Seedbank change following adoption of integrated weed management (IWM) for a mixed population of ACCase-resistant and -susceptible green foxtail (Setaria viridis) in a rotation experiment

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McLennan, Deanna
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The germinable seedbank of a rotation experiment was characterized in the spring of 2017 to determine the response of an ACCase-resistant green foxtail (Setaria viridis (L.) Beauv.) biotype and the total weed community to the crop phases and herbicide omission treatments applied from 2000-2016. Three herbicide omission treatments; herbicide omission during one crop phase, or two crop phases, and no herbicide omissions, were applied to two four-year crop rotations that consisted of annual and annual/perennial crop phases. All herbicide omission treatments in the annual (annual crop phases only) and annual/perennial (annual and perennial crop phases) rotations had the same average total seedbank density. In both rotations, species richness and non-foxtail weed species densities were greater when herbicides were omitted in two years of the annual and annual/perennial rotations. Yellow foxtail (Setaria pumila (L.) Beauv.) and green foxtail were the most abundant species in the seedbank. Yellow foxtail densities were reduced by 50% in the annual rotation, and 80% in the annual/perennial rotation when regular in-crop herbicide applications were made compared with herbicide omissions in two crop phases. Increased in-crop ACCase inhibitor usage from 1/4 years to 2/4 years resulted in 4X total and 17X greater ACCase-resistant green foxtail densities on average and a 52% larger proportion of ACCase-resistant green foxtail in the seedbank. Crop phases that were more competitive (e.g. canola) or strategies that minimized weed seed rain (e.g. cutting for hay) resulted in lower total and ACCase-resistant green foxtail densities. New crop phases and integrated weed management (IWM) treatments were introduced in the rotation experiment in the spring of 2017. The objective was to determine the immediate impact of the IWM treatments on the ACCase-resistant and -susceptible green foxtail biotypes, yellow foxtail, and all other weeds in the seedbank after one year and with in-crop measurements during the field season. The newly implemented IWM treatments did not affect crop yield or weed suppression in the 2017 soybean and canola crops but did reduce Setaria biomass and yield in wheat. After one year of IWM treatments, the IWM treatments in canola, wheat and soybean did not have a greater decrease in green foxtail, yellow foxtail or total seedbank densities than the Control treatment. The 2018 seedbank remained similar to the 2017 seedbank, green foxtail densities were greater where more ACCase inhibitor applications had been made and yellow foxtail densities were greater where more herbicide omissions had occurred. More than one year of the IWM treatments in the annual and annual/perennial rotations may be needed before the effect become evident in the seedbank.
Green foxtail, Setaria viridis, Integrated weed management (IWM), ACCase inhibitors