The weed suppressive ability of selected Australian grain crops; case studies from the Riverina region in New South Wales

Herbicide resistance in both grasses and broadleaf weeds is on the rise across Australia, with an increasing number of cropping weeds experiencing resistance to multiple herbicides. One contributing factor to this issue is the adoption of conservation agriculture (CA). CA is a system of residue management that avoids the use of cultivation for establishment of annual broadacre crops. Another contributing factor is poor management of herbicide mode of action strategies in broadacre farming. One key tool for integrated weed management (IWM) strategies is the use of competitive grain crop cultivars and post-harvest crop residues, which can effectively suppress or delay weed seedling emergence and provide an initial advantage for the crop in terms of early weed suppression. The ability of various dual-purpose grazing or non-grazing grain crops and their residues to suppress weeds until subsequent planting the following year was compared in two successive field experiments in the Riverina region of New South Wales (NSW), Australia. We evaluated 1) the impact of residues of several grain crops on winter and post-harvest summer annual weed establishment from 2012 to 2014 and 2) in-crop and post-harvest weed suppression in 2014–2015 using a genetically diverse set of canola cultivars, including those found to be highly weed-suppressive in the first trial. Replicated field trials were established in Wagga Wagga, in a moderate rainfall zone (mean 572 mm/year) from 2012 to 2015 using commercially available crop cultivars. Differences in in-crop weed infestation and in post-harvest crop fallows associated with grain crop cultivar and species were observed in each of three years. Significant weed suppression associated with grazing and non-grazing wheat residues was observed after harvest, with grazing wheat exhibiting significant suppression of fleabane and witchgrass up to 130 days post-harvest. Grazing and non-grazing canola provided strong and significant suppression of fleabane and witchgrass for up to 140 days following harvest. Grazing cereal cultivars were generally more suppressive of weeds than non-grazing cultivars. Early vigour and ability to intercept light and accumulate biomass resulted in suppression of in-crop weed growth in canola trials, with GT-50 the most weed suppressive canola cultivar. Weed biomass differed with cultivar in both years, and appeared to be inversely related to early crop vigour, suggesting the importance of crop biomass in regulating weed competition in the crop. Cultivars CB Taurus and GT-50 were consistently the most weed suppressive when residues remained in plots 150 days post-harvest. These results indicate that establishment of certain species and cultivars of grain crops may effectively suppress weed growth both in-crop and post-harvest, in the absence of post-emergent herbicides. In addition, the choice of canola cultivar for desired weed suppression impacts the subsequent ability of the crop and its residues to successfully interfere with weed growth.