Secretion of milk and milk components in sheep

The effect of milking intervals of 4-20 h and of milking frequencies of 1-6 times a day on the rate of secretion of milk and milk components was determined in 2 breeds of sheep unselected for dairy production, the Australian Merino and the New Zealand Poll Dorset. The rate of milk secretion was greater after short intervals and after increased milking frequencies, indicating that in sheep the rate of milk secretion in the short term is regulated by a local feedback mechanism. After short intervals between milkings, the fat concentration in the milk was higher (99.5 +/- 6.21 g/kg) and the protein concentration was lower (about 65 +/- 2.89 g/kg) than after long intervals between milkings. This was not due to the presence of residual milk left in the mammary glands as subsequent experiments gave similar results when the residual milk was removed with the aid of oxytocin. We obtained similar responses if ewes were exposed to a series of 4 consecutive milkings to remove possible carry-over effects of the previous interval and achieve a constant volume of residual milk. We concluded that in sheep the rate of secretion of fat decreases while that of protein increases with time after milking and that the synthesis of fat and protein are controlled by at least 2 different mechanisms. This might be due to the fact that, unlike dairy cows and goats, sheep have not been subjected to selective pressure for dairy production.