Effects of long-term thrice-daily milking on mammary enzyme activity, cell population and milk yield in the goat.

In two separate experiments, lactating goats were milked unilaterally thrice daily instead of twice daily for periods of 13 and 37 wk, starting at 2 to 5 wk after parturition. The other gland was milked twice daily throughout. In both experiments thrice-daily milking increased milk yield significantly. In the first experiment, after 37 wk the amounts of RNA and DNA, rate of cell proliferation and activities of several enzymes per cell were greater in the thrice-milked gland (P less than .05). In the second experiment, after 9 or 10 d of thrice-daily milking the activities of several key mammary enzymes per cell measured in biopsy samples had increased in the thrice-milked but not the twice-milked gland. After 13 wk of thrice-daily milking, the DNA content of the glands and the activity per cell of the key enzymes was the same in both glands, showing that the synthetic capacity of the gland was being used more effectively in the treated gland. These results indicate that several mechanisms contributed to the increased milk yield induced by thrice-daily milking.