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Respray Requests on Custom-Applied, Glyphosate-Resistant Soybeans in Illinois: How Many and Why

  • Brian J. Schutte (a1), Aaron G. Hager (a2) and Adam S. Davis (a1)

If an herbicide application fails to control a targeted weed community sufficiently, farmers may try to eliminate surviving weeds with a follow-up application (hereafter “respray”). Despite the implications of resprays on the spread of herbicide-resistant weeds, respray frequencies and causal factors are poorly understood. A two-part survey of glyphosate-resistant soybean fields and custom application services was conducted in Illinois during 2005 and 2006 to determine the relative frequency of respray requests for postemergence glyphosate, and to identify weed community factors associated with glyphosate respray requests. A meta-analysis was then utilized to project the impacts of weed community factors driving respray requests on crop yield. Glyphosate resprays were requested for 14% of surveyed fields in both 2005 (n = 43) and 2006 (n = 90). In 2005, respray requests were highly associated with both population densities of weed communities visible from roadsides and incidences of skips (i.e., rectangular areas of escaped weeds indicating custom application failure). A skip increased the odds of respray request by more than ninefold, and population densities of weed communities visible from roadsides were, on average, 2.5 times greater in respray-requested fields compared with nonrequested fields. In 2006, respray requests were associated with population densities of weed communities identified by walking through fields. Contrary to 2005, requests in 2006 were concentrated in those fields with low weed population densities. Prior to resprays, weed communities capable of causing substantial soybean yield loss were present in both respray-requested and nonrequested fields in 2005 but in only nonrequested fields in 2006. Although this investigation indicated that custom applicators can take actions to reduce respray requests (i.e., avoiding skips), farmers and custom applicators should be prepared to implement additional weed control after postemergence glyphosate applications because damaging weed communities may remain.

Si la aplicación de un herbicida no funciona para controlar suficientemente una comunidad de malezas específica, los agricultores quizás traten de eliminar las malezas sobrevivientes con una aplicación subsecuente (de ahora en adelante re-aplicación). A pesar de las implicaciones de la re-aplicación en la propagación de malezas resistentes al herbicida, las frecuencias de las aplicaciones y de sus factores causales son poco comprendidos. Una encuesta de dos partes de cultivos resistentes al glifosato y de los servicios de aplicación personalizada fue llevada al cabo en Illinois durante 2005 y 2006 para: (1) Determinar la frecuencia relativa de solicitudes de re-aplicación para el glifosato post-emergente y (2) identificar los factores de las comunidades de malezas asociados con las solicitudes de re-aplicación de glifosato. Después, se utilizó un meta-análisis para proyectar los impactos en el rendimiento del cultivo de los factores de la comunidad de malezas que incentivaran las solicitudes de re-aplicación. Las re-aplicaciones de glifosato se solicitaron para el 14% de los campos encuestados tanto en 2005 (n = 43) como en 2006 (n = 90). En el 2005 las solicitudes de re-aplicación se relacionaron en alto grado tanto con las densidades de población de las comunidades de malezas visibles desde la orilla de los caminos como con existencia de “manchones” (o sea áreas rectangulares de malezas que no fueron alcanzadas por la primera aplicación del herbicida). Uno de estos “manchones” incrementó más de 9 veces las probabilidades de que hubiera una solicitud de re-aplicación y en promedio las densidades de población de las comunidades de malezas visibles desde la orilla de los caminos fueron 2.5 veces mayores en campos donde se solicitó la re-aplicación, comparados con los que no la solicitaron. En 2006, las solicitudes de re-aplicación fueron relacionadas con las densidades de población de las comunidades de malezas que fueron identificadas caminando a través de los campos. Contrario al 2005, las solicitudes en 2006 se concentraron en campos con bajas densidades de población de malezas. Anterior a las re-aplicaciones, se encontraron en 2005 comunidades de malezas capaces de causar una pérdida sustancial en el rendimiento de la soya, tanto en campos que solicitaron la re-aplicación como los que no. Sin embargo, en 2006 dichas comunidades solamente estuvieron presentes en los campos donde no se solicitó. Aunque esta encuesta indicó que los aplicadores de herbicidas pueden tomar acciones para reducir las solicitudes de re-aplicación (o sea, evitando los “manchones”), los agricultores y los aplicadores de herbicidas deben estar preparados para implementar controles adicionales de malezas posteriores a las aplicaciones post-emergentes de glifosato ya que algunas comunidades de malezas dañinas pueden persistir.

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