A SURVEY OF FALLOW PRACTICES AND WEED FLORAS IN WHEAT STUBBLE AND GRAIN-SORGHUM IN NORTHERN NEW-SOUTH-WALES

A survey undertaken in northern New South Wales after the 1989 wheat h arvest investigated the effects of cultural practices used by dryland farmers on summer weed flora. Only 50% of fallow paddocks surveyed bet ween December and February were weed-free, and by mid January 65% had inadequate stubble cover (<1000 kg/ha) for protection from soil erosio n, mainly because of excessive tillage. By February, only 10% of paddo cks were being sprayed with a herbicide to control weeds. Examination of 65 uncultivated fallow wheat stubble paddocks and 25 grain sorghum crops identified 87 and 51 different weed species, respectively. In fa llow paddocks where no herbicide had been used, 69 and 61 species were found in ungrazed and grazed paddocks, respectively. Where glyphosate had been used the number of species was 37 (grazed) and 39 (ungrazed) , and for glyphosate plus atrazine, 23 and 18 species. The number of w eed species found in grain sorghum was 29 for cultivated without atraz ine, 35 for cultivated with atrazine, and 35 for no-tillage with atraz ine. The most important weeds found in fallow were liverseed grass, na tive millet, common sowthistle, wireweed, and barnyard grasses. Black bindweed was a problem where atrazine had not been used. Native millet was the most abundant species where atrazine had been applied. In gra in sorghum the most common broadleaf weeds for cultivated paddocks wit h no atrazine treatment were Tribulus spp., Australian bindweed, pigwe ed, and Bathurst burr, while the most common grass weeds were barnyard grasses, liverseed grass, stinkgrass, and native millet. When cultiva tion plus atrazine was used, barnyard grasses, native millet, wild oat s, and liverseed grass were the most common weeds. In minimum tillage or no-tillage paddocks treated with atrazine, native millet, Queenslan d blue grass, liverseed grass, common sowthistle, Australian bindweed, and windmill grass were the most common weeds.