Variation in the yield and composition of milk of grazing Merino sheep

Abstract

A technique for estimating the yield and composition of milk produced by grazing ewes is described. Ewes were milked by machine, after the intravenous injection of 3 i.u. posterior pituitary extract, at the rate of 20 to 30/hr, and then returned to pasture with their lambs which were prevented from sucking by covering the udders. Approximately 4 hr later the ewes were milked again; the yield over the known time interval was recorded and sampled for analysis. There was no significant variation in the yield, or fat and protein contents, or the milk obtained over six different 4 hr periods in a day. It was concluded that the technique gave reliable estimates of the rate of secretion and that these could be converted directly to estimates of yield per day of milk and its constituents. Such estimates, if made once or twice weekly, would also be likely to give reliable data on the course of lactation, which would be determined primarily by the mutual behaviour, largely undisturbed, of ewe and lamb. The lactations of 50 fine-wool Merino ewes grazing improved pastures that provided abundant feed were recorded. For the majority of ewes, daily yield increased until a mean maximum of 1400–1600 g/day was attained at 3 weeks post partum, and then declined at the rate of 15–20 g/day for 6 to 8 weeks. The total yield over 10 weeks was about 80 kg and at 14 weeks was 90–100 kg; half or more of these amounts had been produced by the end of the fifth week. Fat and protein contents declined in the early weeks of lactation and then rose so that the highest values were observed at the end of lactation. Lactose contents showed the opposite trend.