Forage quality and feed intake responses of cattle to improved pastures, tree killing and stocking rate in open eucalypt woodlands of north-eastern Australia

The effects of oversewing introduced grasses and legumes, tree killing and stocking rate on simulated diet quality and feed intake of cattle were studied in an open eucalypt woodland in the semi-arid tropics of north-eastern Australia in 1990. Rainfall during the experimental period was above average, producing good conditions for pasture growth, which resulted in little influence of stocking rate on pasture or dietary attributes. During the growing season, forage quality, as measured by nitrogen concentration and in vitro digestibility, was higher in sown pasture than in native pasture, and higher in plots with live trees than in plots with killed trees. Treatment differences in herbage quality disappeared later in the year when pastures senesced. In sown pasture treatments, there was no effect of tree killing on legume (Stylosanthes spp.) quality. However, Stylosanthes made a greater contribution to the diet of cattle grazing in wooded paddocks than in paddocks where trees had been killed. The proportion of Stylosanthes in the diet reached a peak in May and then declined. Oversewing increased both diet quality and feed intake while, to a lesser extent, killing trees resulted in generally lower diet quality and feed intake. Estimates of liveweight change were made from the quantities of energy and protein consumed by the cattle. As expected, large increases in liveweight gain were predicted to occur in oversewn pastures compared with native pastures. However, estimates indicated that animal performance may be better where trees are left intact rather than killed. While carrying capacity is greatly increased with the removal of trees, this may be at the expense of individual animal performance. Possible reasons for this significant negative response in dietary attributes and predicted liveweight gain to tree killing are discussed.