Julie M. Young

Weed Management in Narrow- and Wide-Row Glyphosate-Resistant Soybean (Glycine max)1

Abstract: Field experiments were conducted over 3 yr at three locations in Illinois to evaluate the efficacy of glyphosate in glyphosate-resistant soybean planted in rows spaced 19, 38, and 76 cm. Minimal soybean injury (less than 10%) was observed from any glyphosate treatment. Glyphosate treatments controlled 82 to 99% of giant foxtail. Common waterhemp control was increased as soybean row spacing was decreased. Applying sequential glyphosate applications or increasing the glyphosate rate from 420 g ae/ha to 840 g/ha frequently increased common waterhemp control in 76-cm rows. Velvetleaf control with glyphosate was variable, ranging from 48 to 99%. Decreasing soybean row spacing, utilizing sequential glyphosate applications, or increasing the glyphosate rate improved velvetleaf control in at least four of eight site-years. Glyphosate treatments generally resulted in weed control and soybean yield equal to or greater than the standard herbicide treatments. However, glyphosate treatments yielded less than the hand-weeded control in four of eight site-years, suggesting that weed control from glyphosate treatments was sometimes inadequate. Nomenclature: Glyphosate; common waterhemp, Amaranthus rudis Sauer #3 AMATA; giant foxtail, Setaria faberi Herrm. # SETFA; velvetleaf, Abutilon theophrasti Medik. # ABUTH; soybean, Glycine max (L.) Merr. Additional index words: Herbicide-resistant crops. Abbreviations: EPOST, early postemergence; LPOST, late postemergence; MPOST, mid-postemergence; NIS, nonionic surfactant; UAN, 28% urea ammonium nitrate; WAT, weeks after treatment.

Influence of Plant Height and Glyphosate on Saflufenacil Efficacy on Glyphosate-Resistant Horseweed (Conyza canadensis)

Abstract A field study was conducted in 2007 and 2008 near Murphysboro, IL to determine the effect of plant height and addition of glyphosate on control of glyphosate-resistant horseweed with saflufenacil. Saflufenacil was applied at rates ranging from 25 to 125 g ai ha−1 alone and in combination with glyphosate at 840 g ae ha−1, and the efficacy compared to paraquat at 840 g ai ha−1. Control of horseweed with glyphosate applied alone was less than 30%, confirming the presence of glyphosate-resistant plants. At 14 d after application, all treatments with saflufenacil or paraquat provided at least 90% control. Saflufenacil applied alone at the lowest rate of 25 g ha−1 provided less control (92%) than all other treatments that included saflufenacil, and efficacy was reduced as horseweed height at application increased. Horseweed control from saflufenacil at 50 g ha−1 was reduced as plant height increased in 2007 but not in 2008. However, saflufenacil applied at 50 g ha−1 or greater resulted in at least 98% control, regardless of horseweed height at application or tank mixture with glyphosate. Combining glyphosate with saflufenacil at 25 g ha−1 increased horseweed control compared with saflufenacil applied alone and resulted in control similar to saflufenacil applied at 50 g ha−1. Control of horseweed from paraquat declined over time as the growth continued from the apical meristem. The extent of horseweed regrowth from applications of saflufenacil alone was less than that observed from paraquat. The addition of glyphosate to saflufenacil further reduced the frequency of horseweed regrowth compared with saflufenacil applied alone. Nomenclature: Glyphosate; paraquat; saflufenacil; horseweed; Conyza canadensis (L.) Cronq. Resumen En 2007 y 2008, se realizó un estudio cerca de Murphysboro, IL para determinar el efecto de la altura de planta y la adición de glyphosate sobre el control de Conyza canadensis resistente a glyphosate con saflufenacil. Saflufenacil fue aplicado a dosis que fueron de 25 a 125 g ai ha−1 solo y en combinación con glyphosate a 840 g ae ha−1, y la eficacia de estos tratamientos se comparó con paraquat a 840 g ai ha−1. El control de C. canadensis con glyphosate aplicado solo fue menor a 30%, confirmando la presencia de plantas resistentes a glyphosate. A 14 d después de la aplicación, todos los tratamientos con saflufenacil o paraquat brindaron al menos 90% de control. Saflufenacil aplicado solo a la dosis más baja de 25 g ha−1 brindó menos control (92%) que todos los demás tratamientos que incluyeron saflufenacil, y la eficacia se redujo al incrementarse la altura de la maleza al momento de la aplicación. El control de C. canadensis con saflufenacil a 50 g ha−1 se redujo al aumentar la altura de las plantas al momento de la aplicación en 2007, pero no en 2008. Sin embargo, saflufenacil aplicado a 50 g ha−1 o más resultó en al menos 98% de control, sin importar la altura de C. canadensis al momento de la aplicación o la mezcla en tanque con glyphosate. El combinar glyphosate con saflufenacil a 25 g ha−1 aumentó el control de C. canadensis en comparación con saflufenacil aplicado solo y resultó en un control similar a saflufenacil aplicado a 50 g ha−1. El control de C. canadensis con paraquat disminuyó conforme pasó el tiempo y continuó el crecimiento a partir del meristemo apical. El nivel de rebrote de plantas de C. canadensis después de aplicaciones de solo saflufenacil fue menor que el nivel observado con paraquat. La adición de glyphosate a saflufenacil disminuyó aún más la frecuencia de rebrotes de C. canadensis en comparación con la aplicación de saflufenacil solo.

Characterization of PPO-Inhibitor-Resistant Waterhemp (Amaranthus tuberculatus) Response to Soil-Applied PPO-Inhibiting Herbicides

Abstract Waterhemp resistance to foliar applications of protoporphyrinogen oxidase (PPO)–inhibiting herbicides has become increasingly disconcerting given the widespread distribution of glyphosate resistance. Fortunately, soil-residual PPO-inhibiting herbicides remain efficacious in waterhemp populations resistant to PPO-inhibiting herbicides; however, these herbicides should theoretically select for the resistant biotype as herbicide concentrations diminish in the soil. Accordingly, the objectives of this research were twofold: (1) evaluate the efficacy of three PPO-inhibiting herbicides, foliar- and soil-applied, on PPO-resistant (PPO-R) and PPO-susceptible (PPO-S) waterhemp, and (2) investigate the differential effects of PPO-inhibiting herbicides on an R biotype and an S biotype during several discrete developmental events relevant to soil–residual herbicide activity (i.e., radicle protrusion, radicle elongation, and waterhemp emergence). Greenhouse and growth chamber experiments indicated that the R biotype was least sensitive to the diphenylether herbicide fomesafen, followed by sulfentrazone and flumioxazin; however, fomesafen plus s-metolachlor improved soil-residual efficacy over fomesafen alone. Growth stage considerably influenced the R : S ratio, decreasing from 38× to 3.4×, when comparing ratios generated from foliar applications and soil-residual applications measuring radicle protrusion, respectively. Overall, this research supports the use of full soil-residual herbicide rates, reinforcing the importance of best management practices to manage the spread of herbicide resistance. Nomenclature: Flumioxazin; fomesafen; glyphosate; s-metolachlor; sulfentrazone; tall waterhemp, Amaranthus tuberculatus (Moq.) Sauer (syn. rudis) AMATA.

Occurrence of an herbicide‐resistant plant trait in agricultural field margins

Abstract Agricultural environments allow study of evolutionary change in plants. An example of evolution within agroecological systems is the selection for resistance to the herbicide glyphosate within the weed, Conyza canadensis. Changes in survivorship and reproduction associated with the development of glyphosate resistance (GR) may impact fitness and influence the frequency of occurrence of the GR trait. We hypothesized that site characteristics and history would affect the occurrence of GR C. canadensis in field margins. We surveyed GR occurrence in field margins and asked whether there were correlations between GR occurrence and location, crop rotation, GR crop trait rotation, crop type, use of tillage, and the diversity of herbicides used. In a field experiment, we hypothesized that there would be no difference in fitness between GR and glyphosate‐susceptible (GS) plants. We asked whether there were differences in survivorship, phenology, reproduction, and herbivory between 2 GR and 2 GS populations of C. canadensis in agrestal and ruderal habitats. We found that geographic location was an important factor in the occurrence of GR C. canadensis in field margins. Although not consistently associated with either glyphosate resistance or glyphosate susceptibility, there were differences in phenology, survivorship, and herbivory among biotypes of C. canadensis. We found equal or greater fitness in GR biotypes, compared to GS biotypes, and GR plants were present in field margins. Field margins or ruderal habitats may provide refugia for GR C. canadensis, allowing reproduction and further selection to occur as seeds recolonize the agrestal habitat. Agricultural practices may select for ecological changes that feed back into the evolution of plants in ruderal habitats.

Soybean (Glycine max) Response to Foliar Applications of Mesotrione1

Field studies were conducted to evaluate soybean injury and yield reduction from foliar applications of mesotrione. Mesotrione was applied at 1.1, 3.2, 11, 35, and 105 g ai/ha to ‘BT 386C’ soybeans at the V1 stage of growth. All rates of mesotrione resulted in visual injury to soybean at 7 and 14 d after treatment (DAT). Overall soybean injury from mesotrione was greatest at 14 DAT, with 25 to 78% injury observed. By 28 DAT, soybean injury was 31 and 66% from mesotrione at 35 and 105 g/ha, respectively, and less than 10% from mesotrione at 1.1, 3.2, and 11 g/ha. Soybean yield was reduced 11 and 22% by mesotrione at 35 and 105 g/ha, respectively. No reduction in soybean yield was observed from mesotrione at rates up to 11 g/ha. Regression analysis indicated that soybean injury from mesotrione at 28 DAT was the best predictor of yield loss (r2 = 0.77), compared with injury evaluations at 7, 14, and 56 DAT. Greenhouse studies were conducted to determine whether soybean injury from mesotrione was affected by soybean growth stage and variety. Soybean varieties BT 386C, ‘Asgrow 4602RR’, ‘Pioneer 94B01’, and ‘LS 930375’ were more sensitive to mesotrione at the VC growth stage than at the V1 and V2 stages. At the V2 stage, Asgrow 4602RR was three to five times more sensitive to mesotrione than the other three varieties. Nomenclature: Mesotrione; soybean, Glycine max L. (Merr.) ‘Asgrow 4602RR’, ‘BT 386C’, ‘LS 930375’, ‘Pioneer 94B01’. Additional index words: Herbicide drift, tank contamination. Abbreviations: DAT, days after treatment; POST, postemergence.

Glyphosate-Induced Antagonism in Rapid Response Giant Ragweed (Ambrosia trifida)

Abstract Glyphosate application to the rapid-response (RR) biotype of glyphosate-resistant (GR) giant ragweed ensues in loss of foliage via rapid tissue death, thereby reducing glyphosate translocation. Experiments were performed to determine if this GR response, in contrast to a non-rapid response (NRR) GR biotype, results in antagonism of the selective herbicides atrazine, cloransulam, dicamba, lactofen, and topramezone. Application of glyphosate at 1,680 g ae ha-1 in the greenhouse resulted in antagonism between all five selective herbicides for the RR biotype, whereas glyphosate applied at 420 g ha-1 was antagonistic only for cloransulam. Application of selective herbicides 2 d prior to glyphosate treatment avoided the antagonism observed in the RR biotype. In the field, glyphosate mixtures with dicamba and topramezone were antagonistic on the RR biotype across both 2015 and 2016 field seasons. Thus, the RR effectively reduces glyphosate efficacy but also has potential to diminish the activity of glyphosate mixtures with selective herbicides, and the degree of antagonism between these mixtures escalates at increasing glyphosate rates. Nomenclature: Atrazine; cloransulam; dicamba; glyphosate; lactofen; topramezone; giant ragweed, Ambrosia trifida L. AMBTR

Timing of Soil-Residual Herbicide Applications for Control of Giant Ragweed (Ambrosia trifida)

Fall-applied residual and spring preplant burn-down herbicide applications are typically used to control winter annual weeds and may also provide early-season residual control of summer annual weed species such as giant ragweed. Field experiments were conducted from 2006 to 2008 in southern Illinois to (1) assess the emergence pattern of giant ragweed, (2) evaluate the efficacy of several herbicides commonly used for soil-residual control of giant ragweed, and (3) investigate the optimal application timing of soil-residual herbicides for control of giant ragweed. Six herbicide treatments were applied at four application timings: early fall, late fall, early spring, and late spring. Giant ragweed first emerged in mid- and late-March in 2007 and 2008, respectively. The duration of emergence varied by year, with 95% of emergence complete in late May of 2008, but not until early July in 2007. Giant ragweed emergence occurred more quickly in plots that received a fall application of glyphosate + 2,4-D compared with the nontreated. Fall-applied residual herbicides did not reduce giant ragweed emergence in 2007 when compared with the nontreated, with the exception of chlorimuron + tribenuron applied in late fall. Giant ragweed control from early- and late-spring herbicide applications was variable by year. In 2007, saflufenacil (50 and 100 g ai ha−1) and simazine applied in early spring reduced giant ragweed densities by 95% or greater through mid-May; however, in 2008, early-spring applications failed to reduce giant ragweed emergence in mid-April. The only treatments that reduced giant ragweed densities by > 80% through early July were late-spring applications of chlorimuron + tribenuron or saflufenacil at 100 g ha−1. Thus, the emergence patterns of giant ragweed in southern Illinois dictates that best management with herbicides would include late-spring applications of soil-residual herbicides just before crop planting and most likely requires subsequent control with foliar or soil-residual herbicides after crop emergence. Las aplicaciones de herbicidas residuales en el otoño y de herbicidas para eliminación general de vegetación antes de la siembra en la primavera son usadas típicamente para el control de malezas anuales de invierno y que pueden además brindar un control residual de malezas anuales de verano tales como Ambrosia trifida, temprano en la temporada. Experimentos de campo fueron realizados entre 2006 y 2008, en el sur de Illinois, para (1) evaluar el patrón de emergencia de A. trifida, (2) evaluar la eficacia de varios herbicidas comúnmente usados para el control residual en el suelo de A. trifida, e (3) investigar el momento de aplicación óptimo para herbicidas residuales en el suelo para el control de A. trifida. Se aplicaron seis tratamientos de herbicidas en cuatro momentos de aplicación: temprano en el otoño, tarde en el otoño, temprano en la primavera, y tarde en la primavera. A. trifida emergió primero durante la mitad y el final de Marzo en 2007 y 2008, respectivamente. La duración de la emergencia varió dependiendo del año, con 95% de la emergencia completándose al final de Mayo de 2008, pero no hasta el inicio de Julio en 2007. La emergencia de A. trifida ocurrió más rápidamente en parcelas que recibieron una aplicación de glyphosate + 2,4-D durante el otoño al compararse con el testigo sin tratamiento. Los herbicidas residuales aplicados en el otoño no redujeron la emergencia de A. trifida en 2007 cuando se compararon con el testigo, con la excepción de chlorimuron + tribenuron aplicados al final del otoño. El control de A. trifida con aplicaciones temprano y tarde durante la primavera fue variable dependiendo del año. En 2007, saflufenacil (50 y 100 g ai ha−1) y simazine aplicados temprano en la primavera redujeron las densidades de A. trifida en 95% o más hasta la mitad de Mayo. Sin embargo, en 2008, aplicaciones realizadas temprano en la primavera fallaron en reducir la emergencia de A. trifida en la mitad de Abril. Los únicos tratamientos que redujeron las densidades de A. trifida > 80% hasta el inicio de Julio fueron las aplicaciones de chlorimuron + tribenuron o saflufenacil a 100 g ha−1 tarde en la primavera. Así, los patrones de emergencia de A. trifida en el sur de Illinois dictan que el mejor manejo con herbicidas debería incluir aplicaciones de herbicidas de suelo residuales tarde en la primavera antes de la siembra del cultivo y muy probablemente requiere un control de seguimiento con herbicidas foliares y de suelo residuales después de la emergencia del cultivo. Nomenclature: 2,4-D; chlorimuron; flumioxazin; glyphosate; saflufenacil; simazine; tribenuron; giant ragweed, Ambrosia trifida L.

Influence of Application Variables on the Foliar Efficacy of Saflufenacil on Horseweed (Conyza canadensis)

Abstract Greenhouse studies were conducted to determine the influence of spray-solution pH, adjuvant, light intensity, temperature, and glyphosate on the efficacy of saflufenacil on horseweed. Control of glyphosate-resistant horseweed from saflufenacil alone was greatest with a spray-solution pH of 5, compared with pH 7 or 9. However, when glyphosate was added to saflufenacil, similar GR50 values were measured with spray solutions adjusted to pH 5 and 9, and horseweed control at pH 9 was 38% greater than at pH 7. The efficacy of saflufenacil on horseweed was 36% greater when crop oil concentrate was used as an adjuvant compared with nonionic surfactant, regardless of the addition of glyphosate or the sensitivity of the horseweed population to glyphosate (resistant vs. susceptible). The addition of glyphosate to low rates of saflufenacil increased control over saflufenacil applied alone on glyphosate-susceptible and -resistant horseweed. Saflufenacil activity was greater under low light intensity (300 &mgr;mol m−2 s−1) than high light intensity (1,000 &mgr;mol m−2 s−1). Although initial horseweed control was greater under high temperature (27 C) compared with low temperature (10 C), by 21 d after treatment horseweed dry weight was similar from saflufenacil applied under high and low temperatures. Nomenclature: Glyphosate, saflufenacil, horseweed, Conyza canadensis (L.) Cronq.

Environmental Factors Moderate Glyphosateinduced Antagonism of POST Herbicides on the Rapid Response Biotype of Glyphosate-Resistant Giant Ragweed (Ambrosia trifida)

Abstract In the rapid response (RR) biotype of glyphosate-resistant (GR) giant ragweed (Ambrosia trifida L.), exposure to glyphosate elicits H2O2 production in mature leaves, resulting in foliage loss and reduced glyphosate translocation. When glyphosate is applied with POST herbicides intended to improve control of A. trifida, the RR to glyphosate has the propensity to antagonize these herbicide combinations. This research documents how transient changes in air temperature, soil moisture, and light intensity during a 6-d period surrounding herbicide application regulate induction of the RR and the effect on POST herbicide interactions with glyphosate. Air temperature had the greatest influence on H2O2 accumulation in leaf disks following treatment with glyphosate, as plants at 30 C produced more than twice the amount of H2O2 at 2.5 h after treatment compared with 10 C. Plants under field capacity conditions accumulated nearly 50% more H2O2 than those at one-third field capacity, while those under no shade had only 18% more H2O2 compared with those in a shaded environment. Despite these initial results, dry weight reduction at 21 d after treatment never differed by more than 8% between levels of environmental factors, thus indicating a negligible influence on glyphosate efficacy. The magnitude of glyphosate-induced antagonism was generally greater at 30 C (12% to 21% less than expected control) versus 10 C (11% to 16%) on atrazine, cloransulam, dicamba, and topramezone and was greater at field capacity (20% to 24%) versus one-third field capacity (11% to 15%) on cloransulam and topramezone. These results indicate air temperatures and soil moisture levels conducive to optimal plant growth accelerate the RR to glyphosate, thereby increasing the likelihood of glyphosateinduced antagonism on several translocated herbicides.

Efficacy of Preplant Corn and Soybean Herbicides on Star-of-Bethlehem (Ornithogalum umbellatum) in No-Till Crop Production

Field research was conducted to evaluate the efficacy of preplant herbicides commonly used in no-till corn and soybean production and to determine the efficacy of three application timings in the spring for star-of-Bethlehem bulb management. A single, preplant application of herbicide treatments that included flumioxazin, sulfentrazone, or paraquat resulted in 91 to 97% control of star-of-Bethlehem at 14 d after treatment (DAT). Star-of-Bethlehem control from atrazine and metribuzin was moderate (70 to 75%) at the Marion location but poor (< 20%) at Murphysboro. Regardless of the initial foliar control at 14 DAT from treatments included in the corn and soybean herbicide screen, only applications containing paraquat resulted in extensive control (75 to 86%) of star-of-Bethlehem foliar regrowth by 1 yr after treatment. Star-of-Bethlehem was most responsive to herbicide applications in mid-March in southern Illinois when compared with applications made March 1 and April 11. The mid-March application timing corresponded to the vegetative reproductive stage, approximately 3 wk prior to flowering. The average density of star-of-Bethlehem bulbs in nontreated plots occupied 7.9% of the field soil volume in the upper 7.6 cm of the soil profile. Spring applications of paraquat (repeated 2 yr consecutively) reduced bulb density in the soil by 88%, compared with 5% or less bulb reduction for consecutive applications of glyphosate or 2,4-D ester applied alone. Overall, paraquat and paraquat tank mixtures provided the most effective and consistent control of star-of-Bethlehem foliage and underground bulbs, which is paramount for long-term management of this invasive species. Una investigación de campo fue realizada para evaluar la eficacia de herbicidas presiembra comúnmente usados en la producción de maíz y soja bajo labranza cero y para determinar su eficacia en tres momentos de aplicación en la primavera para el manejo de bulbos de Ornithogalum umbellatum. Una única aplicación en presiembra de tratamientos de herbicidas que incluyeron ya fuera flumioxazin, sulfentrazone, o paraquat resultaron en 91 a 97% de control de O. umbellatum a 14 d después del tratamiento (DAT). El control de O. umbellatum con atrazine y metribuzin fue moderado (70 a 75%) en la localidad de Marion pero pobre (< 20%) en Murphysboro. Sin importar el control foliar inicial a 14 DAT con los tratamientos incluidos en la evaluación de herbicidas en maíz y soja, solamente las aplicaciones que contenían paraquat resultaron en un control extensivo (75 a 86%) del rebrote foliar de O. umbellatum a 1 año después del tratamiento. O. umbellatum respondió más a las aplicaciones de herbicidas en la mitad de Marzo en el sur de Illinois cuando se comparó con aplicaciones hechas el 1 de Marzo y el 11 de Abril. El momento de aplicación en la mitad de Marzo correspondió con el estadio de reproducción vegetativa, aproximadamente 3 semanas antes de la floración. La densidad promedio de bulbos de O. umbellatum en parcelas sin tratamiento ocupó el 7.9% del volumen del suelo en el campo en los 7.6 cm superiores del perfil del suelo. Las aplicaciones en la primavera de paraquat (repetidas consecutivamente por 2 años) redujeron la densidad de bulbos en el suelo en 88%, al compararse con 5% o menos de reducción de bulbos para aplicaciones consecutivas de glyphosate o 2,4-D ester aplicados solos. En general, paraquat y mezclas en tanque con paraquat brindaron el control más efectivo y consistente del follaje y los bulbos de O. umbellatum, lo que es indispensable para el manejo a largo plazo de esta nueva especie invasiva. Nomenclature: 2,4-D ester; atrazine; flumioxazin; glyphosate; metribuzin; paraquat; sulfentrazone; star-of-Bethlehem, Ornithogalum umbellatum L. OTGUM; corn, Zea mays L.; soybean, Glycine max (L.) Merr.