FARMING SYSTEMS AND THEIR EFFECT ON THE RESPONSE OF SUGARCANE TO NITROGEN

THE AUSTRALIAN sugarcane industry is under increasing pressure to minimise losses of nutrients from the field. Inputs need to be precisely matched to crop requirements. A trial was established at Mackay to determine whether nitrogen (N) rates could be reduced following longterm green-cane trash blanketing (GCTB) and to test whether current recommendations for new farming systems were robust. Four farming system treatments were established: (B) Long-term burnt trash, 1.5 m single rows, bare fallow, conventional cultivation; (GC) Long-term GCTB, 1.5 m single rows, bare fallow, conventional cultivation; (NFSS) 1.8 m single rows, soybean fallow, pre-formed beds, GCTB; (NFSD) 1.8 m dual rows, soybean fallow, pre-formed beds, GCTB. B and GC were possible as the trial was located on a site that was set-up in 1992 to investigate GCTB and the trash treatments had been maintained. Within each farming system treatment four N rates were applied (0, 75, 150 and 225 kg N/ha). Response to N was similar for the B and GC systems. This suggests that, at this stage, N rates cannot be reduced following long-term GCTB. It is possible that soil carbon and N may not be accumulating as predicted under the GCTB system. A 4 t/ha soybean fallow crop appeared to contribute little N to the following plant cane crop as the NFSS and NFSD systems responded in a similar manner to N as the B and GC systems. The soybean fallow crop was defoliated by an insect pest which may have impacted on the amount of N returned to the soil. Current recommendations for N to be applied to plant cane following poor soybean crops appear justified. Similar yields were achieved with all farming systems. Relatively high rainfall during the study period may have negated the soil moisture conservation benefit from a GCTB system. At this stage, the benefit of new farming systems appears to be a lower cost of production. Improvements to soil health reported elsewhere will hopefully also result in increased production in the future.