Amit J. Jhala

Flax (Linum usitatissimum L.)

Flax is a temperate industrial oilseed crop grown primarily in Canada, China, and Russia. Flax is a diploid, autogamous species, and breeding follows traditional methods, enhanced by mutation breeding and the use of genetic markers. Recently available flax genomic resources may hasten the achievement of breeding objectives: increased yield, shorter time to mature, disease resistance, and seed oil quality. Flax oil is unique because it is contains up to 64% α-linolenic acid (ALA). ALA polymerizes rapidly with exposure to oxygen and is therefore useful in varnishes, inks, linoleum, and other traditional industrial applications. ALA is also a metabolic precursor to ω-3 polyunsaturated fatty acids (PUFAs), which have positive effects with respect to cardiovascular health and inflammatory diseases, as well as anticancer properties. In addition to ALS, flax contains antioxidants and phytosterols that may increase health benefits. Flax is being used as a functional food ingredient for humans and animal feed to increase the ω-3 fatty acids in eggs and meat. Considerable progress has been made in understanding and enhancing the metabolic pathways leading to ALA and PUFA synthesis in flax. Further research investment in this niche crop will increase the scope of utilization for industrial, food and feed oil, and fiber byproducts.

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.

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.

Factors Affecting the Germination of Tall Morningglory (Ipomoea purpurea)

Abstract Tall morningglory is an annual broadleaf vine and a problem weed in many annual and perennial crops in several countries including the United States. A better understanding of the germination biology of tall morningglory would facilitate the development of better control strategies for this weed. Experiments were conducted under greenhouse and laboratory conditions to evaluate the effects of various environmental factors, such as temperature, light, planting depth, pH, osmotic and salt stress, and flooding duration, on the germination of tall morningglory. The results suggested that the optimum day/night temperature range for the germination of tall morningglory was 20/12.5 to 35/25 C and maximum germination (89%) was observed at 30/20 C. Temperature higher and lower than the optimum range significantly reduced germination. Alternate light and dark did not have any adverse effect on the germination of tall morningglory seeds. The germination was 10% at an osmotic stress of −0.3 and −0.4 MPa, and above that, no germination was observed. Tall morningglory showed some tolerance to salt stress. The germination was 40% and 12% at salt concentrations of 50 mM and 200 mM, respectively. Germination was affected by pH levels, and maximum germination occurred at pH 6, whereas above or below that level, germination was significantly reduced. Maximum germination of seeds was 83 and 94% when sown at 0 and 2 cm depth in soil, within a week of sowing; however, germination was significantly reduced to 76% when placed at a depth of 4 cm or deeper. Under no flooding treatment, 87% of seed germinated, but flooding delayed and inhibited the germination of tall morningglory seeds. It is concluded that several environmental factors affected the germination of tall morningglory, and this information could help to predict the spread of tall morningglory in new areas such as Florida. Nomenclature: Tall morningglory, Ipomoea purpurea (L.) Roth, PHBPU.

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.

Effect of Water Stress on the Growth and Fecundity of Common Waterhemp (Amaranthus rudis)

Abstract Common waterhemp is one of the most commonly encountered and troublesome weeds in the midwestern United States. It is well known that water stress adversely affects crop growth and yield; however, the effects of water stress on weed growth and seed production are poorly understood. The objective of this study was to determine the effects of degree and duration of water stress on growth, development, and fecundity of two common waterhemp biotypes in greenhouse experiments conducted at the University of Nebraska–Lincoln. No difference was observed in growth, development, and seed production between two biotypes in response to degree and duration of water stress; therefore, data were combined. The degree of water stress study included five treatments, where the amount of water applied to each pot at 2-d interval was equivalent to 100, 75, 50, 25, and 12.5% of pot (soil) water content. The highest plant height (163 cm), number of leaves (231 plant−1), and growth index (4.4 × 105 cm3) were recorded at 100% of pot water content (no water stress). Similarly, aboveground biomass, total leaf area, and seed production reached their maximum at 100% of pot water content treatment, whereas they were reduced as degree of water stress increased. The study of water stress duration included five treatments, where amount of water applied to each pot at 2-, 4-, 6-, 8-, and 10-d intervals was equivalent to 100% of pot water content. The highest plant height (150 cm), number of leaves (210 plant−1), and growth index (3.8 × 105 cm3) were observed at 2-d interval of water stress, whereas seed production was similar at 2-d (36,549 seeds plant−1) and 4-d (34,176 seeds plant−1) intervals. This study shows that common waterhemp has capacity to survive and reproduce even under a higher degree and duration of water stress. Nomenclature: Common waterhemp; Amaranthus rudis Sauer.

Herbicide Programs for Control of Glyphosate-Resistant Volunteer Corn in Glufosinate-Resistant Soybean

Abstract Glyphosate-resistant (GR) volunteer corn is a significant problem weed in soybean grown in rotation with corn in the midwestern United States and eastern Canada. The objective of this study was to evaluate the efficacy of glufosinate applied in single or sequential applications compared with acetyl-coenzyme A carboxylase (ACCase) inhibitors applied alone or tank mixed with glufosinate for controlling GR volunteer corn in glufosinate-resistant soybean. At 15 d after early-POST (DAEP), ACCase inhibitors applied alone controlled volunteer corn 76 to 93% compared to 71 to 82% control when tank mixed with glufosinate. The expected volunteer corn control achieved by tank mixing ACCase inhibitors and glufosinate was greater than the glufosinate alone, indicating that glufosinate antagonized ACCase inhibitors at 15 DAEP, but not at later rating dates. ACCase inhibitors applied alone or tank mixed with glufosinate followed by late-POST glufosinate application controlled volunteer corn and green foxtail ≥ 97% at 30 DAEP. Single early-POST application of glufosinate controlled common waterhemp and volunteer corn 53 to 78%, and green foxtail 72 to 93% at 15 DAEP. Single as well as sequential glufosinate applications controlled green foxtail and volunteer corn greater than or equal to 90%, and common waterhemp greater than 85% at 75 d after late-POST (DALP). Contrast analysis suggested that glufosinate applied sequentially provided greater control of volunteer corn at 15 and 75 DALP compared to a single application. Similar results were reflected in volunteer corn density and biomass at 75 DALP. Volunteer corn interference did not affect soybean yield, partly because of extreme weather conditions (hail and high winds) in both years of this study. Nomenclature: Clethodim; fenoxaprop-P; fluazifop-P; glufosinate; quizalofop-P; sethoxydim; common waterhemp, Amaranthus rudis Sauer; green foxtail, Setaria viridis (L.) Beauv.; soybean, Glycine max (L.) Merr.; volunteer corn, Zea mays L. Resumen El maíz voluntario resistente a glyphosate (GR) es un problema significativo de malezas en soja producida en rotación con maíz en el centro oeste de los Estados Unidos y en el este de Canadá. El objetivo de este estudio fue evaluar la eficacia de glufosinate aplicado solo o en aplicaciones secuenciales comparado con inhibidores de acetyl-coenzyme A carboxylase (ACCase) aplicados solos o en mezclas en tanque con glufosinate para el control de maíz GR voluntario en soja resistente a glufosinate. A 15 d después de la aplicación POST temprana (DAEP), los inhibidores de ACCase aplicados solos controlaron el maíz voluntario 76 a 93% comparado con 71 a 82% de control con la mezcla en tanque con glufosinate. El control esperado de maíz voluntario con las mezclas en tanque con ACCase y glufosinate fue mayor que el de glufosinate solo, lo que indicó que glufosinate antagonizó a los inhibidores de ACCase a 15 DAEP, pero no en fechas de evaluación posteriores. Los inhibidores de ACCase aplicados solos o en mezclas en tanque con glufosinate seguidos de aplicaciones tardías POST de glufosinate controlaron el maíz voluntario y Setaria viridis ≥ 97% a 30 DAEP. Aplicaciones POST tempranas de glufosinate solo controlaron Amaranthus rudis y maíz voluntario 53 a 78%, y S. viridis 72 a 93% a 15 DAEP. Aplicaciones solas y secuenciales de glufosinate controlaron S. viridis y maíz voluntario en 90% o más, y A. rudis más de 85% a 75 d después de la aplicación POST tardía (DALP). Análisis de contrastes sugirieron que glufosinate aplicado secuencialmente brindó mayor control del maíz voluntario a 15 y 75 DALP al compararse con una única aplicación. Resultados similares fueron observados en la densidad y biomasa del maíz voluntario a 75 DALP. La interferencia del maíz voluntario no afectó el rendimiento de la soja, parcialmente porque se presentaron condiciones extremas del estado del tiempo (granizo y vientos fuertes) en los dos años de 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.

Comparison of Glufosinate-Based Herbicide Programs for Broad-Spectrum Weed Control in Glufosinate-Resistant Soybean

Abstract Because of the increasing number of glyphosate-resistant weeds, alternate herbicide-resistant crops and herbicides with different modes of action are required to protect crop yield. Glufosinate is a broad-spectrum POST herbicide for weed control in glufosinate-resistant crops, including soybean. The objective of this study was to compare herbicide programs with glufosinate applied singly at late-POST (LPOST) or sequentially at early POST (EPOST) followed by (fb) LPOST applications and PRE herbicides fb EPOST/LPOST glufosinate alone or tank-mixed with acetochlor, pyroxasulfone, or S-metolachlor in glufosinate-resistant soybean. A field experiment was conducted at the South Central Agriculture Laboratory in Clay Center, NE, in 2012 and 2013. Glufosinate applied in a single LPOST or sequential EPOST fb LPOST application controlled common lambsquarters, common waterhemp, eastern black nightshade, green foxtail, large crabgrass, and velvetleaf ≤ 82% and resulted in a weed density of 6 to 10 plants m−2 by the end of the season. Flumioxazin-, saflufenacil-, or sulfentrazone-based premixes provided 84 to 99% control of broadleaf and grass weeds tested in this study at 15 d after PRE application and a subsequent LPOST application of glufosinate alone controlled broadleaf and grass weeds 69 to 93% at harvest, depending on the herbicide program and weed species being investigated. The PRE application of sulfentrazone plus metribuzin fb EPOST glufosinate tank-mixed with acetochlor, pyroxasulfone, or S-metolachlor controlled the tested broadleaf and grass weeds ≥ 90%, reduced density to ≤ 2 plants m−2, and reduced weed biomass to ≤ 10 g m−2 and produced soybean yields of ≥ 4,450 and 3,040 kg ha−1 in 2012 and 2013, respectively. Soybean injury was 0 to 20% from PRE or POST herbicides, or both and was inconsistent, but transient, during the 2-yr study, and it did not affect soybean yield. Sulfentrazone plus metribuzin applied PRE fb glufosinate EPOST tank-mixed with acetochlor, pyroxasulfone, or S-metolachlor provided the highest level of weed control throughout the growing season and increased soybean yield compared with a single LPOST or a sequential EPOST fb LPOST glufosinate application. Additionally, these herbicide programs provide four distinct mechanisms of action that constitute an effective weed-resistance management strategy in glufosinate-resistant soybean. Nomenclature: Acetochlor; flumioxazin; glufosinate; metribuzin; pyroxasulfone; saflufenacil; S-metolachlor; sulfentrazone; common lambsquarters, Chenopodium album L.; common waterhemp, Amaranthus rudis Sauer; eastern black nightshade, Solanum ptychanthum Dunal; green foxtail, Setaria viridis (L.) Beauv.; large crabgrass, Digitaria sanguinalis (L.) Scop.; velvetleaf, Abutilon theophrasti Medik; soybean, Glycine max (L.) Merr. Resumen Debido al creciente número de malezas resistentes a glyphosate, es necesario alternar cultivos resistente a herbicidas con diferentes modos de acción para proteger los rendimientos de los cultivos. Glufosinate es un herbicida POST de amplio espectro para el control de malezas en cultivos resistentes a glufosinate, incluyendo soja. El objetivo de este estudio fue comparar programas de herbicidas con glufosinate aplicado solo en POST-tarde (LPOST), o secuencialmente en POST-temprano (EPOST) seguido de (fb) aplicaciones LPOST, y herbicidas PRE fb glufosinate solo en EPOST/LPOST, o mezclas en tanque con acetochlor, pyroxasulfone, o S-metolachlor, en soja resistente a glufosinate. Se realizó un experimento de campo en el Laboratorio de Agricultura del Centro Sur, en Clay Center, Nebraska, en 2012 y 2013. Glufosinate aplicado solo LPOST o en secuencia EPOST fb LPOST controló Chenopodium album, Amaranthus rudis, Solanum ptychanthum, Setaria viridis, Digitaria sanguinalis, y Abutilon theophrasti ≤ 82% y resultaron en una densidad de malezas de 6 a 10 plantas m−2 al final de la temporada. Premezclas basadas en flumioxazin, saflufenacil, o sulfentrazone brindaron 84 a 99% de control de malezas de hoja ancha y gramíneas evaluadas en este estudio a 15 d después de la aplicación; PRE fb glufosinate solo (EPOST/LPOST) controlaron malezas de hoja ancha y gramíneas 69 a 93% al momento de la cosecha, dependiendo del programa de herbicidas y las especies de malezas investigadas. La aplicación PRE de sulfentrazone más metribuzin fb EPOST con glufosinate mezclado en tanque con acetochlor, pyroxasulfone, o S-metolachlor controló las especies de malezas de hoja ancha y gramíneas evaluadas ≥ 90%, redujo la densidad ≤ 2 plantas m−2, redujo la biomasa de malezas ≤ 10 g m−2, y produjo rendimientos de soja ≥ 4,450 y 3,040 kg ha−1, en 2012 y 2013, respectivamente. El daño en la soja fue 0 a 20% en los tratamientos PRE, POST, o ambos, y fue inconsistente pero fue transitorio, durante los 2 años del estudio, y no afectó el rendimiento de la soja. Sulfentrazone más metribuzin aplicados PRE fb glufosinate EPOST mezclado en tanque con acetochlor, pyroxasulfone, o S-metolachlor brindó el mayor nivel de control de malezas a lo largo de la temporada de crecimiento e incrementó el rendimiento de la soja al compararse con una aplicación de glufosinate LPOST o aplicaciones secuenciales EPOST fb EPOST. Adicionalmente, estos programas de herbicidas permitieron el uso de cuatro mecanismos de acción distintos lo que constituye una estrategia efectiva para el manejo de resistencia en soja resistente a glufosinate.

Confirmation and Control of HPPD-Inhibiting Herbicide–Resistant Waterhemp (Amaranthus tuberculatus) in Nebraska

Field and greenhouse experiments were conducted in Nebraska to (1) confirm the 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting resistant-waterhemp biotype (HPPD-RW) by quantifying the resistance levels in dose-response studies, and (2) to evaluate efficacy of PRE-only, POST-only, and PRE followed by POST herbicide programs for control of HPPD-RW in corn. Greenhouse dose-response studies confirmed that the suspected waterhemp biotype in Nebraska has evolved resistance to HPPD-inhibiting herbicides with a 2- to 18-fold resistance depending upon the type of HPPD-inhibiting herbicide being sprayed. Under field conditions, at 56 d after treatment, ≥90% control of the HPPD-RW was achieved with PRE-applied mesotrione/atrazine/S-metolachlor+acetochlor, pyroxasulfone (180 and 270 g ai ha−1), pyroxasulfone/fluthiacet-methyl/atrazine, and pyroxasulfone+saflufenacil+atrazine. Among POST-only herbicide programs, glyphosate, a premix of mesotrione/atrazine tank-mixed with diflufenzopyr/dicamba, or metribuzin, or glufosinate provided ≥92% HPPD-RW control. Herbicide combinations of different effective sites of action in mixtures provided ≥86% HPPD-RW control in PRE followed by POST herbicide programs. It is concluded that the suspected waterhemp biotype is resistant to HPPD-inhibiting herbicides and alternative herbicide programs are available for effective control in corn. The occurrence of HPPD-RW in Nebraska is significant because it limits the effectiveness of HPPD-inhibiting herbicides. Nomenclature Acetochlor, atrazine, glyphosate, clopyralid, dicamba, diflufenzopyr, dimethenamid-P, flumetsulam, fluthiacet-methyl, glufosinate, isoxaflutole, mesotrione, metribuzin, pyroxasulfone, S-metolachlor, saflufenacil, rimsulfuron, tembotrione, thiencarbazone-methyl, topramezone, waterhemp, Amaranthus tuberculatus (Moq.) Sauer, corn, Zea mays L. Se realizaron experimentos de campo y de invernadero en Nebraska para (1) confirmar un biotipo de Amaranthus tuberculatus resistente a inhibidores de 4-hydroxyphenylpyruvate dioxygenase (HPPD) (HPPD-RW) cuantificando el nivel de resistencia con estudios de respuesta a dosis, y (2) evaluar la eficacia de programas de herbicidas para el control de HPPD-RW en maíz con sólo herbicidas PRE, sólo POST, y herbicidas PRE seguidos por POST. Los estudios de respuesta a dosis en invernadero confirmaron que el biotipo de A. tuberculatus en Nebraska ha evolucionado resistencia a herbicidas inhibidores de HPPD con 2 a 18 veces mayor resistencia dependiendo del tipo de herbicida inhibidor de HPPD que se aplicó. Bajo condiciones de campo, a 56 d después del tratamiento, se alcanzó ≥90% de control de HPPD RW con aplicaciones PRE de mesotrione/atrazine/S-metolachlor + acetochlor, pyroxasulfone (180 y 270 g ai ha−1), pyroxasulfone/fluthiacet-methyl/atrazine, y pyroxasulfone + saflufenacil + atrazine. Entre los programas de herbicidas con sólo POST, glyphosate, una premezcla de mesotrione/atrazine mezclados en tanque con diflufenzopyr/dicamba, o metribuzin, o glufosinate brindaron ≥92% control de HPPD-RW. Combinaciones de herbicidas efectivos con diferentes sitios de acción en mezclas brindaron ≥86% de control de HPPD-RW en programas de herbicidas PRE seguidos por POST. Se concluyó que el biotipo de A. tuberculatus es resistente a herbicidas inhibidores de HPPD y que hay programas de herbicidas alternativos disponibles para su control efectivo en maíz. La ocurrencia de HPPD-RW en Nebraska es significativa porque limita la efectividad de herbicidas inhibidores de HPPD.