Lactation was induced in three cows and five heifers with 14 subcutaneous injections of estradiol-17β (E2β) and progesterone (P) dissolved in absolute ethanol. Two animals received extended treatment with 10 intramuscular injections of E2β and P dissolved in corn oil. Injections were at consecutive 12-hr intervals and each dose contained .05 mg E2β and .125 P per kilogram of body weight. Blood, urine and milk samples were collected for radioimmunoassay of hormones. Although lactation was induced in all animals, post-induction increases in milk yields were subnormal and lactation yields of 4% fat-corrected milk were 50 to 59% of expectancy. Milk yield of all individuals reached at least 2.5 kg/day 9 to 13 days (x̄ ± 10.6) after the last injection of E2β and P. Concentrations of progesterone and total free estrogen in milk decreased significantly by 4 to 14 days after first milking or a maximum of 18 days after last injection. Each milk hormone was significantly correlated with its blood plasma concentration. Also, milk and plasma estrogen were significantly correlated with urinary estrogen excretion rate. Milk and plasma progesterone never exceeded concentrations reported for untreated cows during pregnancy and milk estrogen was similar after Day 2 of lactation. Plasma and urinary estrogen during treatment equaled or exceeded amounts reported for late pregnancy. Decreases in progesterone and estrogen were rapid during the first 3 days after last treatment and both were low when plasma prolactin significantly increased 4 or 5 days later. Treatment with E2β and P for 7 or 12 days produced hormone profiles similar to 3 to 7 days before parturition. However, when lactation was initiated plasma estrogen and prolactin were lower as compared with data reported for parturient lactogenesis.
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Erb et al. (1976) studied this question.