The Importance of Prolactin and the Milking Stimulus in the Artificial Induction of Lactation in Cows

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Abstract
Lactations were successfully induced in 15 of 18 non-pregnant cows treated with estradiol-17.beta. (0.1 mg/kg body wt) and progesterone (0.25 mg/kg body wt) on days 1-7 (where day 1 was the 1st day of treatment). A further 23 of 26 cows were successfully treated with estradiol-17.beta. and progesterone standardized at 40 and 100 mg/day, respectively. No significant differences in milk yields were obtained by the additional treatments of twice daily milking from day 10, sustained elevation or suppression of plasma prolactin during the induction phase by reserpine and bromocryptine, respectively, or by continuing estradiol-17.beta. injections alone on days 8-11. Levels of plasma prolactin less than 20 mg/ml were adequate for the preparation of the mammary gland for lactation to occur. The induced lactations of 24 monozygotic twin cows ranged from 20-87% of their respective siblings which had normal parturient lactations. All 11 cows treated with the standard estrogen-progesterone treatment plus reserpine had successfully induced lactations. Without estrogen and progesterone treatment, a short period of reserpine treatment followed by dexamethasone trimethylacetate failed to induce lactation in 5 cows. Of induced cows, 90% (39) and 91% of parturient cows (22) conceived and subsequently calved following natural mating.