare ground will likely underestimate what may occur under normal field use conditions when POST applications are made and the crop canopy or weed groundcover is nearly 100% green material. A potential change to enhance the accuracy of current environmental simulation models would be to increase the theoretical findings to allow for the effect of green plant material on dicamba emissions under field conditions.

Nomenclature: Dicamba; cotton; Gossypium hirsutum l.; soybean; Glycine max L. Merr.

Atrazine applied at planting is commonly used for weed control in corn. With global climate change causing an increase in river flooding in the United States over the past decade, producers need information to determine the best course of action in flooded fields treated with atrazine into which they wish to immediately plant soybean. Studies were designed to understand the effect of flooding on atrazine residual activity including atrazine concentration, soybean injury, and soybean yield. In 2012, soybean yield in flooded treatments was reduced by prior atrazine application. In 2014, soybean injury was <10% in all plots, and nonflooded, atrazine-treated soils had yields equal to the nontreated. Findings from this research indicated that it is possible for producers to consider replanting soybean after atrazine application, with appropriate changes to product labeling.

Nomenclature: Atrazine; corn; Zea mays L.; soybean; Glycine max L. Merr.

Glyphosate-tolerant and glyphosate-resistant weeds are becoming increasingly problematic in cotton fields in Australia, necessitating a return from a glyphosate dominated system to a more integrated approach to weed management. The development of an integrated weed management system can be facilitated by identifying the critical period for weed control (CPWC), a model that enables cotton growers to optimize the timing of their weed control inputs. Using data from field studies conducted from 2003 to 2015, CPWC models using extended functions, including weed biomass in the relationships, were developed for the mimic weeds, common sunflower and Japanese millet, in high-yielding, fully irrigated cotton. A multispecies CPWC model was developed after combining these data sets with data for mungbean in irrigated cotton, using weed height and weed biomass as descriptors in the models. Comparison of observed and predicted relative cotton-lint yields from the multispecies CPWC model demonstrated that the model reasonably described the competition from these three very different mimic weeds, opening the possibility for cotton growers to use a multispecies CPWC model in their production systems.

Nomenclature: Cotton; Gossypium hirsutum L. GOSHI; common sunflower; Helianthus annuus L. HELAN; Japanese millet; Echinochloa esculenta (A. Braun) H. Scholz; mungbean; Vigna radiata (L.) R. Wilczek

The use of photosystem II (PSII)-inhibitor and/or protoporphyrinogen oxidase (PPO)-inhibitor PRE herbicides in soybean may, under adverse environmental conditions, result in early season crop injury. A field study was conducted near Brule and North Platte, Nebraska, during the 2016 and 2017 growing seasons with the objective to evaluate the impact of PRE herbicides metribuzin (PSII-inhibitor) and sulfentrazone (PPO-inhibitor) on early season soybean development, final plant stand, and yield using 22 soybean varieties adapted to southwestern Nebraska. Herbicide treatments consisted of metribuzin (560 g ai ha^-1) and sulfentrazone (280 g ai ha^-1) applied within 3 d after planting and a nontreated control (NTC). Sulfentrazone reduced green canopy vegetation at the V2 growth stage by 22% and final plant stand at physiological maturity by 10% compared with the NTC. The number of pods per plant was 16% higher for sulfentrazone and the number of seeds per plant was 15% and 4% higher for sulfentrazone and metribuzin compared with the NTC, respectively. Sulfentrazone and metribuzin resulted in a slightly higher yield (3%) compared with the NTC, thus no yield reduction from PRE herbicides was observed in this study. These results support other findings that sulfentrazone and metribuzin have potential to cause early-season crop injury; however, when applied according to their label recommendations and following regional agronomic management practices, this impact may not translate into soybean yield reduction while such herbicides provide effective soil residual weed control.

Nomenclature: metribuzin; sulfentrazone; soybean; Glycine max L. Merr.

Sulfentrazone was recently granted food-use tolerance approval for use on Brassica head and stem, as well as Brassica leafy vegetables. To date, one sulfentrazone registrant has listed those crops on its use label. In coastal California multiple crops per year including Brassica vegetables are grown in rapid succession; therefore, to avoid injury to rotational crops, herbicides used in those fields must be carefully selected. Given concerns about the relatively long soil persistence of sulfentrazone, studies were conducted to measure the response of direct-seeded carrot, lettuce, onion, spinach, and seeded tomato planted 3, 6, 9, and 12 mo after sulfentrazone application at 0, 112, 224, and 336 g ai ha^–1. Eight plant-back studies were conducted during 2010–11 and 2012–13. Data collected were injury estimates, and stand and dry weights. Results indicate that it is safe to plant carrot and tomato 3 mo after sulfentrazone application at rates up to 336 g ai ha^–1. Lettuce and green onion should not be planted within 9 mo of sulfentrazone application. Spinach should not be planted within 12 mo of sulfentrazone application.

Nomenclature: Sulfentrazone; carrot; Daucus carota subsp. Sativus (Hoffm.) Schübl. & G. Martens; lettuce; Lactuca sativa L.; onion; Allium cepa L.; spinach; Spinacia oleracea L.; tomato; Lycopersicon lycopersicum (L.) Karsten L. esculentum (L.) Mill.

Integrated strategies for management of glyphosate-resistant (GR) horseweed are needed to reduce reliance on herbicides. Planting a cover crop after corn or soybean harvest in the Upper Midwest may reduce horseweed establishment and growth. Experiments were conducted in Michigan to determine if cereal rye and winter wheat, seeded at 67 or 135 kg ha-¹, and terminated with glyphosate at 1.27 kg ae ha-¹ 1 wk before planting (early termination) or 1 wk after soybean planting (planting green) would suppress establishment and growth of GR horseweed. Cover-crop biomass was 212% to 272% higher when termination was delayed by planting green compared with early termination. At the time of termination, cover crops reduced GR horseweed biomass 41% to 89% compared with no cover. Planting green increased the C:N ratio of cover-crop residue, which improved residue persistence and GR horseweed suppression at the time of POST herbicide application, approximately 5 wk after planting. Planting green reduced GR horseweed biomass 46% to 93% compared with no cover at the time of POST herbicide application; early termination provided less consistent suppression. Cover crops alone did not suppress GR horseweed through soybean harvest. Soybean yield was 30% to 108% greater when planting green compared with early termination at 2 site-years. Cereal rye and winter wheat, seeded at 67 or 135 kg ha-¹, provided early-season GR horseweed suppression. Results from this research indicate that the practice of planting green may improve GR horseweed suppression through the time of POST herbicide application.

Nomenclature: Glyphosate; horseweed, Conyza canadensis L. Cronq.; winter wheat, Triticum aestivum L.; cereal rye, Secale cereale L.; soybean, Glycine max (L.) Merr

Glyphosate-resistant horseweed is difficult to manage in no-tillage crop production fields and new strategies are needed. Cover crops may provide an additional management tool but narrow establishment windows and colder growing conditions in northern climates may limit the cover crop biomass required to suppress horseweed. Field experiments were conducted in 3 site-years in Michigan to investigate the effects of two fall-planted cover crops, cereal rye and winter wheat, seeded at 67 or 135 kg ha^–1, to suppress horseweed when integrated with three preplant herbicide strategies in no-tillage soybean. The preplant strategies were control (glyphosate only), preplant herbicide without residuals (glyphosate + 2,4-D), and preplant herbicide with residuals (glyphosate + 2,4-D + flumioxazin + metribuzin). Cereal rye produced 79% more biomass and provided 12% more ground cover than winter wheat in 2 site-years. Increasing seeding rate provided 41% more cover biomass in 1 site-year. Cover crops reduced horseweed density 47% to 96% and horseweed biomass by 59% to 70% compared with no cover at the time of cover crop termination. Cover crops provided no additional horseweed suppression 5 wk after soybean planting if a preplant herbicide with or without residuals was applied, but reduced horseweed biomass greater than 33% in the absence of preplant herbicides. Cover crops did not affect horseweed suppression at the time of soybean harvest or influence soybean yield. Preplant herbicide with residuals and without residuals provided at least 52% and 20% greater soybean yield compared with the control at 2 site-years, respectively. Cereal rye and winter wheat provided early-season horseweed suppression at biomass levels below 1,500 kg ha^–1, lower than previously reported. This could give growers in northern climates an effective strategy for suppressing horseweed through the time of POST herbicide application while reducing selection pressure for horseweed that is resistant to more herbicide sites of action.

Nomenclature: horseweed; Conyza canadensis L. Cronq.; winter wheat; Triticum aestivum L.; cereal rye; Secale cereale L.; soybean; Glycine max (L.) Merr.

A paper-based survey was conducted from 2015 to 2017 among stakeholders of the Texas rice industry on current weed management challenges and factors influencing management decisions. A total of 108 survey questionnaires were completed by stakeholders at the rice Cooperative Extension meetings conducted in the rice-growing counties of Texas. In addition, late-season field surveys were conducted prior to harvest in 2015 and 2016 across the rice-growing counties to understand dominant weed escapes occurring in rice fields. Results from the questionnaire survey revealed that rice–fallow–rice was the most common rotation practiced in Texas rice production. Echinochloa spp., Leptochloa spp., and Cyperus spp. were the top three problematic weed issues faced by the respondents. Among the Leptochloa species, Nealley's sprangletop, a relatively new species in rice fields, was indicated as an emerging concern. Clomazone was the most frequently used PRE herbicide, whereas quinclorac, propanil, imazethapyr, and cyhalofop were the popular POST herbicides. Most respondents (72%) made weed-control decisions on the basis of economic thresholds, whereas 63% made decisions on the basis of weed problems from previous years. Most respondents (88%) expressed moderate to high concern for herbicide-resistant weeds in their operations. Strategies to manage herbicide-resistant weeds and economical weed management practices were among the top suggested research needs. The field survey revealed that jungle rice, Nealley's sprangletop, and hemp sesbania were the top three late-season weed escapes in rice production in Texas, with frequencies of occurrence of 28%, 19%, and 13%, respectively. Furthermore, average field area infested by a species was the greatest for jungle rice (13%), followed by hemp sesbania (11%) and weedy rice (11%). Findings from the stakeholder and field surveys help direct future research and outreach efforts for sustainable weed management in Texas rice.

Nomenclature: Clomazone; cyhalofop; imazethapyr; propanil; quinclorac; hemp sesbania; Sesbania herbacea (Mill.) McVaugh; jungle rice; Echinochloa colona (L.) Link; Nealley's sprangletop; Leptochloa nealleyi Vasey; rice; Oryza sativa L.; weedy rice; Oryza sativa L.

Differential tolerance may be observed among rice cultivars with desiccant exposure events during rice reproduction and ripening. Five field studies were established at the Mississippi State University Delta Research and Extension Center in Stoneville, MS, to determine the effects of exposure to sublethal concentrations of common desiccants across multiple rice cultivars. Rice cultivars in the study were ‘CLXL745', ‘XL753', ‘CL163', ‘Rex’, and ‘Jupiter'. Desiccant treatments included no desiccant, paraquat, or glyphosate and were applied at the 50% heading growth stage respective to cultivar. Differential injury estimates among cultivars and desiccant treatments was observed when glyphosate or paraquat was applied at 50% heading. Injury from glyphosate at 50% heading was nondetectable across all cultivars. However, injury following paraquat applications was >7% across all rating intervals and cultivars. Hybrid cultivars exhibited less injury with paraquat applications than the inbred cultivars in the study. Rice following exposure to glyphosate or paraquat at 50% heading growth stage produced rough rice grain yield decreases ranging from 0% to 20% and 9% to 21%, respectively. Rough rice grain yield decreases were observed across all cultivars following paraquat exposure, and all inbred cultivars following glyphosate exposure. Across desiccant treatment, head rice yield was reduced in three of five cultivars in the study. When pooled across cultivar, paraquat applications cause a head rice yield reduction of 10%, whereas rice yield following glyphosate application remained >95%. Although differential tolerance among cultivars to paraquat or glyphosate exposure was observed, impacts on grain quality coupled with yield reductions suggests extreme rice sensitivity to exposure to sublethal concentrations of these desiccants at the 50% heading growth stage.

Nomenclature: Glyphosate; paraquat; rice; Oryza sativa L.

Information on performance of sequential treatments of quizalofop-P-ethyl with florpyrauxifen-benzyl on rice is lacking. Field studies were conducted in 2017 and 2018 in Stoneville, MS, to evaluate sequential timings of quizalofop-P-ethyl with florpyrauxifen-benzyl included in preflood treatments of rice. Quizalofop-P-ethyl treatments were no quizalofop-P-ethyl; sequential applications of quizalofop-P-ethyl at 120 g ha^–1 followed by (fb) 120 g ai ha^–1 applied to rice in the 2- to 3-leaf (EPOST) fb the 4-leaf to 1-tiller (LPOST) growth stages or LPOST fb 10 d after flooding (PTFLD); quizalofop-P-ethyl at 100 g ha^–1 fb 139 g ha^–1 EPOST fb LPOST or LPOST fb PTFLD; quizalofop-P-ethyl at 139 g ha^–1 fb 100 g ha^–1 EPOST fb LPOST and LPOST fb PTFLD; and quizalofop-P-ethyl at 85 g ha^–1 fb 77 g ha^–1 fb 77 g ha^–1 EPOST fb LPOST fb PTFLD. Quizalofop-P-ethyl was applied alone and in mixture with florpyrauxifen-benzyl at 29 g ai ha^–1 LPOST. Visible rice injury 14 d after PTFLD (DA-PTFLD) was no more than 3%. Visible control of volunteer rice (‘CL151' and ‘Rex') 7 DA-PTFLD was similar and at least 95% for each quizalofop-P-ethyl treatment. Barnyardgrass control with quizalofop-P-ethyl at 120 fb 120 g ha^–1 LPOST fb PTFLD was greater (88%) in mixture with florpyrauxifen-benzyl. The addition of florpyrauxifen-benzyl to quizalofop-P-ethyl increased rough rice yield when quizalofop-P-ethyl was applied at 100 g ha^–1 fb 139 g ha^–1 EPOST fb LPOST. Sequential applications of quizalofop-P-ethyl at 120 g ha^–1 fb 120 g ha^–1 EPOST fb LPOST, 100 g ha^–1 fb 139 g ha–1 EPOST fb LPOST, or 139 g ha^–1 fb 100 g ha^–1 EPOST fb LPOST controlled grass weed species. The addition of florpyrauxifen-benzyl was not beneficial for grass weed control. However, because quizalofop-P-ethyl does not control broadleaf weeds, florpyrauxifen-benzyl could provide broadspectrum weed control in acetyl coenzyme A carboxylase–resistant rice.

Nomenclature: Florpyrauxifen-benzyl, quizalofop-P-ethyl; barnyardgrass, Echinochloa crus-galli L. Beauv. ECHCG; volunteer rice, Oryza sativa L. ORYSA; rice, Oryza sativa L. ‘PVL01'

Establishment of alfalfa by interseeding it with corn planted for silage can enhance crop productivity but weed management is a challenge to adoption of the practice. Although a simple and effective approach to weed management would be to apply a glyphosate-based herbicide, concerns about herbicide resistance and limitations in available alfalfa varieties exist. Field experiments were conducted to compare the efficacy and selectivity of PRE, POST, and PRE followed by POST herbicide programs to a glyphosate-only strategy when interseeding alfalfa with corn. Experiment 1 compared PRE applications of acetochlor, mesotrione, S-metalochlor, metribuzin, and flumetsulam. Results indicate that acetochlor and metribuzin, and S-metalochlor used at a rate of 1.1 kg ai ha^–1 were the most effective and selective PRE herbicides 4 wk after treatment (WAT), but each resulted in greater overall weed cover than glyphosate by 8 WAT. Experiment 2 evaluated applications of bentazon, bromoxynil, 2,4-DB, and mesotrione at early and late POST times. Several herbicides used POST exhibited similar effectiveness and selectivity as glyphosate, including early applications of bromoxynil (0.14 kg ai ha^–1) and 2,4-DB (0.84 or 1.68 kg ai ha^–1), as well as late applications of bromoxynil (0.42 kg ai ha^–1), 2,4-DB (0.84 kg ai ha^–1), and mesotrione (0.05 or 0.11 kg ai ha^–1). A third experiment compared applications of acetochlor PRE, bromoxynil POST, and a combination of acetochlor PRE with bromoxynil POST. All treatments were effective and safe for use in this interseeded system, although interseeded alfalfa provided 65% to 70% weed suppression in corn planted for silage without any herbicide. Herbicide treatments had no observable impacts on corn and alfalfa yields, so weed management was likely of limited economic importance. However, weed competitiveness can vary based on several different factors including weed species, density, and site-specific factors, and so further investigations under different environments and conditions are needed.

Nomenclature: acetochlor; bentazon; bromoxynil; flumetsulam; mesotrione; metribuzin; S-metalochlor; 2,4-DB; alfalfa; Medicago sativa L.; corn; Zea mays L.

Residual herbicides applied PRE provide early season weed control, potentially avoid the need for multiple POST herbicides, and can provide additional control of herbicide-resistant weeds. Thus, field studies were conducted in 2017 and 2018 at Concord, NE, to evaluate the influence of PRE herbicides on critical time for postemergence weed removal (CTWR) in corn. The studies were arranged in a split-plot design that consisted of three herbicide regimes as main plot treatments and seven weed removal timings as subplot treatments in four replications. The herbicide regimes included no PRE herbicide, atrazine, and a premix of saflufenacil/dimethenamid-P mixed with pyroxasulfone. The weed removal timings were at V3, V6, V9, V12, and V15 corn growth stages and then plots were kept weed-free until harvest. A weed-free and nontreated control were included for comparison. The relationship between corn growth or yield, and weed removal timings in growing degree days (GDD) was described by a four-parameter log-logistic model. This model was used to estimate the critical time for weed removal based on 5% crop yield loss threshold. A delay in weed removal until the V2 to V3 corn growth stage (91 to 126 GDD) reduced corn biomass by 5% without PRE herbicide application. The CTWR started at V3 without PRE herbicide in both years. Atrazine delayed the CTWR up to V5 in both years, whereas saflufenacil/dimethenamid-P plus pyroxasulfone further delayed the CTWR up to the V10 and V8 corn growth stages in 2017 and 2018, respectively. Herbicide applied PRE particularly with multiple sites of action can delay the CTWR in corn up to a maximum growth stage of V10, and delay or reduce the need for POST weed management.

Nomenclature: Atrazine, dimethenamid-P; glyphosate; pyroxasulfone; saflufenacil; corn, Zea mays L.

Weed control of paraquat can be erratic and may be attributable to differing species sensitivity and/or environmental factors for which minor guidance is available on commercial labels. Therefore, the objectives of this research were to quantify selectivity of paraquat across select weed species and the influence of environmental factors. Experiments were performed under controlled conditions in the greenhouse and growth chamber. Compared with purple deadnettle (dose necessary to reduce shoot biomass by 50% = 39 g ai ha^–1), waterhemp, Palmer amaranth, giant ragweed, and horseweed were 4.9, 3.3, 1.9, and 1.3 times more sensitive to paraquat, respectively. The injury progression rate over 3 d after treatment (DAT) was a more accurate predictor of final efficacy at 14 DAT than the lag phase until symptoms first appeared. For example, at the 17.5 g ha^–1 dose, the injury rate of waterhemp and Palmer amaranth was, on average, 3.6 times greater than that of horseweed and purple deadnettle. The influence of various environmental factors on paraquat efficacy was weed specific. Applications made at sunrise improved control of purple deadnettle over applications at solar noon or sunset. Lower light intensities (200 or 600 µmol m^–2 s^–1) surrounding the time of application improved control of waterhemp and horseweed more than 1,000 µmol m^–2 s^–1. Day/night temperatures of 27/16 C improved horseweed and purple deadnettle control compared with day/night temperatures of 18/13 C. Though control was positively associated with injury rates in the application time of day and temperature experiments, a negative relationship was observed for waterhemp in the light-intensity experiment. Thus, although there are conditions that enhance paraquat efficacy, the specific target species must also be considered. These results advocate paraquat dose recommendations, currently based on weed height, be expanded to address sensitivity differences among weeds. Moreover, these findings contrast with paraquat labels stating temperatures of 13 C or lower do not reduce paraquat efficacy.

Nomenclature: Paraquat; giant ragweed, Ambrosia trifida L.; horseweed, Conyza canadensis (L.) Cronq; Palmer amaranth, Amaranthus palmeri S. Watson; purple deadnettle, Lamium purpureum L.; waterhemp, Amaranthus tuberculatus (Moq.) J.D. Sauer

POST goosegrass and other grassy weed control in bermudagrass is problematic. Fewer herbicides that can control goosegrass are available due to regulatory pressure and herbicide resistance. Alternative herbicide options that offer effective control are needed. Previous research demonstrates that topramezone controls goosegrass, crabgrass, and other weed species; however, injury to bermudagrass may be unacceptable. The objective of this research was to evaluate the safening potential of topramezone combinations with different additives on bermudagrass. Field trials were conducted at Auburn University during summer and fall from 2015 to 2018 and 2017 to 2018, respectively. Treatments included topramezone mixtures and methylated seed oil applied in combination with five different additives: triclopyr, green turf pigment, green turf paint, ammonium sulfate, and chelated iron. Bermudagrass bleaching and necrosis symptoms were visually rated. Normalized-difference vegetative index measurements and clipping yield data were also collected. Topramezone plus chelated iron, as well as topramezone plus triclopyr, reduced bleaching potential the best; however, the combination of topramezone plus triclopyr resulted in necrosis that outweighed reductions in bleaching. Masking agents such as green turf paint and green turf pigment were ineffective in reducing injury when applied with topramezone. The combination of topramezone plus ammonium sulfate should be avoided because of the high level of necrosis. Topramezone-associated bleaching symptoms were transient and lasted 7 to 14 d on average. Findings from this research suggest that chelated iron added to topramezone and methylated seed oil mixtures acted as a safener on bermudagrass.

Nomenclature: Topramezone; triclopyr; crabgrass; Digitaria spp.; goosegrass; Eleusine indica (L.) Gaertn.; bermudagrass; Cynodon dactylon (L.) Pers.

Buckhorn plantain populations purportedly resistant to 2,4-D were identified in Pennsylvania following long-term, continual applications of the active ingredient to turfgrass. The research objectives of this study were to 1) confirm 2,4-D resistance with dose-response experiments, 2) confirm field resistance of buckhorn plantain to 2,4-D in Pennsylvania, and 3) evaluate alternative herbicides for 2,4-D-resistant buckhorn plantain. Greenhouse dose-response experiments evaluated the sensitivity of buckhorn plantain biotypes that were resistant or susceptible to 2,4-D, and to halauxifen-methyl, two synthetic auxin herbicides from different chemical families. The resistant biotype was ≥11.3 times less sensitive to 2,4-D than the susceptible biotype and required a 2,4-D dosage ≥4.2 times greater than the standard application rate to reach 50% necrosis. No cross-resistance was observed to halauxifen-methyl because both resistant and susceptible populations demonstrated similar herbicide sensitivity. Field experiments confirmed previous reports of ineffectiveness (≤30% reduction) with 2,4-D and other phenoxycarboxylic herbicides in potentially resistant buckhorn plantain biotypes. Treatments containing halauxifen-methyl resulted in a ≥70% reduction in resistant biotypes. This is the first known report of synthetic auxin herbicide resistance in any weed species in Pennsylvania and highlights emerging herbicide resistance challenges in turfgrass systems.

Nomenclature: 2, 4-D; halauxifen-methyl; buckhorn plantain, Plantago lanceolata L. PLALA

Eucalyptus species are grown for fiber, fuel, and other uses on more than 17.8 million ha worldwide, yet some species are considered invasive and may have adverse environmental or social impacts outside their native range. Aminocyclopyrachlor (AMCP) and standard applications of imazapyr and triclopyr herbicides were compared for eucalyptus control using a basal stem application method. At 6 and 12 mo after treatment (MAT), basal stem applications using 5% (vol/vol) AMCP (120 g ae L^–1) in methylated soybean oil (MSO) resulted in 97% to 99% eucalyptus crown reduction and generally provided greater control across all diameter classes than standard treatments of 28% imazapyr (240 g ae L^–1) or 75% triclopyr ester (480 g ae L^–1). AMCP at 5% was as effective as 40% vol/vol. Increases in stem live height at 24 MAT suggest that the effect of triclopyr ester basal stem treatment may be impermanent. AMCP treated trees did not have regrowth by 24 MAT.

Nomenclature: Aminocyclopyrachlor; imazapyr; triclopyr; Camden white gum; Nepean River gum; Eucalyptus benthamii Maiden et Cambage

Annual grass weeds reduce profits of wheat farmers in the Pacific Northwest. The very-long-chain fatty acid elongase (VLCFA)-inhibiting herbicides S-metolachlor and dimethenamid-P could expand options for control of annual grasses but are not registered in wheat, because of crop injury. We evaluated a safener, fluxofenim, applied to wheat seed for protection of 19 soft white winter wheat varieties from S-metolachlor, dimethenamid-P, and pyroxasulfone herbicides; investigated the response of six varieties (UI Sparrow, LWW 15-72223, UI Magic CL+, Brundage 96, UI Castle CL+, and UI Palouse CL+) to incremental doses of fluxofenim; established the fluxofenim dose required to optimally protect the varieties from VLCFA-inhibiting herbicides; and assessed the impact of fluxofenim dose on glutathione S-transferase (GST) activity in three wheat varieties (UI Sparrow, Brundage 96, and UI Castle CL+). Fluxofenim increased the biomass of four varieties treated with S-metolachlor or dimethenamid-P herbicides and one variety treated with pyroxasulfone. Three varieties showed tolerance to the herbicides regardless of the fluxofenim treatment. Estimated fluxofenim doses resulting in 10% biomass reduction of wheat ranged from 0.55 to 1.23 g ai kg^-1 seed. Fluxofenim doses resulting in 90% increased biomass after treatment with S-metolachlor, dimethenamid-P, and pyroxasulfone ranged from 0.07 to 0.55, 0.09 to 0.73, and 0.30 to 1.03 g ai kg^-1 seed, respectively. Fluxofenim at 0.36 g ai kg^-1 seed increased GST activity in UI Castle CL+, UI Sparrow, and Brundage 96 by 58%, 30%, and 38%, respectively. These results suggest fluxofenim would not damage wheat seedlings up to three times the rate labeled for sorghum, and fluxofenim protects soft white winter wheat varieties from S-metolachlor, dimethenamid-P, or pyroxasulfone injury at the herbicide rates evaluated.

Nomenclature: Dimethenamid-P; fluxofenim; pyroxasulfone; S-metolachlor; sorghum, Sorghum bicolor (L.) Moench; wheat, Triticum aestivum L.

Weeds are managed in Florida strawberry production systems with plastic mulches, fumigants, and herbicides. There are limited post-transplant options to control weeds that emerge in the planting holes in the plastic-covered beds, but flumioxazin at 107 g ai ha^–1 can be applied pretransplant under the plastic mulch to control broadleaf and grass weeds. Three research trials were conducted in Balm and Dover, FL, in 2017 and 2018 to evaluate tolerance of the strawberry cultivar ‘Radiance’ to flumioxazin rates ranging from 54 to 6,854 g ha^–1 and to estimate herbicide persistence under the plastic mulch. Shoot damage was observed at 428 to 857 g ha–1 (4× and 8× the label rate, respectively), but a significant increase in the number of dead plants was not observed until the treatment rate was 857 g ha^–1 at one site and 3,427 g ha^–1 at a second site (8× and 32× the label rate, respectively). Berry yields were unaffected by rates lower than 857 g ha^–1. Flumioxazin persisted throughout the growing season (approximately 150 d) with no reduction in soil concentration. We conclude that applied at the label rate, flumioxazin is a safe pretransplant weed management option for season-long weed control in strawberry with no yield reduction at rates below 8× the label rate. Caution is recommended for growers who plant a second crop on the same bed.

Nomenclature: flumioxazin; strawberry; Fragaria × ananassa

Horseweed and giant ragweed are competitive, annual weeds that can negatively impact crop yield. Biotypes of glyphosate-resistant (GR) giant ragweed and horseweed were first reported in 2008 and 2010 in Ontario, respectively. GR horseweed has spread throughout the southern portion of the province. The presence of GR biotypes poses new challenges for soybean producers in Canada and the United States. Halauxifen-methyl is a recently registered selective herbicide against broadleaf weeds for preplant use in corn and soybean. There is limited literature on the efficacy of halauxifen-methyl on GR horseweed and giant ragweed when combined with currently registered products in Canada. The purpose of this study was to determine the effectiveness of halauxifen-methyl applied alone and tank-mixed to control GR giant ragweed and GR horseweed in glyphosate and dicamba-resistant (GDR) soybean in southwestern Ontario. Six field experiments were conducted separately for each weed species over 2018 and 2019. Halauxifen-methyl applied alone offered 72% control of GR horseweed at 8 wk after application (WAA). Control was improved to >91% when halauxifen-methyl applied in combination with metribuzin, saflufenacil, chlorimuron-ethyl + metribuzin, and saflufenacil + metribuzin. At 8 WAA, halauxifen-methyl provided 11% control of GR giant ragweed, and 76% to 88% control when glyphosate/2,4-D choline, glyphosate/dicamba, glyphosate/2,4-D choline + halauxifen-methyl, and glyphosate/dicamba + halauxifen-methyl were used. We conclude that halauxifen-methyl applied preplant in a tank-mixture can provide effective control of GR giant ragweed and horseweed in GDR soybean.

Nomenclature: Giant ragweed, Ambrosia trifida L.; horseweed, Conyza canadensis (L.) Cronq.; soybean, Glycine max (L.) Merr.

Hair fescue is a common perennial grass that reduces yields in lowbush blueberry fields. This grass is suppressed with nonbearing-year foramsulfuron applications, though suppression may be improved through use of sequential glufosinate and foramsulfuron applications. The objective of this research was to determine the main and interactive effects of fall bearing-year glufosinate applications, spring nonbearing-year glufosinate applications, and spring nonbearing-year foramsulfuron applications on hair fescue. The experiment was a 2 by 2 by 2 factorial arrangement of fall bearing-year glufosinate application (0, 750 g ai ha–¹), spring nonbearing-year glufosinate application (0, 750 g ai ha–¹), and spring nonbearing-year foramsulfuron application (0, 35 g ai ha–¹) arranged in a randomized complete block design at lowbush blueberry fields located in Parrsboro and Portapique, NS, Canada. Fall bearing-year glufosinate applications, spring nonbearing-year glufosinate applications, and spring nonbearing-year foramsulfuron applications alone provided inconsistent hair fescue suppression. Fall bearing-year glufosinate applications followed by spring nonbearing-year foramsulfuron applications, however, reduced nonbearing-year total tuft density, flowering-tuft density, and flowering-tuft inflorescence number at each site and reduced seed production at Portapique. Sequential fall bearing-year and spring nonbearing-year glufosinate applications or sequential spring nonbearing-year glufosinate and foramsulfuron applications reduced flowering-tuft density and flowering-tuft inflorescence number at each site but did not consistently reduce total tuft density. Sequential herbicide treatments reduced bearing-year seedling density and may therefore contribute to hair fescue seed bank management in lowbush blueberry.

Nomenclature: Foramsulfuron; glufosinate; hair fescue; Festuca filiformis Pourr; FESTE; lowbush blueberry; Vaccinium angustifolium Ait.

Previous research has shown that glufosinate and nicosulfuron at low rates can cause yield loss to grain sorghum. However, research has not been conducted to pinpoint the growth stage at which these herbicides are most injurious to grain sorghum. Therefore, field tests were conducted in 2016 and 2017 to determine the most sensitive growth stage for grain sorghum exposure to both glufosinate and nicosulfuron. Field test were designed with factor A being the herbicide applied (glufosinate or nicosulfuron). Factor B consisted of timing of herbicide application including V3, V8, flagleaf, heading, and soft dough stages. Factor C was glufosinate or nicosulfuron rate where a proportional rate of 656 g ai ha^-1 of glufosinate and 35 g ai ha^-1 of nicosulfuron was applied at 1/10×, 1/50×, and 1/250×. Visible injury, crop canopy heights (cm), and yield were reported as a percent of the nontreated. At the V3 growth stage visible injury of 32% from the 1/10× rate of glufosinate and 51% from the 1/10× rate of nicosulfuron was observed. This injury was reduced by 4 wk after application (WAA) and no yield loss occurred. Nicosulfuron was more injurious than glufosinate at a 1/10× and 1/50× rate when applied at the V8 and flagleaf growth stages resulting in death of the shoot, reduced heading, and yield. Yield losses from the 1/10× rate of nicosulfuron were observed from V8 through early heading and ranged from 41% to 96%. Yield losses from the 1/50× rate of nicosulfuron were 14% to 16% at the flagleaf and V8 growth stages respectively. The 1/10× rate of glufosinate caused 36% visible injury 2 WAA when applied at the flagleaf stage, which resulted in a 16% yield reduction. By 4 WAA visible injury from either herbicide at less than the 1/10× rate was not greater than 4%. Results indicate that injury can occur, but yield losses are more probable from low rates of nicosulfuron at V8 and flagleaf growth stages.

Nomenclature: glufosinate; nicosulfuron; grain sorghum; Sorghum bicolor L. Moench ssp. bicolor