251 FOLLICLE DEVIATION AND OVULATORY CAPACITY IN BOS INDICUS HEIFERS

Abstract

In Holstein cows, the diameter of the dominant follicle (DF) at the time of follicle deviation is 8.5 mm and the subordinate follicle (SF) is 7.2 mm (Ginther et al. 1996 Biol. Reprod. 55, 1187–1194). However, follicular responsiveness to an ovulatory treatment occurs only with 10.0-mm-diameter follicles (Sartori et al. 2001 Biol. Reprod. 65, 1403–1409). The current study tested the hypothesis that, in Bos indicus (Nelore and crossbred Nelore × Gir) females, the follicular diameters at the time of deviation and ovulation responsiveness are smaller than those in Holstein cows. The experiment was performed in two phases. In the first phase, 12 Nelore heifers were previously synchronized with a protocol using progestagen and estradiol benzoate. After implant removal, all heifers were evaluated by transrectal ultrasonography (Aloka SSD-500, Tokyo, Japan) every 12 h until Day 5 of the estrous cycle (Day 0 = Day of the ovulation) to assess the time of ovulation, the time of follicle deviation, and the follicular diameter at the deviation. In the second phase, 29 Bos indicus heifers (Nelore and crossbred Nelore × Gir) were previously synchronized with the same protocol as cited above. After the ovulations (Day 0), the follicles were evaluated by transrectal ultrasonography every 24 h, until they reached the diameter of 7.0–8.4 mm (n = 9); 8.5–10.0 mm (n = 10); and >10.0 mm (n = 10). In order to assess the ovulatory capacity, all animals were treated with 25 mg of LH (Lutropin-V^®; Bioniche Animal Health, Inc., Belleville, Ontario, Canada) at these follicle diameter ranges. After the LH treatment, all animals were monitored by ultrasonography every 12 h for 48 h. ANOVA, Bartlett, and chi-square tests were used in the statistical analyses. In the first phase, the diameters of the DF and SF at the time of follicular deviation (61.9 ± 4.9 h after ovulation) were 6.2 ± 0.2 and 5.8 ± 0.2 mm, respectively. In the second phase, the the average follicular diameters at the time of LH administration in the groups 7.0–8.4 mm, 8.5–10.0 mm, and >10.0 mm were 7.6^a ± 0.1 mm, 9.6^b ± 0.1 mm, and 10.9^c ± 0.2 mm; and their ovulation rates were 33.3%^a (3/9), 80.0%^b (8/10), and 90.0%^b (9/10), respectively (P < 0.05). The interval from LH treatment to ovulation was 38.0 ± 4.0 h, 31.5 ± 2.7 h, and 30.0 ± 2.0 h, respectively (P > 0.05). In conclusion, in Bos indicus heifers, follicle deviation occurred with smaller diameters than previously reported in Bos taurus breeds. In addition, Bos indicus heifers are able to ovulate in response to 25 mg of LH with smaller diameters compared to those of Bos taurus breeds. Moreover, in Bos indicus heifers, ovulatory capacity is acquired by follicles as small as 7.0–8.4 mm, but this responsiveness significantly increases after follicles reach 8.5–10.0 mm.

This work was supported by FAPESP (Proc:03/10203-4); Bioniche Animal Health, Inc., Belleville, Ontario, Canada; and Tecnopec, São Paulo, Brazil.