EFFECT OF CROP-ROTATION, TILLAGE PRACTICE, AND HERBICIDES ON THE POPULATION-DYNAMICS OF WILD OATS IN WHEAT

The effects of crop rotation, tillage practice, and herbicide use on t he population dynamics of wild oats (Avena fatua and A. sterilis ssp. ludoviciana) were studied in a field experiment in northern New South Wales. In the third and fourth years of a continuous wheat rotation, c ultivated fallow using tines increased wild oat density and reduced gr ain yield compared with a no-tillage fallow. Tillage did not affect th e vertical distribution of wild oat seeds in the soil, and about 80% o f wild oat seeds were in the top 5 cm of soil in both tillage treatmen ts. The seed reservoir at the end of the experiment was smaller under a no-tillage fallow regime. The half-life of wild oat seeds in the soi l was about 6 months, and rotation of wheat with sorghum was the most effective means of reducing the wild oat seed reservoir. Rotational st rategies for weed control are also likely to be effective in delaying or minimising the development of herbicide resistance, particularly wh ere the average seed bank life time, as shown for wild oats in this st udy, is short. Annual use of either tri-allate or flamprop-methyl in 4 successive wheat crops did not prevent a massive build-up of wild oat seed. The poor performance of herbicides was partly attributed to bel ow-average rainfall in autumn and early winter in 1985 and 1986. Howev er, wild oats are well adapted to continuous cropping with wheat, wher e recruitment of 3-6% of the soil seed reservoir maintained the popula tion despite the use of selective herbicides. The results of this expe riment indicate that a continuous wheat rotation using herbicides to c ontrol wild oats is likely to be much less effective in reducing the w ild oat seed reservoir.