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 synchroniza tion scheme, gonadotropin injection protocol and presence of a large o varian follicle on response to superstimulation of follicular developm ent and the ensuing superovulation. Estrus was synchronized with eithe r a progestin compound (MGA) or by the use of a luteolytic agent (PGF) . Superstimulation was induced with 280 mg equivalents of pFSH adminis tered either by a single subcutaneous injection or by a series of 8 in tramuscular injections over 4 d. Follicular development was followed f or 5 d with real-time ultrasound, and the heifers were retrospectively classified as to the presence or absence of a large follicle (greater than or equal to 8 mm; morphologically dominant follicle) at the star t 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 s cheme, injection protocol and morphologically dominant follicle (P=0.0 5). The number of large follicles on Day 5 (Day 0 = day of start of su perstimulation) and ovarian score (scale 1 to 5 based on extent of fol licular development; 1=least, 5=most) on Day 5 were significantly corr elated (P<0.05) with ovulation rate. In Trial II (20 heifers), the ovu lation rate, number of embryos recovered, number of transferable embry os 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 (gre ater than or equal to 8 mm) on Days 3, 4 and 5 and ovarian scores on D ays 4 and 5 were all significantly correlated (P<0.05) with ovulation rate. In both trials, differences in follicle populations were not see n until Day 3 of the superstimulation procedure. Collectively, these t rials 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 PG F as a means of enhancing the ovulation rate or embryo quality.