Field evaluation of Australian canola genotypes for in-crop and post-harvest weed suppression

In 2014 to 2016, replicated field trials were performed to evaluate mechanisms of weed suppression in selected Australian canola genotypes in moderate to low rainfall zones at Wagga Wagga and Condobolin NSW, respectively, in commercially managed paddocks. In 2015-16, a split plot design with and without trifluralin (Tri) as the main plot and cultivar as the sub plot was employed for canola trials; 8 cultivars including hybrid and open-pollinated cultivars were assessed. At each site, crop and weed growth were monitored at various phenological stages including early season, vegetative, grain-filling, harvest and post-harvest. Data collection focused upon those traits leading to both in-crop and potential post-harvest suppression of weeds.
Early vigour and ability to intercept light and accumulate biomass rapidly resulted in suppression of in-crop weed growth in canola trials, as well as post-harvest weed suppression associated with remaining crop residues. GT-50, Hyola 600RR and Hyola 50 were the most weed suppressive and consistently high yielding canola cultivars in both locations in all trial years. Cultivars CB Taurus and GT-50 provided greatest weed suppression when only residues remained in plots for 150 days post-harvest. Pre-emergence trifluralin treatment resulted in improved crop yields in contrast to untreated plots for most cultivars, but not all. In this case, these weed suppressive cultivars possessed rapid early crop growth and vigour and reduced light at the soil surface, potentially limiting weed growth in the absence of trifluralin.
Our results show that establishment of certain canola cultivars may effectively result in enhanced in-crop and post-harvest weed suppression, with or without the use of post-emergent herbicides during the growing season, especially when considering common spring and summer annual weeds which are problematic post-harvest. Therefore, canola cultivar choice may be an economical form of weed management due to competition by the crop and possibly other factors, such as production of allelochemicals by decomposing crop residues. Further investigation is now underway in Australian paddocks to determine the allelopathic mechanisms in canola, particularly the identification of allelochemical(s) associated with post-harvest weed suppression in field soils.