LONG-TERM EFFECTS OF ROTATION, TILLAGE AND STUBBLE MANAGEMENT ON WHEAT PRODUCTION IN SOUTHERN NSW

A long term field experiment began in 1979 at Wagga Wagga, N.S.W., to compare the sustainability of a range of rotation, tillage and stubble management systems on a red earth th. This paper reports yield, yield components and grain protein of wheat for 1979-90. Rotations considered were alternating lupin-wheat (LW), lupin-wheat-wheat (LWW), continuous wheat (WW) with and without N fertilizer (100 kg N/ha), and alternating sub-clover-wheat (CW). Soil N supply at the start of the experiment was high following many years of sub-clover based pasture. From 1979 to 1983, there was a negative grain yield response to N fertilizer and no response to a legume in rotation except in the drought of 1982 when low yields were recorded from LW. Thereafter, a positive grain yield response was usually produced to N fertilizer in WW rotations, until 1989 and 1990, when these crops displayed aluminium toxicity symptoms. Overall, average grain yields from legume I rotations were higher than WW with added N fertilizer. Since 1983, LW rotations consistently produced higher mean grain yields than CW, but mean grain protein and total N uptake were lower. Yields and N uptake by the second wheat crop in a LWW rotation indicated little carryover of benefits from the lupins. Slightly higher mean grain yield and harvest index, but lower mean grain protein, were produced by direct drilling, compared with cultivation before sowing, following lupins or sub-clover. However, retaining stubble rather than burning in autumn consistently reduced grain yields. There was no evidence that early burial of wheat stubble following summer rain, rather than incorporation in autumn, improved grain yield or total N uptake. The build-up of giant brome grass and diseases, particularly where stubble was retained and crops direct-drilled, casts some doubt on the long term sustainability of these short term rotations in this environment.