A simulation study of wheat crop response to water supply, nitrogen nutrition, stubble retention, and tillage

A series of simulations was made with a fallow-wheat crop simulation model to examine the effects of stubble retention and tillage on yield for 2 semi-arid environments in north-western Victoria over a 100-200-year period. The fallow treatments comprised 4 combinations of stubble management and tillage in 18-month-long fallows of a fallow-wheat rotation. The 2 sites are representative of the major soil types of the region: a chromic vertisol at Dooen and a calcic xerosol at Walpeup. Stubble retention had the greatest effect on yield, providing a median increase of 0.8 and 0.6 t/ha at Dooen and Walpeup, respectively. Zero tillage had an important but small advantage in the presence of stubble at Dooen (0.2 t/ha) and no significant advantage under any system at Walpeup. Yield response was primarily caused through increased fallow-stored water of approximately 89 and 25 mm at Dooen and Walpeup, respectively. The accumulation of soil mineral nitrogen (SMN) at sowing in fallows did not show the same treatment differences as with water or yield. There was a significant interaction between soil type and fallow method. At Dooen, stubble depressed SMN by about 24 kg N/ha (163-185 kg N/ha) below those without stubble (188-207 kg N/ha). Tillage reduced SMN with or without stubble by about 20 kg N/ha. At Walpeup, on the other hand, the highest SMN was associated with tillage and was enhanced by stubble, but the differences were small (<10 kg N/ha). Nitrogen fertiliser produced a small increase in yield (median 0.2 t/ha) at Dooen, increasing to about 0.4 t/ha in wetter years when stubble was retained and fallows were maintained with zero tillage. Over a 100-year period, nitrogen fertiliser reduced the rate of yield decline. The use of nitrogen fertiliser with stubble retention and zero tillage offers an opportunity for arresting declining yield and quality in intensive cropping systems in the Wimmera region of Victoria.