Feed‐back control of milk secretion in the goat by a chemical in milk.

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Abstract
The nature of the inhibition of milk secretion during a long milking interval was investigated in goats treated in 3 possible ways: (i) milked twice daily at 08.00 h and 16.00 h or (ii) milked 3 times daily at 00.00 h, 08.00 h and 16.00 h, or (iii) milked 3 times daily at 00.00 h, 08.00 h and 16.00 h, but at 00.00 h the milk removed was replaced with an equal volume of isosmotic sucrose solution. The latter treatment was carried out in order to subject the gland to a degree of physical distension equivalent to that on treatment (i). On either 3 times daily milking or 3 internal daily milking with sucrose replacement, milk secretion rate over the 16.00-08.00 h period was significantly higher (by .apprx. 10% in both cases) than on twice daily milking. Secretion rates of lactose, milk protein, citrate and Ca2+ during the 00.00-08.00 h period were similar on either 3 times daily milking or 3 internal daily milking with sucrose replacement; the secretion rate of fat was significantly higher on 3 internal daily milking with sucrose replacement. Secretion rates of Na+, K+ and Cl- were significantly higher on 3 internal daily milking with sucrose replacement. In the case of Na+, the increased Na+ secretion rate was sufficient to create a normal Na+ concentration in the milk/sucrose mixture removed at the next milking. In the cases of K+ and Cl-, their secretion rates were not sufficient to restore their concentrations to normal by the next milking. Physical distension evidently does not cause the reduction in milk secretion which normally takes place in the latter part of a long milking interval. There may be a locally-active chemical inhibitor in milk, which reduces milk secretion by negative feed-back during this time.