Tillage practices and the growth and yield of wheat in southern New South Wales: Yanco, in a 425 mm rainfall region

Abstract

In a comparison of tillage treatments over a 3 year cropping phase (1982-84) following clover ley on a red-brown earth soil, long (9 months) and short (3 months) cultivated fallow gave the highest average wheat yields (2.80 t/ha). Cultivated fallow in year 1, followed by direct drill without fallow in years 2 and 3 yielded 2.66 t/ha, while direct drill without fallow every year averaged 2.43 t/ha (full disturbance at sowing) or 2.25 t/ha (minimum disturbance). Stubble retention when combined with direct drilling reduced yield further. Herbicide fallow followed by direct drilling gave the same yield as cultivated fallow in 1 year but a lower yield in another. Long fallow accumulated significantly more total soil water and mineral nitrogen in 2 years out of 3, while short fallow had no effect in any year. Plant population and early growth were reduced each year by the absence of fallow and by direct drilling, and also by residue retention; plant nitrogen and phosphorus status also tended to be reduced. In the absence of soil water differences at sowing and in the wetter 2 years, final grain yield differences, although always relatively smaller than early growth differences, were related to these differences in early growth which, it is concluded, reflect the main underlying cause of reduced yield with direct drilling. Reductions occurred even with a dry spring when direct drilled crops had clearly better soil and plant water status. In the driest year (1982) yield differences were small despite early growth differences. Sowing of all treatments was delayed in 2 years out of 3 because of top soil dryness in no-fallow plots. Some cultivated fallow crops showed a tendency to 'hay off' (i.e. markedly reduced harvest index and kernel weights) when the grain filling period was dry. The incidence of yellow leaf spot (Pyrenophora tritici repentis) and root diseases, although low, was greater in direct drilled crops, even after stubble burning.