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 m odel 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 rot ation. The 2 sites are representative of the major soil types of the r egion: a chromic vertisol at Dooen and a calcic xerosol at Walpeup. St ubble retention had the greatest effect on yield, providing a median i ncrease of 0.8 and 0.6 t/ha at Dooen and Walpeup, respectively. Zero t illage 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, respe ctively. The accumulation of soil mineral nitrogen (SMN) at sowing in fallows did not show the same treatment differences as with water or y ield. 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 reduce d 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 enhanc ed 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 Doo en, increasing to about 0.4 t/ha in wetter years when stubble was reta ined and fallows were maintained with zero tillage. Over a 100-year pe riod, nitrogen fertiliser reduced the rate of yield decline. The use o f nitrogen fertiliser with stubble retention and zero tillage offers a n opportunity for arresting declining yield and quality in intensive c ropping systems in the Wimmera region of Victoria.