Abstract
Eight nonpregnant dairy cows, one dry and seven producing daily 10 to 22 kg of 4.0% fat-corrected milk (FCM), were subcutaneously injected 15 times at 12-hr intervals (first injection—Day 0) with a mixture of estradiol-17β (E2β) and progesterone (P) to evaluate effects on lactation and milk composition. Each injected dose contained .05 mg of E2β and .125 mg of P per kilogram of body weight. Average daily yield of FCM was depressed from 14 kg on Day 0 to 4 kg on Day 18 (P<.01) and then increased to average of pretreatment rates by Day 60. Compared to pretreatment, subsequent lactation was not enhanced in three cows lactating pretreatment due to calving and was enhanced in two of four cows lactating pretreatment due to treatment with E2β and P. Average milk lactose concentration was lowest on Day 18 and percentages of fat and total protein were highest on Day 20 as compared to Days 0, 43 or 50 (P<.05). Changes in milk composition caused by treatment with E2β and P were characteristic of normal mammary gland regression and subsequent parturient lactogenesis. Concentrations of total free estrogen and progesterone in plasma were elevated during treatment (Days 5 and 8) and each was decreased on Day 16 (P<.05). Concentration of estrogen in milk was increased about 10-fold on Days 5 and 8 as compared to Day 0 (P<.05). On Day 16 and thereafter, milk estrogen concentrations and total secreted were nonsignificantly different from Day 0. Even on Day 5 of treatments, only .007% of the injected E2β was recovered in milk. Blood and milk estrogen concentrations were correlated (P<.01). Compared to Day 0, average plasma prolactin was elevated on the day that milk yield in each cow increased at least 1 kg (P<.05). Copyright © 1976. American Society of Animal Science . Copyright 1976 by American Society of Animal Science.