Weed suppression by crop competition in three crop species in Western Australia.

To examine the competitive ability of crop cultivars to suppress weeds in barley, canola and wheat crops, four field experiments were conducted in two locations within the Western Australian (WA) wheatbelt during 2008 and 2009. In 2008, four cultivars of each crop were sown under in-crop herbicide or no in-crop herbicide treatments. In 2009, two selected cultivars of each crop species were sown at two seeding rates under in-crop herbicide and no in-crop herbicide treatments. Initial weed control by in-crop herbicides was more effective in barley and wheat than in canola. Of the three crop species studied, barley and wheat crops appeared more competitive than canola in suppressing annual ryegrass (Lolium rigidum Gaud.) heads. Within each crop, some cultivars showed greater competitive ability than others, in terms of grain yield and annual ryegrass suppression. However, the competitive ability of cultivars of a single crop type (barley, canola or wheat) was highly variable, probably due to variation between seasons and sites. In barley, cv. ‘Roe’ under no in-crop herbicide produced greater grain yield than cv. ‘Baudin’. In wheat, cv. ‘EGA Bonnie Rock’ under no in-crop herbicide produced greater grain yield and suppressed more annual ryegrass than cv. ‘Wyalkatchem’. The cereal cultivars that were more competitive also had a larger canopy area at maximum vegetative growth stage and intercepted more light. In canola, the difference in competitive ability between cultivars was not consistent.