THE RESPONSES OF MAIZE AND GRAIN-SORGHUM HYBRIDS TO NITROGEN-FERTILIZER IN SOUTH EAST QUEENSLAND

We investigated, over 3 seasons, 1973-74 to 1975-76, the responses of 5 selected maize hybrids and 5 sorghum hybrids to applied nitrogen (N) under irrigation on N-deficient, black earth soils in South East Queensland. Five levels of N (0, 60, 120, 180 and 240 kg/ha) were applied as urea at sowing to 5 maize hybrids, XL81, Q739, PQ500, PX50, RX404, at 1 location. The same rates were applied to sorghum hybrids NK212, P846, E57, Goldfinger and Goldrush at another location. Higher grain yields were obtained from maize hybrids XL81 and RX404 than from Q739, PX50 and PQ500. A mean grain yield of 6660 kg/ha was produced from XL81 over the 3 seasons for the application of 180 kg N/ha. A hybrid x N interaction occurred only in the year with highest yields. Over all years, grain yield of each hybrid was linearly related to mean site yield and slopes were not significantly (P < 0.05) different. Hybrids XL81 and RX404 showed significantly (P < 0.05) higher grain yields than other hybrids over the range of grain N concentrations obtained. We conclude that 180 kg N/ha applied at or prior to sowing would optimise maize yields on N-depleted soils in South East Queensland. Highest grain yields of sorghum were obtained from all hybrids when N application was in the range 120-180 kg N/ha. The grain yields of all 5 hybrids converged in the 5000-7000 kg/ha yield range, indicating no superior yielding potential in any 1 hybrid. Goldrush, however, showed significantly (P < 0.05) higher grain N concentrations than the other hybrids over the range of grain yields.