Interactions between crop sequences, weed populations and herbicide use in Western Australian broadacre farms: findings of a six-year survey

Abstract. Six years of survey data taken from 184 paddocks spanning 14 million ha of land used for crop and pasture production in south-west Western Australia were used to assess weed populations, herbicide resistance, integrated weed management (IWM) actions and herbicide use patterns in a dryland agricultural system. Key findings were that weed density within crops was low, with 72% of cropping paddocks containing fewer than 10 grass weeds/m2 at anthesis. Weed density and herbicide resistance were not correlated, despite the most abundant grass weed species (annual ryegrass, Lolium rigidum Gaudin) testing positive for resistance to at least one herbicide chemistry in 92% of monitored paddocks. A wide range of herbicides were used (369 unique combinations) suggesting that the diversity of herbicide modes of action may be beneficial for reducing further development of herbicide resistance. However, there was a heavy reliance on glyphosate, the most commonly applied active ingredient. Of concern, in respect to the evolution of glyphosate resistant weeds, was that 45% of glyphosate applications to canola were applied as a single active ingredient and area sown to canola in Western Australia expanded from 0.4 to 1.4 million hectares from 2005 to 2015. In order to minimise the weed seed bank within crops, pastures were used infrequently in some regions and in 50% of cases pastures were actively managed to reduce weed seed set, by applying a non-selective herbicide in spring. The use of non-selective herbicides in this manner also kills pasture plants, consequently self-regenerating pastures were sparse and contained few legumes where cropping intensity was high. Overall, the study indicated that land use selection and utilisation of associated weed management actions were being used successfully to control weeds within the survey area. However, to successfully manage herbicide resistant weeds land use has become less diverse, with pastures utilised less and crops with efficacious weed control options utilised more. Further consideration needs to be given to the impacts of these changes in land use on other production factors, such as soil nutrient status and plant pathogens to assess sustainability of these weed management practices in a wider context.