The nutrition of ruminants grazing native and improved pastures. I. Seasonal variation in the diet selected by grazing sheep and cattle

Abstract

The chemical composition of the forage on offer on a native pasture dominated by Poa sieberana and Themeda australis, on a similar native pasture dressed with superphosphate and oversown with white clover, and on two Phalaris aquatica (formerly tuberosa)–Trifolium repens pastures dressed with superphosphate and grazed at different stocking rates during summer, was determined at monthly intervals during 1974 and 1975. Samples were also plucked to simulate grazing in 1975, and extrusa were collected from oesophageally fistulated cattle in both years and from fistulated sheep in 1975. Digestibility and nitrogen content of the diet were determined, and in 1974 the extrusa were manually separated into green and senescent fractions. Samples of rumen liquor were also collected through rumen fistulae, and analysed for ammonia, and total and individual volatile fatty acids.

The application of superphosphate and white clover, and the replacement of native grasses with Phalaris aquatica, both induced large increases in nutritive value measured in terms of digestibility, protein, macro- and micro-elements in the forage on offer and in the diet selected. Changes in the composition of the forage on offer were attributable to an increase in green material, and to the greater nutritive value of both green and senescent fractions as the extent of pasture improvement increased. Digestibility, nitrogen and mineral content of the diet selected by cattle and sheep increased with pasture improvement, the nutritive value of the diet selected by sheep being greater on all pastures although the difference was greatest on native pasture. The increase in nutritive value of the diet selected by cattle on improved pasture was associated with an increase in the green component of the diet and in the nutritive value of the green material selected.