Abstract 174

McCosker,TH; O'Rourke,-PK; Eggington,-AR; Doyle,-FW

Mt Bundey Sta., Adelaide River, Northern Territory, Australia.

Australian-Rangeland-Journal. 1988, 10: 1, 18-29; 32 ref.

In a large supplementation experiment on Mount Bundey Station in the Darwin pastoral district of the Northern Territory, the pregnancy rates of lactating heifers were positively related to yields of Themeda triandra, Sorghum plumosum, Eriachne burkittii and Eragrostis spp., the first 3 of which declined in yield following heavy grazing pressure. Quantity of pasture was more closely correlated with animal production than was quality due to the wide range in yields and narrow range in quality attributes. Pregnancy rate in lactating heifers was more sensitive to yield of pasture species than was lactating cow pregnancy rate, cow mortality, calf wt/cow mated or steer av. daily liveweight gain. Heifers consumed less supplement relative to their requirements than mature cows and steers and their response was not confounded by previous history. These factors probably account for their greater sensitivity to pasture variations. Pregnancy rate of cows was positively related to the proportion of lowland soil types and negatively to proportion of lithosols (hills). An equation incorporating the percentages of these soils, explained 63%, 42% and 39% of the variation in lactating cow pregnancy rate in 1982, 1983 and 1984, resp. Variation in pregnancy rate among paddocks was therefore largely attributed to variation in the proportions of 'upland' and 'lowland' soil types. Sensitivity to grazing pressure of species within the native pasture was indicated by localized damage to T. triandra and S. plumosum and a large reduction in yield of S. intrans after 5 years of continuous grazing with access to non-protein N supplements at one breeder to 14 ha. Native pastures in the Adelaide River area could not sustain this stocking rate.