Estrus synchronization and pregnancy rates in beef cattle given CIDR-B, prostaglandin and estradiol, or GnRH

Two experiments were conducted to determine estrous response and pregnancy rate in beef cattle given a controlled internal drug release (CIDR-B) devic e plus prostaglandin F-2 alpha (PGF) at CIDR-B removal, and estradiol or go nadotropin releasing hormone (GnRH). In Experiment I, crossbred beef heifer s received a CIDR-B device and 1 mg estradiol benzoate (EB), plus 100 mg pr ogesterone (E + P group; n = 41), 100 mu g gonadotropin releasing hormone ( GnRH group; n = 42), or no further treatment (Control group; n = 42), on Da y 0. On Day 7, CIDR-B devices were removed and heifers were treated with PG F. Heifers in the E + P group were given 1 mg EB, 24 h after PGF, and then inseminated 30 h later. Heifers in the GnRH group were given 100 mu g GnRH, 54 h after PGF, and concurrently inseminated. Control heifers were insemin ated 12 h after onset of estrus. The estrous rate was lower (P < 0.01) in t he GnRH group (55%) than in either the E + P (100%) or Control (83%) groups . The mean interval from CIDR-B removal to estrus was shorter (P < 0.01) an d less variable (P < 0.01) in the E + P group than in the GnRH or Control g roups. Pregnancy rate in the E + P group (76%) was higher (P < 0.01) than i n the GnRH (48%) or Control (38%) groups. In Experiment II, 84 cows were tr eated similarly to the E + P group in Experiment I. Cows received 100 mg pr ogesterone and either I mg EB or 5 mg estradiol-17 beta (E-17 beta) on Day 0 and either 1 mg of EB or 1 mg of E-17 beta on Day 8 (24 h after CIDR-B re moval), in a 2 x 2 factorial design, and were inseminated 30 h later. There were no differences among groups for estrous rates or conception rates. Th e mean interval from CIDR-B removal to estrus was 44.2 h, s = 11.2. Concept ion rates were 67%, 62%, 52%, and 71% in Groups E-17 beta/E-17 beta, E-17 b eta/EB, EB/E-17 beta, and EB/EB, respectively. In cattle given a CIDR-B dev ice and estradiol plus progesterone, treatment with either EB or E-17 beta effectively synchronized estrus and resulted in acceptable conception rates to fixed-time artificial insemination.