Kevin W. Bradley

Reducing the Risks of Herbicide Resistance: Best Management Practices and Recommendations

Herbicides are the foundation of weed control in commercial crop-production systems. However, herbicide-resistant (HR) weed populations are evolving rapidly as a natural response to selection pressure imposed by modern agricultural management activities. Mitigating the evolution of herbicide resistance depends on reducing selection through diversification of weed control techniques, minimizing the spread of resistance genes and genotypes via pollen or propagule dispersal, and eliminating additions of weed seed to the soil seedbank. Effective deployment of such a multifaceted approach will require shifting from the current concept of basing weed management on single-year economic thresholds.

Glyphosate and Multiple Herbicide Resistance in Common Waterhemp (Amaranthus rudis) Populations from Missouri

Abstract Field and greenhouse experiments were conducted to determine the level of glyphosate resistance in common waterhemp populations from Platte County (MO1) and Holt County, Missouri (MO2), and to determine the level and distribution of resistance to glyphosate, acetolactate synthase (ALS)–inhibiting herbicides, and protoporophyrinogen oxidase (PPO)–inhibiting herbicides across the MO1 site. Results from greenhouse experiments revealed that the MO1 and MO2 waterhemp populations were 19 and 9 times more resistant to glyphosate, respectively, than a susceptible waterhemp population. In field experiments, greater than 54% of waterhemp at the MO1 site survived 1.7 kg glyphosate ae ha−1 (twice the labeled rate) 6 wk after treatment. Tank-mix combinations of ALS- and PPO-inhibiting herbicides with glyphosate also failed to provide complete control of the waterhemp population at the MO1 site. Collection and screening of seed from individual female waterhemp accessions revealed multiple resistance to glyphosate, ALS-, and PPO-inhibiting herbicides across the MO1 site. All 14 waterhemp accessions collected across the MO1 site exhibited greater than 65% survival to 2× rates of glyphosate and thifensulfuron, and these accessions were spread across a 5-km2 (503-ha) area. Four waterhemp accessions collected across a 0.9-km2 (87-ha) area also exhibited 26 to 38% survival to 2× rates of lactofen. The results from these experiments provide evidence and confirmation of the first glyphosate-resistant waterhemp population in the United States and reveal that multiple resistance to glyphosate, ALS-, and PPO-inhibiting herbicides can occur in waterhemp. Nomenclature: Glyphosate; lactofen; thifensulfuron; common waterhemp, Amaranthus rudis Sauer.

Influence of Herbicide Programs on Weed Management in Soybean with Resistance to Glufosinate and 2,4-D

Abstract A field experiment was conducted in Boone and Callaway counties in Missouri in 2010 and 2011 to investigate herbicide programs for the management of summer annual grass and broadleaf weeds in soybean resistant to 2,4-D and glufosinate. Results revealed that the addition of 0.56, 0.84, or 1.12 kg ha−1 2,4-D (amine) to either or both POST applications of glufosinate in a two-pass POST herbicide program increased control of common waterhemp compared to two POST applications of glufosinate alone. Similar levels of common cocklebur, giant foxtail, large crabgrass, and barnyardgrass control were achieved with any of the two-pass POST programs that contained 2,4-D compared to two-pass POST programs containing glufosinate alone. Similar control of these species was also achieved with the inclusion of 2,4-D in either the first or second pass of glufosinate. Two-pass programs resulted in the highest levels of weed control (90%). Annual grass and broadleaf weed control was as much as 59% lower when one-pass POST herbicide programs were compared to PRE followed by (fb) POST or two-pass POST programs. However, one-pass POST programs were sufficient to obtain exceptional control of common cocklebur and giant foxtail. Across all site years, soybean yields ranged from 2,680 to 3,100 kg ha−1 for all herbicide treatments, but did not differ statistically. Overall, results from these experiments indicate that compared to glufosinate alone, PRE fb POST or two-pass POST herbicide programs that incorporate 2,4-D amine with glufosinate in 2,4-D–resistant soybean enhance control of common waterhemp, while providing similar levels of control of other summer annual grass and broadleaf weeds. Nomenclature: 2,4-D; glufosinate barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG; common cocklebur, Xanthium strumarium L. XANST; common waterhemp, Amarathus rudis Sauer AMATA; giant foxtail, Setaria faberi Herrm. SETFA; large crabgrass, Digitaria sanguinalis (L.) Scop. DIGSA; soybean, Glycine max (L.) Merr. Resumen Se realizó un experimento de campo en los condados Boone y Callaway en Missouri en 2010 y 2011 para investigar programas de herbicidas para el manejo de malezas anuales de verano de hoja ancha y gramíneas en soya resistente a 2,4-D y glufosinate. Los resultados revelaron que la adición de 0.56, 0.84 ó 1.12 kg ha−1 2,4-D (amine) a cualquiera o ambas aplicaciones POST de glufosinate en un programa de herbicidas POST con dos pases incrementó el control de Amaranthus rudis comparado a dos aplicaciones POST de glufosinate solo. Niveles similares de control de Xanthium strumarium, Setaria faberi, Digitaria sanguinalis y Echinochloa crus-galli fueron alcanzados con cualquiera de los programas POST de dos pases que contenían 2,4-D al compararse con los programas POST de dos pases que contenían glufosinate solo. Un control similar de estas especies se alcanzó con la inclusión de 2,4-D en ya sea el primer o segundo pase con glufosinate. Los programas de dos pases resultaron en los niveles de control más altos (90%). El control de malezas anuales gramíneas y de hoja ancha fue hasta 59% menor cuando programas de herbicidas POST de un pase fueron comparados con programas PRE seguidos por (fb) POST o programas POST de dos pases. Sin embargo, programas POST de un pase fueron suficiente para obtener un control excepcional de X. strumarium y S. faberi. A través de todos los sitios-años, el rendimiento de la soya varió entre 2,680 y 3,100 kg ha−1 para todos los tratamientos con herbicidas, pero estos no difirieron estadísticamente. En general, los resultados de estos experimentos indican que al compararse con glufosinate solo, PRE fb POST o programas POST de dos pases que incorporan 2,4-D amine con glufosinate en soya resistente al 2,4-D mejoran el control de A. rudis, mientras brindan niveles similares de control de otras malezas anuales de hoja ancha y gramíneas.

Glyphosate-Resistant Waterhemp (Amaranthus Rudis) Control and Economic Returns With Herbicide Programs in Soybean

Abstract Field experiments were conducted in Platte County, Missouri, during 2006 and 2007 to evaluate PRE, POST, and PRE followed by (fb) POST herbicide programs for the control of glyphosate-resistant waterhemp in soybean. All PRE fb POST treatments resulted in at least 66 and 70% control of glyphosate-resistant waterhemp in 2006 and 2007, respectively. Control of glyphosate-resistant waterhemp was less than 23% with lactofen and acifluorfen in 2006, but at least 64% in 2007. Variability in control likely resulted from differences in trial locations and a population of protoporphyrinogen oxidase (PPO)–resistant waterhemp at the Platte County site in 2006 compared with 2007. In both years, glyphosate resulted in less than 23% control of glyphosate-resistant waterhemp and provided the least control of all herbicide programs. Programs containing PRE herbicides resulted in waterhemp densities of less than 5 plants/m2, whereas the POST glyphosate treatment resulted in 38 to 70 plants/m2. Waterhemp seed production was reduced at least 78% in all PRE fb POST programs, from 55 to 71% in POST programs containing lactofen and acifluorfen and by only 21% in the POST glyphosate treatment. Soybean yields corresponded to the level of waterhemp control achieved in both years, with the lowest yields resulting from programs that provided poorest waterhemp control. PRE applications of S-metolachlor plus metribuzin provided one of the highest net incomes in both years and resulted in

Wide Distribution of the Waterhemp (Amaranthus tuberculatus) ΔG210 PPX2 Mutation, which Confers Resistance to PPO-Inhibiting Herbicides

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Influence of Weed Height and Glufosinate plus 2,4-D Combinations on Weed Control in Soybean with Resistance to 2,4-D

340/ha greater net income than the glyphosate-only treatment. Collectively, the results from these experiments illustrate the effectiveness of PRE herbicides for the control of glyphosate-resistant waterhemp in glyphosate-resistant soybean and the inconsistency of PPO-inhibiting herbicides or PPO-inhibiting herbicide combinations for the control of waterhemp populations with multiple resistance to glyphosate and PPO-inhibiting herbicides. Nomenclature: Acifluorfen; glyphosate; lactofen; metribuzin; S-metolachlor; common waterhemp, Amaranthus rudis Sauer; soybean Glycine max L

A Multistate Study of the Association Between Glyphosate Resistance and EPSPS Gene Amplification in Waterhemp (Amaranthus tuberculatus)

Resistance in waterhemp to herbicides that inhibit protoporphyrinogen oxidase (PPO) previously was shown to result from the deletion of a glycine codon at position 210 (&Dgr;G210) in the PPO-encoding gene, PPX2. Research was conducted to determine if this same mechanism accounted for resistance in geographically separated populations—from Illinois, Kansas, and Missouri—and, if so, to determine if the mutation conferring resistance was independently selected. A dose–response study with lactofen indicated that the resistant populations had different levels of resistance. These differences, however, could be accounted for by different frequencies of resistant individuals within populations and, therefore, the dose–response data were consistent with the hypothesis that the populations contained the same resistance mechanism. Direct evidence in support of this hypothesis was provided by DNA sequencing, which showed that nearly all resistant plants evaluated contained the &Dgr;G210 mutation. A variable region of the PPX2 gene was sequenced and resulting sequences were aligned and organized into a phylogenetic tree. The phylogenetic tree did not reveal clear clustering by either geography or phenotype (resistant vs. sensitive). Possibly recombination within the PPX2 gene has masked its evolutionary history. Nomenclature: Common waterhemp, Amaranthus tuberculatus (Moq.) Sauer var. rudis (Sauer) Costea and Tardif AMATU.

Herbicide Program Approaches for Managing Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri) and Waterhemp (Amaranthus tuberculatus and Amaranthus rudis) in Future Soybean-Trait Technologies

Abstract The introduction of transgenic crops with resistance to 2,4-D will provide growers with new weed management options in soybean. Field and greenhouse experiments were conducted in 2010 and 2011 to determine the influence of weed height on compatibility of glufosinate and 2,4-D in resistant soybean. Results from these experiments indicate a significant increase in the rate of glufosinate or 2,4-D is needed when applied alone in order to achieve similar levels of weed control as lower rates of glufosinate and 2,4-D combinations. Mixtures of glufosinate and 2,4-D were more effective in controlling common waterhemp, Asiatic dayflower, and barnyardgrass than either herbicide alone, and with the magnitude of difference greater as the height of common waterhemp increased. Large crabgrass control decreased when glufosinate was applied with 2,4-D compared with glufosinate alone, with the magnitude of the reduction greater as weed height increased. Sequential application of glufosinate and 2,4-D eliminated the antagonism noted with tank mixtures. Soybean yields were also reduced by approximately 3% in response to 30 to 35–cm compared to 10 to 15–cm herbicide applications because of the combination of weed interference and reduced control. Overall, results from these experiments indicate that glufosinate plus 2,4-D combinations are likely to provide increased control of problematic weeds like Asiatic dayflower and common waterhemp while providing similar grass control as herbicide programs that contain glufosinate alone. Nomenclature: 2,4-D; glufosinate; Asiatic dayflower, Commelina communis L.; barnyardgrass, Echinochloa crus-galli (L.) Beauv.; common cocklebur, Xanthium strumarium L.; common waterhemp, Amaranthus rudis Sauer; giant foxtail, Setaria faberi Herrm.; ivyleaf morningglory, Ipomoea hederacea Jacq.; large crabgrass, Digitaria sanguinalis (L.) Scop.; soybean, Glycine max (L.) Merr. Resumen La introducción de cultivos transgénicos con resistencia a 2,4-D dará a los productores opciones nuevas de manejo en soya. Experimentos de campo y de invernadero fueron realizados en 2010 y 2011 para determinar la influencia de la altura de las malezas sobre la compatibilidad de glufosinate y 2,4-D en soya resistente. Los resultados de estos experimentos indican que cuando glufosinate o 2,4-D son aplicados individualmente, se necesita un aumento en la dosis para alcanzar niveles de control de malezas similares a dosis bajas de combinaciones de glufosinate y 2,4-D. Las mezclas de glufosinate y 2,4-D fueron más efectivas para el control de Amaranthus rudis, Commelina communis, y Echinochloa crus-galli que cualquiera de estos herbicidas solos, y la magnitud de esta diferencia aumentó cuando la altura de A. rudis incrementó. El control de Digitaria sanguinalis disminuyó cuando glufosinate fue aplicado con 2,4-D al compararse con glufosinate solo, con una mayor reducción del control al aumentar la altura de las malezas. Aplicaciones secuenciales de glufosinate y 2,4-D eliminaron el antagonismo observado con mezclas en tanque. También, los rendimientos de la soya se redujeron en aproximadamente 3% en respuesta a aplicaciones de herbicidas a 30 a 35 cm al compararse con 10 a 15 cm debido a la combinación de la interferencia de las malezas y el control reducido. En general, los resultados de estos experimentos indican que las combinaciones de glufosinate más 2,4-D pueden incrementar el control de malezas problemáticas como C. communis y A. rudis al tiempo que brindan un control de malezas gramíneas similar a los programas de herbicidas que contienen glufosinate solo.

Wheat (Triticum aestivum) Yield Response to Henbit (Lamium amplexicaule) Interference and Simulated Winterkill1

Abstract Waterhemp is an increasingly problematic weed in the U.S. Midwest, having now evolved resistances to herbicides from six different site-of-action groups. Glyphosate-resistant waterhemp in the Midwest is especially concerning given the economic importance of glyphosate in corn and soybean production. Amplification of the target-site gene, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) was found to be the mechanism of glyphosate resistance in Palmer amaranth, a species closely related to waterhemp. Here, the relationship between glyphosate resistance and EPSPS gene amplification in waterhemp was investigated. Glyphosate dose response studies were performed at field sites with glyphosate-resistant waterhemp in Illinois, Kansas, Kentucky, Missouri, and Nebraska, and relative EPSPS copy number of survivors was determined via quantitative real-time polymerase chain reaction (qPCR). Waterhemp control increased with increasing glyphosate rate at all locations, but no population was completely controlled even at the highest rate (3,360 g ae ha−1). EPSPS gene amplification was present in plants from four of five locations (Illinois, Kansas, Missouri, and Nebraska) and the proportion of plants with elevated copy number was generally higher in survivors from glyphosate-treated plots than in plants from the untreated control plots. Copy number magnitude varied by site, but an overall trend of increasing copy number with increasing rate was observed in populations with gene amplification, suggesting that waterhemp plants with more EPSPS copies are more resistant. Survivors from the Kentucky population did not have elevated EPSPS copy number. Instead, resistance in this population was attributed to the EPSPS Pro106Ser mutation. Results herein show a quantitative relationship between glyphosate resistance and EPSPS gene amplification in some waterhemp populations, while highlighting that other mechanisms also confer glyphosate resistance in waterhemp. Nomenclature: Glyphosate; common waterhemp, Amaranthus tuberculatus (Moq.) Sauer var. rudis (Sauer) Costea and Tardif; Palmer amaranth, Amaranthus palmeri S. Wats AMAPA; corn, Zea mays L.; soybean, Glycine max (L.) Merr.

Impact of Herbicides and Application Timing on Weed Control, Yield, and Nutritive Value of Tall Fescue Pastures and Hayfields

Herbicide-resistant Amaranthus spp. continue to cause management difficulties in soybean. New soybean technologies under development, including resistance to various combinations of glyphosate, glufosinate, dicamba, 2,4-D, isoxaflutole, and mesotrione, will make possible the use of additional herbicide sites of action in soybean than is currently available. When this research was conducted, these soybean traits were still regulated and testing herbicide programs with the appropriate soybean genetics in a single experiment was not feasible. Therefore, the effectiveness of various herbicide programs (PRE herbicides followed by POST herbicides) was evaluated in bare-ground experiments on glyphosate-resistant Palmer amaranth and glyphosate-resistant waterhemp (both tall and common) at locations in Arkansas, Illinois, Indiana, Missouri, Nebraska, and Tennessee. Twenty-five herbicide programs were evaluated; 5 of which were PRE herbicides only, 10 were PRE herbicides followed by POST herbicides 3 to 4 wks after (WA) the PRE application (EPOST), and 10 were PRE herbicides followed by POST herbicides 6 to 7 WA the PRE application (LPOST). Programs with EPOST herbicides provided 94% or greater control of Palmer amaranth and waterhemp at 3 to 4 WA the EPOST. Overall, programs with LPOST herbicides resulted in a period of weed emergence in which weeds would typically compete with a crop. Weeds were not completely controlled with the LPOST herbicides because weed sizes were larger (≥ 15 cm) compared with their sizes at the EPOST application (≤ 7 cm). Most programs with LPOST herbicides provided 80 to 95% control at 3 to 4 WA applied LPOST. Based on an orthogonal contrast, using a synthetic-auxin herbicide LPOST improves control of Palmer amaranth and waterhemp over programs not containing a synthetic-auxin LPOST. These results show herbicides that can be used in soybean and that contain auxinic- or HPPD-resistant traits will provide growers with an opportunity for better control of glyphosate-resistant Palmer amaranth and waterhemp over a wide range of geographies and environments. Nomenclature: 2,4-D; dicamba; glufosinate; glyphosate; HPPD, 4-hydroxyphenylpyruvate dioxygenase; isoxaflutole; and mesotrione; Palmer amaranth, Amaranthus palmeri S. Wats; waterhemp (tall and common, respectively), Amaranthus tuberculatus (Moq.) Sauer, and Amaranthus rudis Sauer; soybean, Glycine max (L.) Merr. Amaranthus spp. resistentes a herbicidas continúan causando problemas de manejo en soja. Nuevas tecnologías para soja que están actualmente en desarrollo y que incluyen resistencia a varias combinaciones de glyphosate, glufosinate, dicamba, 2,4-D, isoxaflutole, y mesotrione, harán posible el uso de sitios de acción que no están actualmente disponibles para uso en soja. Cuando se realizó esta investigación, estas tecnologías estaban todavía bajo regulación y la evaluación de programas de herbicidas con la genética apropiada de soja en un solo experimento no era factible. Por esto, se evaluó la efectividad de varios programas de herbicidas (herbicidas PRE seguidos de herbicidas POST) en experimentos con suelo desnudo en localidades en Arkansas, Illinois, Indiana, Missouri, Nebraska, y Tennessee que tenían Amaranthus palmeri resistente a glyphosate y Amaranthus rudis y Amaranthus tuberculatus resistentes a glyphosate. Se evaluaron 25 programas de herbicidas; 5 de los cuales fueron solamente herbicidas PRE, 10 fueron herbicidas PRE seguidos por herbicidas POST 3 a 4 semanas después (WA) de la aplicación PRE (EPOST), y 10 fueron herbicidas PRE seguidos por herbicidas POST 6 a 7 WA de la aplicación PRE (LPOST). Los programas con herbicidas EPOST brindaron 94% de control o más de A. palmeri, A. tuberculatus, y A. rudis a 3 a 4 WA de la aplicación EPOST. En general, los programas con herbicidas LPOST resultaron en un período de emergencia de malezas en el cual las malezas típicamente competirían con el cultivo. Las malezas no fueron controladas completamente con los herbicidas LPOST porque el tamaño de las malezas fue mayor (≥15 cm) al compararse con su tamaño en la aplicación EPOST (≥7 cm). La mayoría de los programas con herbicidas LPOST brindaron 80 a 95% de control a 3 a 4 WA de la aplicación LPOST. Con base en un contraste ortogonal, el usar un herbicida de tipo auxina sintética LPOST mejoró el control de A. palmeri, A. rudis, y A. tuberculatus al compararse con programas que no contenían un herbicida de tipo auxina sintética LPOST. Estos resultados muestran que los herbicidas que pueden ser usados en soja resistente a herbicidas de tipo auxina o HPPD brindarán a los productores una oportunidad para controlar mejor A. palmeri, A. rudis, y A. tuberculatus resistentes a glyphosate, en un amplio rango de ambientes y geografías.