Gyr is the most important zebu breed for dairy herds in Brazil and in other tropical countries. Superovulatory responses in this breed have been shown to be lower than in European or beef zebu breeds such as Nelore and Brahman. The aims of this study were to investigate the effect of the presence of a dominant follicle on the superovulatory response, and to determine endocrine patterns in superstimulated and non-superstimulated Gyr cows. The first experiment was designed to evaluate the effect of the dominant follicle on embryo yield. Multiparous, non-lactating Gyr cows were treated with a conventional superovulation (SOV) protocol [300 IU of Pluset (Serono, Roma, Italy) in 8 decreasing doses] starting on either Day 10 (G10, n = 14) or Day 8 (G8, n = 16) of the estrous cycle or 48 h after dominant follicle removal by ultrasound-guided follicle aspiration (G48, n = 10). Ovarian follicle populations were monitored daily by ultrasonography. Data were analyzed by ANOVA, and means were compared by Tukey's test. Dominant follicle removal resulted in a larger number of small follicles before SOV (27.1 ± 2.7 vs. 14.7 ± 1.5 and 13.1 ± 1.2; P < 0.05), but the number of follicles reaching a diameter larger than 9 mm after superstimulation (17.4 ± 1.3 vs. 14.4 ± 2.0 and 11.4 ± 2.0; P > 0.05), and the number of viable (IETS grades 1 and 2) embryos (3.1 ± 0.8 vs. 3.0 ± 0.7 and 3.3 ± 0.8; P > 0.05) did not differ from G10 and G8 groups, respectively. There was great variation in superovulatory response, and the Pearson correlation between follicle numbers at the time of initiating superstimulatory treatments and response was low (r = 0.49; P > 0.05). In the second experiment, endocrine patterns in superstimulated (n = 32) and non-superstimulated (n = 24) Gyr cows were compared. Blood samples were collected on Day 14 of the estrous cycle or after 4 days of FSH treatment when follicular fluid was also obtained from both groups by ultrasound-guided follicle aspiration. Plasma and follicular fluid samples were stored at −20°C until assay for progesterone (P4), androstenedione (A2), and estradiol (E2) by RIA, using commercial kits (MedLab, Auckland, New Zealand, and Diagnostic Systems Laboratories, Inc., Webster, MN, USA). Mean plasma E2 concentrations did not differ between FSH-treated and control cows (3.7 ± 0.4 vs. 3.2 ± 0.4 pg mL−1; P > 0.05). Intrafollicular concentrations of E2, A2, and P4 in FSH-stimulated follicles (mean size of 14.0 ± 1.2 mm) were 193.5 ± 83.0, 55.7 ± 17.0, and 54.8 ± 28.1 ng mL−1, respectively, lower (P < 0.05) than those found in non-superstimulated growing dominant (mean size of 11.6 ± 0.5 mm) follicles (501.2 ± 83.8, 122.2 ± 22.5, and 97.0 ± 21.9 ng mL−1, respectively), but similar to concentrations in non-superstimulated, non-dominant (mean size of 7.2 ± 0.4) follicles (152.6 ± 99.2, 37.7 ± 20.3, and 37.3 ± 6.8 ng mL−1 for E2, A2, and P4, respectively). Results suggest that factors other than follicle dominance adversely affect superovulatory responses in the Gyr breed, and that follicular steroidogenesis may also be adversely affected following treatment with exogenous FSH. Results also failed to support the use of ultrasonography to predict superovulatory response in the Gyr breed.
Pre- and post-weaning injections of bovine somatotropin to optimize puberty achievement of Bos indicus beef heifers1
