R.T. COWAN¹, T.M. DAVISON¹ and R.K. SHEPHARD²

¹Queensland Department of Primary industries, Mutdapilly Research Station, M.S. 825, Ipswich, 4305.
²Queensland Department of Primary industries, Oonoonba Veterinary Laboratory, Townsville, 4810.

Abstract

The Composition of the diet selected by Friesian cows grazing pure tropical grass pasture containing Panicum maximum cv. Gatton and mixed tropical pasture containing P. maximum cv. Gatton and the legumes Neonotonia wightii cv. Tinaroo and Desmodium intortum cv. Greenleaf was measured during summer and winter on the Atherton Tablelands, north Queensland. The grass pasture was stocked at 2.0 and 3.5 cows ha^-1, and received 200 and 400 kg N ha^-1 year^-1 at each stocking rate. The grass–legume pasture was stocked at 2.0 cows ha^-1 and received no nitrogen fertiliser. All pastures were grazed on a rot of one week grazing, three weeks spelling, and samples of the diet determined with oesophageally fistulated heifers.
With pure grass pastures there was a rapid decline in the leaf content of the diet during the first three days of the week, and an associated increase in stem and dead material. Crude protein content of the diet exceeded 12% in the dry matter only during the first two days of each week. The effects of stocking rate and fertiliser level were evident early and late in the week, but were small in relation to the changes as the pasture was grazed down.
There was less variation in the composition of the diet of cows grazing grass–legume pastures, but the crude protein content of the diet again only exceeded 12% in the first two days of the week. For grass pastures the results show a close association between leaf percentage in the diet and both leaf yield on offer and total pasture yield on offer. Grazing time was reduced when yield of grass leaf was less than 1.0 t DM ha^-1. Milk production varied by up to 25% during the week, with peak values being on days 2 and 3 and the minimum value on day 7.