The number of follicles recruited in successive waves are consistent in postpubertal cattle (Singh et al. 2004 Theriogenology 62, 227), but ovarian response to gonadotropin stimulation is highly variable among animals. We tested the hypotheses that the number of follicles present at wave emergence are repeatable and are predictive of superstimulatory response in prepubertal calves; therefore, we expected that calves with higher antral follicular counts (AFC; follicles = 1mm) at wave emergence will result in a greater number of follicles available for oocyte collection after a conventional 4-day gonadotropin treatment. Hereford crossbreed calves (n = 52; 5.1 to 6.8 months of age) were ranked according to the number of follicles = 1 mm detected in transrectal ultrasound video recordings of both ovaries at the time of wave emergence (First AFC; range: 12 to 53 follicles). Calves in the bottom (Low AFC; <20 follicles; n = 6) and top (High AFC; >32 follicles; n = 5) quartiles were selected for ovarian superstimulation. Emergence of a new follicular wave (Day 0) was induced by transvaginal follicle ablation (14 to 57 days after first AFC; 5.7 to 7.1 months of age), AFC were performed again (Second AFC), and calves were given eight 12-hourly IM injections of 25 mg of pFSH (Folltropin-V®, Bioniche Animal Health Inc., Belleville, Canada) beginning on Day 0.5. All calves were given 12.5 mg of pLH (Lutropin-V®, Bioniche Animal Health Inc.) IM 12 h after the last FSH and number of follicles equal to 3, 3 to 5, 6 to 8, and equal to 6 and 9 were counted 24 h after LH treatment (at the time of oocyte collection). A t-test was used to compare the number of follicles and ovarian response (Low v. High AFC). Values of Pearson (0.8; P < 0.001) and Spearman (0.9; P < 0.001) correlation coefficients between First and Second AFC indicate strong repeatability of numbers of follicles present at the time of wave emergence. As expected, mean number of follicles were greater (P = 0.01) in the High- than Low-AFC group (24.2 ± 2.0 v. 15.7 ± 1.0) at the Second AFC. The High-AFC group had a greater number of follicles at oocyte collection than Low AFC for 6 to 8 mm (13.4 ± 2.1 v. 5.3 ± 1.7; P = 0.01), but not for 3 to 5 mm (9.4 ± 2.5 v. 5.3 ± 2.1; P = 0.2) or 9 mm (7.6 ± 2.9 v. 4.8 ± 2.0; P = 0.4) size categories. However, High AFC resulted in a greater total number of follicles 3 (30.4 ± 3.1 v. 15.5 ± 3.2; P = 0.009) and 6 mm (21.0 ± 4.1 v. 10.2 ± 2.9; P = 0.05). The number of 6-mm follicles at the end of superstimulation represented 80 and 60% of 1-mm follicles at wave emergence in the High- and Low-AFC groups (P = 0.3). In conclusion, the number of follicles at the beginning of a wave are predictive of follicles recruited into subsequent waves in 7-month-old calves, and higher AFC at wave emergence resulted in a greater number of 3- and 6-mm follicles available for oocyte collection.
Research was supported by the Natural Science and Engineering Council of Canada (NSERC). Hormones provided by Vetoquinol Inc. ARTK funded by CNPq-Science Without Borders, Brazil.
Manpower Assessment Brief No. 32: Underrepresentation of women and minorities in natural science and engineering bachelor`s degrees
