Incidence of Herbicide Resistance, Seedling Emergence, and Seed Persistence of Smooth Barley (Hordeum glaucum) in South Australia

Smooth barley has emerged as a problematic weed in cereal crops of South Australia. After the recent reports of herbicide resistance and increase in seed dormancy in smooth barley, it was considered important to determine the herbicide resistance status and seedbank behavior of field populations of this weed species. A field survey was undertaken in the Upper North and Eyre Peninsula regions of South Australia in October 2012. Of the 90 smooth barley populations screened for resistance to quizalofop, 15% exhibited some level of resistance and 85% were susceptible. Resistance to acetolactate synthase (ALS)-inhibiting herbicides was low, with only 3 and 12% of populations classified as developing resistance to imazamox + imazapyr and sulfosulfuron, respectively. No multiple resistance patterns were observed; however, two ALS-inhibiting herbicide-resistant populations had sulfonylurea-to-imidazolinone cross-resistance. At the start of the growing season, the majority of smooth barley populations emerged rapidly (median 50% time to emergence [T50] = 8 d). In contrast, some populations of smooth barley displayed an extremely slow emergence pattern, with T50 of > 20 d. No direct linkage between seed dormancy and herbicide resistance was observed. However, two acetyl coenzyme A carboxylase-inhibiting herbicide-resistant populations were highly dormant and exhibited delayed emergence. The majority of smooth barley populations showed low-level or no seedbank persistence, but a few populations persisted for 1 yr. However, some weed populations had up to 20% seedbank persistence from 1 yr to the next. Overall there was a strong negative relationship between smooth barley seedling emergence and the level of seed persistence (R2 = 0.84, P < 0.05). This association indicated that greater seed dormancy could be responsible for extended persistence of the seedbank of this weed species. The study provides valuable insights into the general pattern of herbicide resistance and the behavior of the seedbank of smooth barley populations on South Australian farms. Nomenclature: Imazamox + imazapyr; quizalofop; sulfosulfuron; smooth barley, Hordeum glaucum (Steud.) Tzvelev. Hordeum glaucum ha emergido como una maleza problemática en los cultivos de cereales en el Sur de Australia. Después de reportes recientes de resistencia a herbicidas y el incremento en la dormancia de la semilla en H. glaucum, se consideró importante determinar el estatus de la resistencia a herbicidas y el comportamiento del banco de semillas de poblaciones de campo de esta especie. Se realizó un estudio observacional de campo en las regiones Alta Norte y de la península Eyre en el Sur de Australia, en Octubre 2012. De las 90 poblaciones de H. glaucum evaluadas por resistencia a quizalofop, 14% exhibieron algún nivel de resistencia y 86% fueron susceptibles. La resistencia a herbicidas inhibidores de acetolactate synthase (ALS) fue baja, ya que solamente 3 y 12% de las poblaciones fueron clasificadas como desarrollando resistencia a imazamox + imazapyr y sulfosulfuron, respectivamente. No se observó ningún patrón de resistencia múltiple. Sin embargo, dos poblaciones resistentes a herbicidas inhibidores de ALS tuvieron resistencia cruzada de sulfonylurea a imidazolinone. Al inicio de la temporada de crecimiento, la mayoría de las poblaciones de H. glaucum emergieron rápidamente (mediana del tiempo de 50% de emergencia [T50] = 8 d). En contraste, algunas poblaciones de H. glaucum mostraron un patrón de emergencia extremadamente tardío, con T50 de > 20 d. No se observó ninguna relación directa entre la dormancia de la semilla y la resistencia a herbicidas. Sin embargo, dos poblaciones resistentes a herbicidas inhibidores de acetyl coenzyme A carboxylase tuvieron una alta dormancia y exhibieron un retraso en la emergencia. La mayoría de las poblaciones de H. glaucum mostraron de bajo a ninguna persistencia del banco de semillas, pero algunas poblaciones persistieron por 1 año. Sin embargo, algunas poblaciones tuvieron hasta 20% de persistencia del banco de semillas de un año al otro. En general, hubo una fuerte relación negativa entre la emergencia de plántulas de H. glaucum y el nivel de persistencia de la semillas (R2 = 0.84, P < 0.05). Esta asociación indicó que una mayor dormancia de la semilla podría ser responsable por la persistencia extendida del banco de semillas de esta especie de maleza. Este estudio brinda una observación valiosa sobre el patrón general de resistencia a herbicida y el comportamiento del banco de semillas de poblaciones de H. glaucum en fincas del Sur de Australia.

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