Hormonal and Reproductive Profiles of Early Postpartum Beef Heifers after Prolactin Suppression or Steroid-Induced Luteal Function3

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Abstract
Two trials were conducted with 24 primiparous suckled beef heifers to determine independent and combined effects of a prolactin-suppressing agent and exogenous steroids on serum hormones and reproductive activity. Heifers were assigned in groups of six to one of four treatments: 1. (C)- subcutaneous control injections administered once a day for 2 days, followed by 2 days of no injections; sequence repeated from day 5 through day 40 postpartum or until first naturally occurring estrus; 2. (CB)- subcutaneous injections of 80 mg CB-154 (2-bromo-α-ergokryptine) (Sandoz) in ethanol once a day for 2 days, followed by 2 days of no injections; sequence repeated from day 5 through day 40 postpartum or until first naturally occurring estrus; 3. (PE)- intramuscular (IM) injection of 25 mg progesterone in sesame oil on day 15 postpartum, followed by IM injection of 4 mg estradiol-17 in sesame oil 48 hr later, or 4. (CBPE)-treatments 2 (CB) plus 3 (PE). A marked reduction in serum prolactin was observed after a single injection CB-154 in all CB and CBPE treated heifers, reaching minimum concentrations by day 7 postpartum after a second injection on day 6. Continued treatment decreased (P<.01) prolactin concentrations over the 35 day blood sampling period to an average of .9 ± .2 ng/ml as compared to 19.2 ± 5.6 ng/ml for non-CB-154 treated (C and PE) heifers. In non-CB-154-treated heifers, serum prolactin was higher during the spring (33.5 ± .9 ng/ml) than the fall (2.6 ± .9 ng/ml). Treatment CB had no effect on serum concentrations of LH, estradiol-17β or progesterone. The resumption of cyclic activity in C and CB heifers prior to day 40 postpartum was characterized in four of six animals by 4- to 5-day elevations of serum progesterone and 7- to 8-day cycles, which were preceded in every case by peaks of estradiol-17β (9 ± 1.9 pg/ml) and nonstanding estrous behavior (three of four) or no behavior (one of four). Palpable corpora lutea were detected in two of four animals during these progesterone increases. Subnormal luteal function was followed by another estradiol-17β peak (7 ± 2 pg/ml) and estrus (three of four) or nonstanding estrus (one of four). Injection of progesterone followed by injection of estradiol-17β resulted in a preovulatory discharge of LH in 12 of 12 PE and CBPE heifers, with eight of 12 initiating luteal function within 72 hr after the LH surge. On the basis of endocrine data, suppression of endogenous prolactin release did not (P<.05) potentiate steroid treatment effects. Treatments PE and CBPE resulted in a reproductive trend which reflected endocrine findings. Data fail to provide evidence that prolactin is antigonado-tropic in bovine heifers. Copyright © 1980. American Society of Animal Science . Copyright 1980 by American Society of Animal Science.