Hormone Induced Lactation in the Bovine. II. Response of Nulligravida Heifers to Modified Estrogen-Progesterone Treatment

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Abstract
Subcutaneous injection of 17/3-estradiol (60 rag/600 kg body weight per day) and progesterone (150 rag/600 kg body weight per day) for 7 days initiated lac- tation in nulligravida heifers. Lactation was initiated with 17/3-estradiol-proges- terone ratios of 1:2.5, 1:2, and 1:1 when constant 17/~-estradiol (60 rag/600 kg body weight per day) was maintained. Varying the dose of 17/3-estradiol (20, 40, or 60 rag/600 kg body weight per day) while maintaining a constant 1713- estradiol:progesterone ratio of 1:2.5 also initiated lactation at all rates of 1713- estradiol. Number of days of injection (7 or 10) and, thus, the total amount of hormone injected had little effect on the ability of the treatment to initiate lactation. Lactation was not successfully induced following intramuscular injection of the hormones or when a suspension of es- trone rather than dissolved 17fl-estradioI was injected subcutaneously. Regardless of treatment the mammary gland did not change during hormone injection. Major glandular growth oc- curred between the last (7th) day of hormone injection and 14 days after the last hormone injection. Mammary g!ands of heifers that lactated continued to grow until approx'mately peak lactation. Neither amount nor ratio of 17/3- estradiol and progesterone is critical in initiation of laetation. Failure to initiate lactation following intramuscular injec- tion or injection of a suspension of estrone may reflect an altered absorption pattena from the injection site and sug-
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