Maintenance energy requirement of grazing sheep in relation to herbage availability. II.* Observations on grazing intake

Abstract

Grazing pressure on three pastures was adjusted so that the mean liveweights (W) of three groups of 10 Merino wethers, initially uniform, were kept at nominally 45, 35, and 25 kg (groups H, M, and L respectively). Grazing intakes of each sheep were estimated over eight 7-day periods during 6 months.

The organic matter (OM) content of the faeces decreased to less than 40% with dccreasing W, but concentrations of nitrogen (FN) and chromium sesquioxide (Cr₂O₃) in the OM increased. Sheep in groups M and L re-ingested with herbage, Cr₂O₃ they had previously excreted on their pastures. Faecal Cr₂O₃ concentrations were adjusted for this recycling, which otherwise would have resulted in underestimation of faecal OM outputs by up to 10% (group M) or 15% (group L).

The digestibility in vitro was determined of herbage grazed from each pasture by a sheep with an oesophageal fistula. The values obtained for group H showed some differences, but not in a consistent manner, from those predicted from FN. For groups M and L, the in vitro digestibility values were consistently lower than those predicted from FN, on average by 12.5 percentage units with a maximum discrepancy of 18.5 units.

It is shown that when sheep graze sparse pastures, as did groups M and L, the ingestion of large amounts of soil can lead to biased estimates of digestibility from FN, resulting in gross overestimation of digestible OM intakes. It is suggested that this bias may account for the apparently high maintenance feed intakes reported by other workers for sheep in these conditions, and that the calorimetric studies made on groups H, M, and L reported earlier gave more reliable estimates of maintenance requirements.

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*Part I, Aust. J. Agric. Res., 23: 57 (1972)