Weed Suppressive Potential of Selected Pasture Legumes against Annual Weeds in Southeastern Australia

Poor adaptation of Trifolium subterraneum L. (subterranean clover) and Medicago sativa L. (lucerne) subterranean clover across southeastern Australia due to acidic soils, insufficient rainfall resulting in false breaks, high input costs, soil acidification and the emergence of herbicide resistant weeds have reduced establishment. To overcome these challenges, numerous novel pasture species have been released but limited knowledge exists regarding their competitive or allelopathic ability to suppress weeds. Field trials were therefore conducted at Wagga Wagga NSW over a three year period to evaluate (a) the suppressive potential of selected pasture legumes grown as monocultures and in mixed stands against volunteer weeds, and (b) the impact of summer (March) and winter (June) sowing dates on establishment. Weed and crop cover and biomass was measured from 2015 to 2017. Results suggest species mixtures with more than one pasture legume did not impact (P<0.05) establishment and subsequent weed infestation. However, summer sowing of arrowleaf clover, yellow serradella cv. Avila and French serradella/bladder clover generally resulted in increased (P<0.05) crop cover in all three growing seasons. Interestingly, arrowleaf clover and biserrula cv. Casbah produced significantly greater crop biomass and less weed biomass. Weed suppression and subsequent biomass was not always dependent on crop biomass however, as was the case for yellow serradella cv. Santorini over the period of 2016-2017. In summary, results have demonstrated that arrowleaf clover, biserrula cv. Casbah and yellow serradella cv. Santorini were the most reliable and competitive annual legumes with respective to yearly regeneration and consistent suppression of pasture weed species in the Riverina.