The summer nutrition of immature sheep: The nitrogen excretion of grazing sheep in relation to supplements of available energy and protein in a Mediterranean environment.

Abstract

The nitrogen excretion of 24 yearling Merino sheep grazing the summer herbage of annual grass-subterranean clover (Trifolium subterraneum L.) sown pastures was measured over four consecutive periods of 28 days in relation to supplementation with available energy (E) and nitrogen (N). The experiment was of change-over design and included nine combinations of supplement E1, E2, E3 x N1, N2, N3 and an unsupplemented (control) treatment; the respective daily allowance of E was 11 3, 227, and 340 g total digestible nutrients daily, and of N supplied as protein 4,8, and 12 g/day. Unsupplemented sheep grazing mature pasture herbage excreted 23.4 g N/day in urine and faeces. Supplementary energy produced an N-sparing effect whereas added protein was associated with an increased excretion of N. Urinary N accounted for all the changes in N excretion, the faecal N component remaining constant throughout the four periods. Relative changes in herbage intake were estimated: as the level of E (dry matter) increased the estimated herbage intake of sheep was observed to fall linearly, but no significant change in estimated herbage intake was associated with changing N. Significant increases in wool production (P<0.05) were associated with supplementary N. Direct and indirect estimates of the N turnover of the grazing sheep are compared, and a crude N balance sheet is presented. Supporting data are presented which suggest that the N intakes of sheep grazing the mature herbage of sown clover-grass pastures in southern Australia are commonly of the order of 12-18 g N/day.