240 RELATIONSHIP BETWEEN PEAK NUMBER OF ANTRAL FOLLICLES AND FOLLICULAR WAVES, HORMONE CONCENTRATIONS, SUPEROVULATORY RESPONSE, AND EMBRYO QUALITY IN BEEF HEIFERS

Abstract

Recent studies in dairy cattle demonstrate that the number of antral follicles during follicular waves is highly variable among animals, but highly repeatable within individuals, and the numbers of follicles during waves are inversely associated with serum FSH concentrations (Burns et al. 2005 Biol. Reprod. 73, 54-62). The aims of this study were to determine (1) serum FSH and estradiol concentrations during follicular waves in beef heifers categorized according to peak number of antral follicles per wave, and (2) the relationship between this categorization and superovulatory response. Estrous cycles of crossbred beef heifers (n = 90) were synchronized with two i.m. injections of PGF2± 11 days apart; animals were scanned daily for 5 days beginning 24 h after the second PG injection to determine the number of follicles during the first follicle wave in two or three consecutive estrous cycles. Heifers were grouped based on the peak number of follicles e 3 mm per wave in diameter (Low: d 15 follicles, n = 12; High: e 25 follicles, n = 11). Intermediate animals were excluded. Heifers were then re-synchronized and blood samples taken every 8 h up to Day 7 to characterize FSH and estradiol profiles. Subsequently, animals were superovulated (pFSH, Folltropin^®; Bioniche Animal Health, Belleville, Ontario, Canada), inseminated, and flushed on Day 7; this procedure was repeated twice. All values for hormone concentrations were aligned relative to the peak FSH value. A mixed model, repeated-measures approach was used to determine if serum FSH and estradiol concentrations were different between groups. Differences in recovery rate (number of oocytes/embryos recovered compared to number of corpora lutea) and proportion of transferable embryos were analyzed using Chi-square analysis. The mean (±SE) peak postovulatory FSH concentration was lower (P d 0.03) for animals with high numbers of follicles. No differences in estradiol concentrations were observed. The mean superovulatory response (number of corpora lutea) was higher (P d 0.05) in the High group than in the low group (17.6 ± 3.6 vs. 8.5 ± 1.1). While there was no difference in the recovery rate of embryos (62 vs. 60%), significantly more (P d 0.05) oocytes/embryos (10.6 ± 2.7 vs. 4.7 ± 0.7) and transferable embryos (5.4 ± 1.3 vs. 3.8 ± 0.8) were recovered per animal in the High group. Of the embryos recovered, the proportion of transferable quality was higher for animals in the Low group (80 vs. 51%; P d 0.05). In conclusion, the number of antral follicles during a follicular wave in beef heifers is inversely related to peak postovulatory FSH concentration, but is unrelated to estradiol concentration. In addition, while the superovulatory response and number of transferable embryos were greater in animals with a high number of antral follicles, embryo quality, measured as the proportion of transferable embryos per donor, was reduced. We conclude that cattle with relatively high numbers of follicles per wave respond best to standard superovulation protocols. However, the reason the proportion of high quality embryos is reduced in cattle with high vs. low numbers of follicles per wave is unknown.