The effect of estrus synchronization scheme, injection protocol and large ovarian follicle on response to superovulation in beef heifers.

Two trials were conducted to examine the effects of estrus synchronization scheme, gonadotropin injection protocol and presence of a large ovarian follicle on response to superstimulation of follicular development and the ensuing superovulation. Estrus was synchronized with either a progestin compound (MGA) or by the use of a luteolytic agent (PGF). Superstimulation was induced with 280 mg equivalents of pFSH administered either by a single subcutaneous injection or by a series of 8 intramuscular injections over 4 d. Follicular development was followed for 5 d with real-time ultrasound, and the heifers were retrospectively classified as to the presence or absence of a large follicle (> or = 8 mm; morphologically dominant follicle) at the start of superstimulation. The 2 trials differed by season of the year and genetic origin of the heifers. In Trial I (20 heifers), the ovulation rate was influenced by the 3-way interaction of the synchronization scheme, injection protocol and morphologically dominant follicle (P = 0.05). The number of large follicles on Day 5 (Day 0 = day of start of superstimulation) and ovarian score (scale 1 to 5 based on extent of follicular development; 1 = least, 5 = most) on Day 5 were significantly correlated (P < 0.05) with ovulation rate. In Trial II (20 heifers), the ovulation rate, number of embryos recovered, number of transferable embryos and ovarian weights were all greater (P < 0.05 to P < 0.01) with the 8-injection protocol than the 1-injection protocol. The number of medium follicles (5 to 7 mm) on Days 2 and 3, number of large follicles (> or = 8 mm) on Days 3, 4 and 5 and ovarian scores on Days 4 and 5 were all significantly correlated (P < 0.05) with ovulation rate. In both trials, differences in follicle populations were not seen until Day 3 of the superstimulation procedure. Collectively, these trials do not provide strong support for a single injection of FSH, as used here, nor does it indicate a clear advantage for either MGA or PGF as a means of enhancing the ovulation rate or embryo quality.