Changes in fallow weed species in continuous wheat in northern New South Wales, 1981-90

Four experiments in northern New South Wales comparing fallow management treatments of no-tillage, cultivated with the stubble retained, and cultivated with the stubble burned, from 1981 to 1990, were sampled for weeds between wheat harvest and seeding on a number of occasions during this period. Eighty weed species were identified, 23 of which were found at all 4 sites but only 13 were recorded in the samples taken. These were dwarf amaranth, wild turnip, spear thistle, Australian bindweed, fleabane, bladder ketmia, prickly lettuce, turnip weed, variegated thistle, common sowthistle, dandelion, wild oats and native millet. The density of weeds during the fallow period decreased by 97% from 1981 to 1990 with some species eliminated, probably because of the herbicides used, particularly glyphosate and chlorsulfuron. These included Boggabri and redroot amaranth, button grass, caustic weed, dense crassula, fat hen, legumes, common peppercress, hedge mustard, London rocket, shepherd's purse, wild mustard, sorghum-almum, paradoxa grass, wild zinnia, and wireweed. Twenty-four weed species were classified as a problem or a potential problem in the future at 1 or more sites. Seventeen weed species, including perennial grasses, often were tolerant to glyphosate at the rates used. The densities of common sowthistle and wild oat were reduced after 10 years, but still remained at all 4 sites. Plant density of species with wind-blown seeds was greater in no-tillage plots than stubble-retained or stubble-burned plots. Weeds whose seeds are dispersed by wind and weeds tolerant to glyphosate were the most troublesome. The perennial grass native millet increased at all 4 sites.