Induction of lactation: lactational, physiological, and hormonal responses in the bovine.

Milk yields, physiological responses, and concentrations of plasma hormones were evaluated in 24 attempts to induce lactation in nonlactating dairy cows. Subcutaneous injections of estradiol-17beta and progesterone (.10 and .25 mg/kg body weight per day) for 7 consecutive days were used. Dexamethasone injections (.028 mg/kg body weight per day) on days 18 to 20 were given during 12 attempts at induction. Milking was initiated on day 21. All cows showed proestrus activity within 2 days after the first steroid injection; this subsided, then reappeared in many animals between days 16 to 20. In 14 of 24 attempts mean daily milk production was greater than 5 kg. Actual or projected 305-day lactation milk yields were between 1859 and 5354 kg. However, milk yields of seven induced cows averaged only 73% (32% to 136% range) of their previous natural lactations. Dexamethasone injections increased the number of cows that produced more than 5 kg/day; however, milk yields were not improved. Concentrations of estradiol, estrone, and progesterone in plasma were unaffected by dexamethasone, but concentrations of glucocorticoids in plasma were depressed on days 19 to 22. Concentrations of prolactin (peak and mean) in plasma for six cows each that produced greater or less than 5 kg/day did not differ. However, concentrations of prolactin increased in the week following steroid injections (days 8 to 15) only in those cows that produced greater than 5 kg/day but were elevated in all cows during the 3rd wk.