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.

Sustainability of nutrient management in grain production systems of south-west Australia

“…no legume for the duration of the survey), 35% 5 years, 22% 4 years, 14% 3 years, 9% 2 years, and 1% of fields with only 1 year without a legume over the duration of the study. These results are similar to industry level data (ABS 2016;Planfarm and Bankwest 2016); for more detail on land use see Harries et al (2020).…”

“…However, pasture legume biomass ranged widely from 2 to 7252 kg/ha such that the range of Nfix was 0-276 kg N/ha. The large variability in legume DM, and consequently Nfix, from self-regenerating pastures is to be expected as pasture years become less frequent and seed reserves of pastures are depleted by weed control actions undertaken throughout long cropping phases (Gill 1996;Heap 2000;Walsh and Powles 2014;Harries et al 2020). These findings reinforce reports of declining productivity of pastures on Australian dryland farms since the early 1990s (Heap 2000;Angus and Peoples 2012).…”

Are herbicide mixtures unaffected by resistance? A case study with Lolium rigidum

“…A very recent survey study, with samples collected across the whole Australian cropping system, has re‐confirmed a much lower (up to 10‐fold lower) frequency of resistance to herbicide mixtures in L. rigidum populations highly resistant to trifluralin (Busi R and Beckie HJ, unpublished). Recent field studies and historical field surveys have consistently indicated that herbicide mixtures and combinations of herbicide MOA are deliberately chosen by farmers to effectively diversify the inherent selection pressure of herbicidal weed control operations (Brunton et al ., 2020a; Harries et al ., 2020). The current use of PRE herbicides to control L. rigidum include at least five MOA, with a few more of recent introduction (e.g.…”

“…cinmethylin) or under development (e.g. aclonifen, bixlozone, carbetamide, trifludimoxazin) (Busi et al ., 2020a; Harries et al ., 2020). Recent studies indicate that the mechanistic genetic basis of resistance to PRE herbicides (e.g.…”

“…As the use of herbicide mixtures (pre-formulated and/or 'tank' mixtures) has become an increasingly common practice (i.e. the (Brunton et al, 2020a;Harries et al, 2020). The cation (Yuan et al, 2007;Busi et al, 2014;Yu and Powles, 2014;Brunton et al, 2020b).…”