Effect of Repeated Injection of Estrogen on Milk Yield of Nulliparous Heifers

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Abstract
Twelve dairy heifers of several breeds varying in age from 7 months to 3 years were stimulated to secrete milk by the daily injection of estradiol benzoate (E.B.) for 2 wk at a level of 0.3 mg/100 lb body weight. Hand and machine milking was initiated twice daily and continued for 6 wks. Maximum daily milk yield based on a weekly average was used as an index of the extent of mammary glandular tissue. Although little milk was secreted by the calves, the heifers of increasing age secreted considerable milk. These data further confirm the suggestion that lobular-alveolar growth is stimulated by the secretion of ovarian hormones during recurring estrus cycles. The 10 older heifers were again administered E. B. at the same level for 1 wk starting the 7th wk. At the beginning of the 2nd course of injections, the mean maximum milk yield was 12.7 lb/day, whereas, following restimulation, mean maximum production increased to 24.5 lb/day, an increase of 92.9%. Four heifers given a 3rd course of injections of E.B. after 15 wk of experimental lactation were stimulated to secrete 22.4% more milk.