Lowell D. Sandell

Confirmation and Control of Triazine and 4-Hydroxyphenylpyruvate Dioxygenase-Inhibiting Herbicide-Resistant Palmer Amaranth (Amaranthus palmeri) in Nebraska

Abstract Palmer amaranth is a difficult-to-control broadleaf weed that infests corn and soybean fields in south-central and southwestern Nebraska and several other states in the United States. The objectives of this research were to confirm triazine and 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicide-resistant Palmer amaranth in Nebraska and to determine sensitivity and efficacy of POST-applied corn herbicides for control of resistant and susceptible Palmer amaranth biotypes. Seeds from a putative HPPD-resistant Palmer amaranth biotype from Fillmore County, NE were collected from a seed corn production field in fall 2010. The response of Palmer amaranth biotypes to 12 rates (0 to 12×) of mesotrione, tembotrione, topramezone, and atrazine was evaluated in a dose–response bioassay in a greenhouse. On the basis of the values at the 90% effective dose (ED90) level, the analysis showed a 4- to 23-fold resistance depending upon the type of HPPD-inhibiting herbicide being investigated and susceptible biotype used for comparison. This biotype also had a 9- to 14-fold level of resistance to atrazine applied POST. Results of a POST-applied herbicide efficacy study suggested a synergistic interaction between atrazine and HPPD-inhibiting herbicides that resulted in > 90% control of all Palmer amaranth biotypes. The resistant biotype had a reduced sensitivity to acetolactate synthase inhibiting herbicides (halosulfuron and primisulfuron), a photosystem-II inhibitor (bromoxynil), and a protoporphyrinogen oxidase inhibitor (fluthiacet-methyl). Palmer amaranth biotypes were effectively controlled (≥ 90%) with glyphosate, glufosinate, and dicamba, whereas 2,4-D ester provided 81 to 83% control of the resistant biotype and > 90% control of both susceptible biotypes. Nomenclature: 2,4-D; atrazine; bromoxynil; dicamba, fluthiacet-methyl; glufosinate; glyphosate; halosulfuron-methyl; lactofen; mesotrione; primisulfuron-methyl; pyrasulfotole; tembotrione; thiencarbazone-methyl; topramezone; Palmer amaranth, Amaranthus palmeri S. Wats; corn, Zea mays L; soybean, Glycine max (L.) Merr. Resumen Amaranthus palmeri es una maleza de hoja ancha difícil de controlar que infesta campos de maíz y soya en el centro y oeste del sur de Nebraska y en varios otros estados en los Estados Unidos. Los objetivos de esta investigación fueron confirmar la existencia de A. palmeri resistente a triazine y herbicidas inhibidores de 4-hydroxyphenylpyruvate dioxygenase (HPPD) en Nebraska y determinar la sensibilidad y la eficacia de herbicidas para maíz aplicados POST para el control de biotipos de A. palmeri susceptibles y resistentes. Semillas de A. palmeri con resistencia putativa a HPPD provenientes del condado Fillmore, NE fueron colectadas de un campo de producción de maíz en el otoño de 2010. La respuesta de los biotipos de A. palmeri a 12 dosis (0 a 12×) de mesotrione, tembotrione, topramezone, y atrazine fue evaluada en un bioensayo de respuesta a dosis en un invernadero. Con base en los valores del nivel de dosis efectiva de 90%, los análisis mostraron una resistencia de 4 a 23 veces mayor dependiendo del tipo de herbicida inhibidor de HPPD investigado y del biotipo susceptible usado como comparación. Este biotipo también tuvo un nivel de resistencia a atrazine POST de 9 a 14 veces mayor. Los resultados del estudio de eficacia de herbicidas aplicados POST sugirieron una interacción sinérgica entre atrazine y herbicidas inhibidores de HPPD que resultó en >90% de control de todos los biotipos de A. palmeri. El biotipo resistente tuvo una sensibilidad reducida a herbicidas inhibidores de acetolactate synthase (halosulfuron y primisulfuron), a un inhibidor del fotosistema II (bromoxynil) y a un inhibidor de protoporphyrinogen oxidase (fluthiacet-methyl). Los biotipos de A. palmeri fueron controlados efectivamente (≥90%) con glyphosate, glufosinate, y dicamba, mientras que 2,4-D ester brindó un control de 81 a 83% del biotipo resistente y >90% de los dos biotipos susceptibles.

Predicting Emergence of 23 Summer Annual Weed Species

Abstract First- and second-year seedbank emergence of 23 summer annual weed species common to U.S. corn production systems was studied. Field experiments were conducted between 1996 and 1999 at the Iowa State University Johnson Farm in Story County, Iowa. In the fall of 1996 and again in 1997, 1,000 seeds for most species were planted in plastic crates. Seedling emergence was counted weekly for a 2-yr period following seed burial (starting in early spring). Soil temperature at 2 cm depth was estimated using soil temperature and moisture model software (STM2). The Weibull function was fit to cumulative emergence (%) on cumulative thermal time (TT), hydrothermal time (HTT), and day of year (DOY). To identify optimum base temperature (Tbase) and base matric potential (&psgr;base) for calculating TT or HTT, Tbase and &psgr;base values ranging from 2 to 17 C and −33 to −1,500 kPa, respectively, were evaluated for each species. The search for the optimal model for each species was based on the Akaikes Information Criterion (AIC), whereas an extra penalty cost was added to HTT models. In general, fewer seedlings emerged during the first year of the first experimental run (approximately 18% across all species) than during the second experimental run (approximately 30%). However, second-year seedbank emergence was similar for both experimental runs (approximately 6%). Environmental effects may be the cause of differences in total seedling emergence among years. Based on the AIC criterion, for 17 species, the best fit of the model occurred using Tbase ranging from 2 to 15 C with four species also responding to &psgr;base  =  −750 kPa. For six species, a simple model using DOY resulted in the best fit. Adding penalty costs to AIC calculation allowed us to compare TT and HTT when both models behaved similarly. Using a constant Tbase, species were plotted and classified as early-, middle-, and late-emerging species, resulting in a practical tool for forecasting time of emergence. The results of this research provide robust information on the prediction of the time of summer annual weed emergence, which can be used to schedule weed and crop management.

Glyphosate-Resistant Giant Ragweed (Ambrosia trifida) Control in Glufosinate-Resistant Soybean

Abstract Glyphosate-resistant giant ragweed is one of the most competitive weeds of agronomic crops in the United States. Early emergence and rapid growth rate makes giant ragweed a competitive weed early in the season and reduces crop yields. Therefore, early spring control of giant ragweed using a preplant herbicide is critical. Glufosinate is an alternative POST herbicide for weed control in glufosinate-resistant soybean. Field experiments were conducted at David City, NE, in 2012 and 2013 to evaluate the efficacy of preplant herbicides followed by glufosinate applied alone or in tank mixes for control of glyphosate-resistant giant ragweed in glufosinate-resistant soybean. Preplant treatments containing 2,4-D, flumioxazin, glufosinate, paraquat, saflufenacil, and sulfentrazone provided 79 to 99% control of giant ragweed 21 d after treatment (DAT), and subsequent application of glufosinate alone or in tank mixes resulted in 90 to 99% control at 21 DAT. Preplant application of S-metolachlor plus metribuzin or chlorimuron, flumioxazin plus thifensulfuron followed by glufosinate resulted in < 40% control of giant ragweed, and soybean yields were < 870 kg ha−1. Although statistically comparable to several other treatments, preplant application of 2,4-D or saflufenacil tank mixes followed by glufosinate resulted in the highest level of control (> 97%) and soybean yield (2,624 to 3,378 kg ha−1). This study confirms that preplant herbicide options are available for control of glyphosate-resistant giant ragweed, and a follow-up application of glufosinate will provide season-long control in glufosinate-resistant soybean. Nomenclature: 2,4-D amine; acetochlor; chlorimuron; cloransulam; dimethenamid-P; flumioxazin, fomesafen; glufosinate; glyphosate; imazethapyr; lactofen; metribuzin; paraquat, S-metolachlor; saflufenacil; sulfentrazone; thifensulfuron-methyl; giant ragweed, Ambrosia trifida L.; soybean, Glycine max (L.) Merr. Resumen Ambrosia trifida resistente a glyphosate es una de las malezas más competitivas en cultivos agronómicos en Estados Unidos. Su emergencia temprana y tasa rápida de crecimiento hacen A. trifida una maleza competitiva temprano durante la temporada de crecimiento, y que reduce el rendimiento de los cultivos. De esta forma, el control de A. trifida, temprano en la primavera, usando herbicidas pre-siembra es crítico. Glufosinate es un herbicida POST alternativo para el control de malezas en soya resistente a glufosinate. Se realizaron experimentos de campo en David City, Nebraska, en 2012 y 2013 para evaluar la eficacia de herbicidas pre-siembra seguidos de glufosinate aplicado solo o en mezclas en tanque para el control de A. trifida resistente a glyphosate, en soya resistente a glufosinate. Los tratamientos pre-siembra con 2,4-D, flumioxazin, glufosinate, paraquat, saflufenacil, y sulfentrazone brindaron 79 a 99% de control de A. trifida 21 d después del tratamiento (DAT), y la subsecuente aplicación de glufosinate solo o en mezclas en tanque resultaron en 90 a 99% de control a 21 DAT. La aplicación pre-siembra de S-metolachlor más metribuzin o chlorimuron, flumioxazin más thifensulfuron seguidos de glufosinate resultaron en <40% de control de A. trifida, y los rendimientos de la soya fueron <870 kg ha−1. Aunque fue estadísticamente comparable a otros tratamientos, la aplicación pre-siembra de mezclas en tanque de 2,4-D o saflufenacil seguidas de glufosinate resultaron en el mayor nivel de control (>97%) y el mayor rendimiento de la soya (2,624 a 3,378 kg ha−1). Este estudio confirma que hay opciones de herbicidas pre-siembra disponibles para el control de A. trifida resistente a glyphosate, y que aplicaciones posteriores de glufosinate brindarán control a lo largo de toda la temporada de crecimiento en soya resistente a glufosinate.

Confirmation and control of glyphosate-resistant common waterhemp (Amaranthus rudis) in Nebraska.

Abstract Glyphosate-resistant common waterhemp is a difficult-to-control annual broadleaf weed that has become a serious management challenge for growers in Nebraska and other states in the United States. The objectives of this study were to confirm glyphosate-resistant common waterhemp in Nebraska by quantifying level of resistance in a dose-response study, and to determine the sensitivity and efficacy of POST soybean herbicides for controlling suspected glyphosate-resistant common waterhemp biotypes. Seeds of suspected glyphosate-resistant common waterhemp biotypes were collected from seven eastern Nebraska counties. Greenhouse dose-response experiments were conducted to evaluate the response of common waterhemp biotypes to nine rates of glyphosate (0 to 16×). Common waterhemp biotypes were 3- to 39-fold resistant to glyphosate depending on the biotype being investigated and the susceptible biotype used for comparison. Results of the POST soybean herbicides efficacy experiment suggested that glyphosate-resistant biotypes, except a biotype from Pawnee County, had reduced sensitivity to acetolactate synthase (ALS)–inhibiting herbicides (chlorimuron-ethyl, imazamox, imazaquin, imazethapyr, and thifensulfuron-methyl). Glufosinate and protoporphyrinogen oxidase (PPO)–inhibiting herbicides (acifluorfen, fluthiacet-methyl, fomesafen, and lactofen) provided ≥ 80% control of glyphosate-resistant common waterhemp at 21 d after treatment (DAT). This study confirmed the first occurrence of glyphosate-resistant common waterhemp in Nebraska, and also revealed reduced sensitivity to ALS-inhibiting herbicides in most of the biotypes tested in this study. Nomenclature: Acifluorfen; chlorimuron-ethyl; fluthiacet-methyl; fomesafen; glufosinate; glyphosate; imazamox; imazaquin; imazethapyr; lactofen; thifensulfuron-methyl; common waterhemp, Amaranthus rudis Sauer; soybean, Glycine max (L.) Merr. Resumen Amaranthus rudis resistente a glyphosate es una maleza anual de hoja ancha difícil de controlar y que se ha convertido en un reto serio de manejo para productores en Nebraska y otros estados en los Estados Unidos. Los objetivos de este estudio fueron confirmar la resistencia a glyphosate de A. rudis en Nebraska, cuantificando el nivel de resistencia mediante estudios de respuesta a dosis, y determinar la sensibilidad y la eficacia de herbicidas POST para soja para el control de biotipos de A. rudis que se sospecha son resistentes a glyphosate. En siete condados del este de Nebraska, se colectaron semillas de biotipos de A. rudis que se sospechaba eran resistentes a glyphosate. Se realizaron experimentos de respuesta a dosis en invernadero, para evaluar la respuesta de biotipos de A. rudis a nueve dosis de glyphosate (0 a 16X). Biotipos de A. rudis fueron de 3 a 39 veces más resistentes a glyphosate, dependiendo del biotipo investigado y del biotipo susceptible usados en la comparación. Los resultados del experimento sobre la eficacia de herbicidas POST para soja sugirieron que los biotipos resistentes a glyphosate, con la excepción del biotipo proveniente del condado Pawnee, tuvieron una sensibilidad reducida a los herbicidas inhibidores de acetolactate synthase (ALS) (chlorimuron-ethyl, imazamox, imazaquin, imazethapyr, y thifensulfuron-methyl). Glufosinate y los herbicidas inhibidores de protoporphyrinogen oxidase (PPO) (acifluorfen, fluthiacet-methyl, fomesafen, y lactofen) brindaron ≥ 80% control de A. rudis resistente a glyphosate a 21 d después del tratamiento (DAT). Este estudio confirmó el primer caso de A. rudis resistente a glyphosate en Nebraska, y también reveló la sensibilidad reducida a herbicidas inhibidores de ALS en la mayoría de los biotipos evaluados en este estudio.

Integrated Management of Glyphosate-Resistant Giant Ragweed (Ambrosia trifida) with Tillage and Herbicides in Soybean

Giant ragweed is one of the most competitive annual broadleaf weeds in soybean production fields in the midwestern United States and eastern Canada because of its early emergence, rapid growth rate, high plasticity, and resistance to glyphosate and acetolactate synthase inhibitors. Therefore, early-season management of giant ragweed is critical to avoid yield loss. The objectives of this study were to evaluate control of glyphosate-resistant giant ragweed through the integration of preplant tillage or 2,4-D; PRE or early POST (EPOST) followed by (fb) late POST (LPOST) herbicide programs with or without preplant tillage or 2,4-D; and their effect on soybean injury and yield. A field study was conducted in 2013 and 2014 in David City, NE in a field infested with glyphosate-resistant giant ragweed. Preplant tillage or 2,4-D application provided > 90% control of glyphosate-resistant giant ragweed 14 d after preplant treatment. Giant ragweed control and biomass reduction were consistently > 90% with preplant tillage or 2,4-D fb sulfentrazone plus cloransulam PRE or glyphosate plus cloransulam EPOST fb glyphosate plus fomesafen or lactofen LPOST compared with ≤ 86% control with same treatments without preplant tillage or 2,4-D. PRE or EPOST fb LPOST herbicide programs preceded by preplant treatments resulted in giant ragweed density < 2 plants m−2 and soybean yield > 2,400 kg ha−1 compared with the density of ≥ 2 plants m−2 and soybean yield < 1,800 kg ha−1 under PRE or EPOST fb LPOST herbicide programs. The contrast analysis also indicated that preplant tillage or 2,4-D fb a PRE or POST program was more effective for giant ragweed management compared with PRE fb POST herbicide programs. Integration of preplant tillage would provide an alternative method for early-season control of giant ragweed; however, a follow up application of herbicides is needed for season-long control in soybean. Nomenclature: 2,4-D amine; cloransulam; fomesafen; lactofen; sulfentrazone; giant ragweed, Ambrosia trifida L.; soybean, Glycine max (L.) Merr. Ambrosia trifida es una de las malezas de hoja ancha más competitivas en campos de producción de soja en el centro-oeste de los Estados Unidos y en el este de Canada debido a su emergencia temprana, alta tasa crecimiento, alta plasticidad, y resistencia a glyphosate e inhibidores de acetolactate synthase. Por esta razón, el manejo de A. trifida, temprano durante la temporada de crecimiento, es crítico para evitar las pérdidas de rendimiento. Los objetivos de este estudio fueron evaluar el control de A. trifida resistente a glyphosate mediante la integración de labranza en pre-siembra o 2,4-D; aplicaciones PRE o POST temprana (EPOST) seguido por (fb) programas de herbicidas en POST tardía (LPOST) con o sin labranza en pre-siembra o 2,4-D; y su efecto sobre el daño y rendimiento de la soja. Se realizó un estudio de campo en 2013 y 2014 en David City, Nebraska en un campo infestado con A. trifida resistente a glyphosate. La labranza en pre-siembra o la aplicación de 2,4-D brindó >90% de control de A. trifida resistente a glyphosate 14 d después del tratamiento pre-siembra. El control y la reducción en la biomasa de A. trifida fueron >90% consistentemente con la labranza pre-siembra o 2,4-D fb sulfentrazone más cloransulam PRE o glyphosate más cloransulam EPOST fb glyphosate más fomesafen o lactofen POST al compararlo con ≤86% de control con los mismos tratamientos sin labranza pre-siembra o 2,4-D. Programas de herbicidas PRE o de EPOST fb LPOST precedidos por tratamientos pre-siembra resultaron en una densidad de A. trifida <2 plantas m−2 y un rendimiento de soja >2,400 kg ha−1 al compararse con la densidad de ≥2 plantas m−2 y un rendimiento de soja ≤1,800 kg ha−1 con programas de herbicidas PRE o EPOST fb LPOST. El análisis de contrastes también indicó que la labranza pre-siembra o 2,4-D fb de un programa PRE o POST fue más efectiva para el manejo de A. trifida al compararse con programas de herbicidas PRE fb POST. La integración de labranza pre-siembra brindaría una alternativa de manejo para el control de A. trifida, temprano en la temporada. Sin embargo, se requiere que a esto le siga una aplicación de herbicidas para obtener un control a lo largo de toda la temporada en soja.

Management of Pigweed (Amaranthus spp.) in Glufosinate-Resistant Soybean in the Midwest and Mid-South

Pigweeds are among the most abundant and troublesome weed species across Midwest and mid-South soybean production systems because of their prolific growth characteristics and ability to rapidly evolve resistance to several herbicide sites of action. This has renewed interest in diversifying weed management strategies by implementing integrated weed management (IWM) programs to efficiently manage weeds, increase soybean light interception, and increase grain yield. Field studies were conducted across 16 site-years to determine the effectiveness of soybean row width, seeding rate, and herbicide strategy as components of IWM in glufosinate-resistant soybean. Sites were grouped according to optimum adaptation zones for soybean maturity groups (MGs). Across all MG regions, pigweed density and height at the POST herbicide timing, and end-of-season pigweed density, height, and fecundity were reduced in IWM programs using a PRE followed by (fb) POST herbicide strategy. Furthermore, a PRE fb POST herbicide strategy treatment increased soybean cumulative intercepted photosynthetically active radiation (CIPAR) and subsequently, soybean grain yield across all MG regions. Soybean row width and seeding rate manipulation effects were highly variable. Narrow row width (≤ 38 cm) and a high seeding rate (470,000 seeds ha−1) reduced end-of-season height and fecundity variably across MG regions compared with wide row width (≥ 76 cm) and moderate to low (322,000 to 173,000 seeds ha−1) seeding rates. However, narrow row widths and high seeding rates did not reduce pigweed density at the POST herbicide application timing or at soybean harvest. Across all MG regions, soybean CIPAR increased as soybean row width decreased and seeding rate increased; however, row width and seeding rate had variable effects on soybean yield. Furthermore, soybean CIPAR was not associated with end-of-season pigweed growth and fecundity. A PRE fb POST herbicide strategy was a necessary component for an IWM program as it simultaneously managed pigweeds, increased soybean CIPAR, and increased grain yield. Nomenclature: Glufosinate; pigweed, Amaranthus spp.; soybean, Glycine max (L.) Merr. Las especies del género Amaranthus están entre las especies de malezas más abundantes y problemáticas en los sistemas de producción de soja en el medio oeste y el sur medio debido a sus características de crecimiento prolífico y su habilidad para evolucionar rápidamente resistencia a varios sitios de acción de herbicidas. Esto ha renovado el interés en la diversificación de estrategias de manejo de malezas implementando programas de manejo integrado de malezas (IWM) para manejar eficientemente a las malezas, que incluyan una mayor intercepción de luz por parte de la soja a la vez que se aumente el rendimiento de grano. Se realizaron estudios de campo a lo largo de 16 sitios-años para determinar la efectividad de la distancia entre hileras, densidad de siembra, y la estrategia de herbicidas, como componentes de un IWM en soja resistente a glufosinate. Los sitios fueron agrupados de acuerdo a las zonas óptimas de adaptación según los grupos de madurez (MGs) de la soja. Al promediar todas las regiones MG, la densidad y altura de Amaranthus, al momento de la aplicación POST del herbicida, y la densidad, la altura y la fecundidad de Amaranthus al final de la temporada, fueron reducidas en programas IWM que usaron una estrategia de herbicidas PRE seguidos por (fb) POST. Además, un tratamiento con una estrategia de herbicidas PRE fb POST aumentó la intercepción acumulativa de radiación fotosintéticamente activa (CIPAR) de la soja y subsecuentemente el rendimiento de grano de la soja al promediar todas las regiones MG. Los efectos de la distancia entre hileras y la densidad de siembra de la soja fueron altamente variables. Hileras angostas (≤ 38 cm) y una alta densidad de siembra (470,000 semillas ha−1) redujeron la altura y la fecundidad al final de la temporada en forma variable entre las regiones MG al compararse con hileras anchas (≥ 76 cm) y densidades de siembra de moderadas a bajas (322,000 a 173,000 semillas ha−1). Sin embargo, las hileras angostas y las altas densidades de siembra no redujeron la densidad de Amaranthus al momento de la aplicación de herbicida POST o al momento de la cosecha de la soja. Al promediar todas las regiones MG, la CIPAR de la soja aumentó al disminuir la distancia entre hileras e incrementar la densidad de siembra. Sin embargo, la distancia entre hileras y la densidad de siembra tuvieron efectos variables sobre el rendimiento de la soja. Adicionalmente, la CIPAR de la soja no estuvo asociada con el crecimiento ni la fecundidad de Amaranthus al final de la temporada. Una estrategia que use herbicidas PRE fb POST fue un componente necesario para que el programa IWM simultáneamente manejara malezas Amaranthus e incrementara la CIPAR de la soja y su rendimiento de grano.

Control of Glyphosate-Resistant Common Waterhemp (Amaranthus rudis) in Glufosinate-Resistant Soybean

Glyphosate-resistant (GR) common waterhemp has become a significant problem weed in Nebraska and several Midwestern states. Several populations of GR common waterhemp are also resistant to acetolactate synthase (ALS)-inhibiting herbicides, making them difficult to control with POST herbicides in GR soybean. Glufosinate-resistant (GFR) soybean is an alternate system for controlling GR common waterhemp, justifying the need for evaluating glufosinate-based herbicide programs. The objectives of this study were to compare POST-only herbicide programs (including one-pass and two-pass POST programs) with PRE followed by (fb) POST herbicide programs for control of GR common waterhemp in GFR soybean and their effect on common waterhemp density, biomass, and soybean yield. Field experiments were conducted in 2013 and 2014 near Fremont, NE in a growers field infested with GR common waterhemp. Glufosinate applied early- and late-POST provided 76% control of GR common waterhemp at 14 d after late-POST (DALPOST) compared with 93% control with a PRE fb POST program when averaged across treatments. The PRE application of chlorimuron plus thifensulfuron plus flumioxazin, S-metolachlor plus fomesafen or metribuzin, saflufenacil plus dimethenamid-P fb glufosinate provided ≥95% control of common waterhemp throughout the growing season, reduced common waterhemp density to ≤2.0 plants m−2, caused ≥94% biomass reduction, and led to 1,984 to 2,210 kg ha−1 soybean yield. Averaged across treatments, the PRE fb POST program provided 82% common waterhemp control at soybean harvest, reduced density to 23 plants m−2 at 14 DALPOST, and caused 86% biomass reduction and 1,803 kg ha−1 soybean yield compared with 77% control, 99 plants m−2, 53% biomass reduction, and 1,190 kg ha−1 yield with POST-only program. It is concluded that PRE fb POST programs with multiple effective modes of action are available for control of GR common waterhemp in GFR soybean. Nomenclature: Acetochlor; alachlor; cloransulam; chlorimuron; dimethenamid; flumioxazin; fomesafen; glufosinate; glyphosate; imazethapyr; metribuzin; saflufenacil; S-metolachlor; sulfentrazone; thifensulfuron; common waterhemp, Amaranthus rudis Sauer; soybean, Glycine max (L.) Merr. Amaranthus rudis resistente a glyphosate (GR) se ha convertido en un problema de malezas significativo en Nebraska y en varios estados del Medio Oeste. Varias poblaciones de A. rudis GR también son resistentes a herbicidas inhibidores de acetolactate synthase, lo que las hace difíciles de controlar con herbicidas POST en soja GR. Soja resistente a glufosinate es un sistema alternativo para el control de A. rudis GR, lo que justifica la necesidad de evaluar programas de herbicidas basados en glufosinate. Los objetivos de este estudio fueron comparar programas con sólo herbicidas POST (incluyendo programas POST con uno y dos pases) con programas de herbicidas PRE seguidos por (fb) POST para el control de A. rudis GR en soja GFR y sus efectos sobre la densidad y biomasa de A. rudis y el rendimiento de la soja. En 2013 y 2014, se realizaron experimentos de campo cerca de Fremont, Nebraska en un campo comercial infestado con A. rudis GR. Glufosinate aplicado en POST temprano y tardío brindó 76% de control de A. rudis GR a 14 d después del POST tardío (DALPOST), comparado con 93% de control con un programa PRE fb POST, cuando se promediaron los tratamientos. Las aplicaciones PRE de chlorimuron más thifensulturon más flumioxazin, S-metolachlor más fomesafen o metribuzin, saflufenacil más dimethenamid-P fb glufosinate brindaron ≥95% de control de A. rudis a lo largo de la temporada de crecimiento, redujeron la densidad de A. rudis a ≤2 plantas m−2, causaron ≥94% de reducción de biomasa, y permitieron un rendimiento de soja de 1,984 a 2,210 kg ha−1. Al promediarse los tratamientos, el programa PRE fb POST brindó 82% de control de A. rudis al momento de la cosecha, redujo la densidad a 23 plantas m−2 a 14 DALPOST, causó 86% de reducción de biomasa, y el rendimiento de la soja fue 1,803 kg ha−1, comparado con 77% de control, 99 plantas m−2, 53% de reducción de biomasa, y un rendimiento de 1,190 kg ha−1 con el programa de sólo herbicidas POST. Se concluyó que hay programas de herbicidas PRE fb POST disponibles con modos de acción efectivos para el control de A. rudis GR en soja GFR.

Influence of Tillage on Common Ragweed (Ambrosia artemisiifolia) Emergence Pattern in Nebraska

Spring tillage is a component of an integrated weed management strategy for control of early emerging glyphosate-resistant weeds such as common ragweed; however, the effect of tillage on common ragweed emergence pattern is unknown. The objectives of this study were to evaluate whether spring tillage during emergence would influence the emergence pattern or stimulate additional emergence of common ragweed and to characterize common ragweed emergence in southeast Nebraska. A field experiment was conducted for three years (2014 to 2016) in Gage County, Nebraska in a field naturally infested with glyphosate-resistant common ragweed. Treatments consisted of a no-tillage control and three spring tillage timings. The Soil Temperature and Moisture Model (STM2) software was used to estimate soil temperature and moisture at a 2-cm depth. The Weibull function was fit to total common ragweed emergence (%) with day of year (DOY), thermal time, and hydrothermal time as independent variables. Tillage treatments and year had no effect on total common ragweed emergence (P = 0.88 and 0.35, respectively) and time to 10, 25, 50, 75, and 90% emergence (P = 0.31). However, emergence pattern was affected by year (P = <0.001) with 50% total emergence reached on May 5 in 2014, April 20 in 2015, and April 2 in 2016 and 90% total emergence reached on May 12, 2014, May 8, 2015, and April 30, 2016. According to the corrected information-theoretic model comparison criterion (AICc), the Weibull function with thermal time and base temperature of 3 C best explained the emergence pattern over three years. This study concludes that spring tillage does not stimulate additional emergence; therefore, after the majority of the common ragweed has emerged and before the crop has been planted, tillage could be used as an effective component of an integrated glyphosate-resistant common ragweed management program in Nebraska. Nomenclature: Glyphosate; common ragweed, Ambrosia artemisiifolia L.