Comparison of four synchronization protocols for fixed-time bovine embryo transfer in Bos indicus × Bos taurus recipients

Abstract

The objective was to evaluate the effects of 400 IU of eCG given on Days 5 or 8 of an estrus synchronization protocol with progesterone-releasing intravaginal devices (PRID) and estradiol benzoate (EB), in recipients for fixed-time embryo transfer. A secondary objective was to determine the effects of injectable progesterone (given concurrent with EB treatment). Three-hundred-and-four crossbred Bos taurus × Bos indicus beef heifers were randomly assigned to one of four treatment groups (2 × 2 factorial design). At unknown stages of the estrous cycle (Day 0), all heifers received a progesterone-releasing intravaginal device (PRID), plus 2 mg of EB i.m., with or without a concurrent treatment of 50 mg of progesterone i.m. Heifers were further subdivided to receive 0.15 mg of d-cloprostenol (PGF) i.m. and 400 IU of eCG i.m. on Days 5 or 8. In all heifers, intravaginal devices were removed on Day 8 and 1 mg of EB was given i.m. on Day 9 (Day 10 was arbitrarily considered the day of estrus). On Day 17, all heifers with >1 CL or a single CL with a diameter ≥18 mm (based on ultrasonographic examination), received an in vitro produced (IVP) embryo by non-surgical transfer. On Day 17, there was an effect of day of eCG administration on the number of CL (1.35±0.08 versus 1.13±0.04, for Day 5 versus Day 8, respectively; P=0.02) and (in a subset of 154 heifers) mean (±S.E.M.) plasma progesterone concentrations (2.41±0.26 versus 1.74±0.19 ng/mL; P=0.03). Although the proportion of recipients transferred/treated and pregnant/transferred did not differ among groups, the proportion of recipients pregnant/treated tended ( P=0.1) to be higher in heifers treated with eCG on Day 5 versus Day 8 (47.0% versus 40.7%, respectively). Progesterone treatment had no significant effect. In conclusion, treatment with eCG (and d-cloprostenol) on Day 5 significantly increased the number of CL and plasma progesterone concentrations and tended to increase pregnancy rates, although progesterone treatment had no significant effect.