Follicular growth, estrus and pregnancy after fixed-time insemination in beef cows treated with intravaginal progesterone inserts and estradiol benzoate

An experiment was performed to compare the effects of 3 short-term treatmen ts with progesterone and estradiol benzoate (EB) on follicular growth, sync hrony of estrus and pregnancy rate after fixed-time insemination in lactati ng postpartum beef cows. In Treatment 1 (n = 46), each cow received a proge sterone-containing intravaginal insert for 7 d with injection of EB (2 mg, im) at the time of device insertion. In Treatment 2 (n = 46), the insert wa s used for only 5 d with injection of EB (2 mg, im) at the time of insertio n; Cows in Treatment 3 (n = 47) received an insert for 5 d with no EB at th e time of insertion. Each cow in the 3 groups received PGF(2)alpha (25 mg, im) at the time of insert removal, followed by EB (1 mg, im) 30 h later. Th e cows were then inseminated 28 to 30 h after treatment with EB (58 to 60 h after insert removal). Treatment with 2 mg EB terminated the growth of the largest ovarian follicle (>5 mm in diameter) at device insertion in 16/16 and 14/15 cows in Treatments 1 and 2, respectively. Estrus was detected wit hin an 8-h target period (48 to 56 h after insert removal) in 93, 87 and 81 % of cows in Treatments 1, 2 and 3, respectively (P>0.05). Pregnancy rates at 39 d post insemination were 60, 50 and 51% for Treatments 1, 2 and 3, re spectively (P>0.05). The pregnancy rates did not differ between cows that w ere anovulatory or those that had ovulated before the initiation of treatme nts (54%), or among cows that were 28 to 40, 41 to 60 or >60 days post part um at insemination (43, 59 and 54%, respectively). Treatment with progester one inserts for 5 or 7 d, PGF2a at the time of insert removal and 1 mg EB 3 0 h later induced the high degree of synchrony of estrus and ovulation nece ssary for fixed-time insemination. (C) 1999 by Elsevier Science Inc.