Maintenance energy requirement of grazing sheep in relation to herbage availability. I. Calorimetric estimates

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. Daily rates of energy expenditure were calculated by measuring the respiratory gaseous exchanges of tracheostomized sheep in each group, and from estimates of CO2 entry rate determined during constant infusion with NaH14CO3. These measurements were made during a period of 3 weeks when the sheep had been at constant W for 9 months, and during a further 3 weeks beginning 30 days after the sheep were shorn. Further measurements were made in two periods of 7 days after animals had been interchanged between groups so that W was increasing in some animals and decreasing in others. Maintenance requirements of all sheep, indicated by the energy expenditures during the periods at constant W, were described by the equation M = 45.1 W + 256, where M is the estimated metabolizable energy requirement in kilocalories per 24 hr. Similar results were obtained during the two periods when W was changing. The requirements were in general 60–70% greater than those for housed sheep of similar W and are discussed in relation to the climatic environment, the condition of the sheep, and the availability of herbage.