Performance Story: Mitigating Herbicide Resistance - Investigating Novel Integrated Weed Management Systems
Overall there is some concern that the integrated weed management techniques had limited impact on the weed populations when herbicides were not used, particularly on the grass weeds. However, we were able to show that it is possible to grow crops without using herbicides for three years and maintain wild oat, and wild buckwheat populations at a statistically similar level to a canola-wheat full herbicide rotation. It was necessary to include silage barley and a competitive winter cereal, at increased seeding rates in those rotations for success. A diversified spring annual rotation with increased seeding rates was not enough to maintain those populations. One caveat is that in this study winter wheat is essentially being considered a weakly competitive winter cereal – this is due to high levels of winter kill in our study. Winter wheat is known to provide excellent levels of competition as well, but the crop needs to survive well to provide that competition which was a challenge in our plots. Research to increase winter survivability of winter wheat would aid in utilization of this crop as a weed management strategy. As it is there is a lot of inherent risk into incorporating winter wheat as a weed management strategy when the crop may not survive the winter.
Effects of the IWM strategies on broadleaves were more limited, particularly for twining weeds such as wild buckwheat. This is an area of significant concern, particularly for weeds like cleavers which do seem to be increasing in frequency and prevalence of herbicide resistance with somewhat limited herbicide options. Additional IWM strategies need to be developed for these weeds in the near future.
Across locations crop yield was increased in the final year when it followed 3 years of a perennial forage, and or had chaff collection added to the rotation. This indicates likely the double benefit of alfalfa as a perennial (weed management and residual nitrogen), but also indicates the potential efficacy of adding chaff collection to a effective herbicide management strategy.
We do see indications in our results that impact of IWM strategies is mediated or governed by the density of the weed population being dealt with. Higher densities showed limited improvements in weed populations or seedbanks when IWM was incorporated, but merely maintenance of the density, versus lower density populations where the weed seedbank was actually reduced.
Overall, this project clearly demonstrates benefits to diversity in IWM tactics studied, and the ability to eliminate herbicide application for three years although it may require significant compromises to the desired rotation. Lower responsiveness from twining weeds like wild buckwheat and cleavers illustrate a gap in our current IWM strategies based on the growth morphology of these weeds. In addition there is a need to understand the lack of impact of perennials and silaging on specific weed species and their weed seedbank densities. Impacts of some of the strategies like chaff collection and their impact on nutrient cycling and potentially greenhouse gas emissions also require further investigation. The IWM systems studied to date are not a silver bullet or the solution to herbicide resistance. They can aid in management of the weeds, but increasing diversity in management strategy early, while herbicides are still effective, is crucial to maintaining control of our weed populations long term. This study has added to our understanding of these strategies’ effectiveness, but have also highlighted a number of gaps and needed future research to best implement these strategies in western Canadian agriculture.