171 ADMINISTRATION OF GnRH ON DAY 6 ALTERS THE NUMBER OF FOLLICULAR WAVES FROM 2 TO 3 DURING THE ESTROUS CYCLE OF HOLSTEIN COWS

Abstract

It is suggested that pregnancy rate is greater in lactating cows inseminated following ovulation of a third-wave follicle compared with a second-wave follicle. The number of follicular waves is not apparent during the estrous cycle. However, GnRH injection on Day 6 is supposed to initiate a new follicular wave earlier; as a result, the number of cows with 3 follicular waves will be increased. This study was done to change the 2-follicular-wave cycles to 3 follicular waves during the estrous cycle. The estrous cycles of 10 cows were synchronized with 2 i.m. injections of prostaglandin F_2α given 11 days apart. The cows were randomly assigned to 1 of 2 treatments. Cows in the control treatment received no treatment, whereas GnRH6 cows received a GnRH injection on Day 6 of the estrous cycle (estrus = Day 0). Ovaries of all cows were scanned daily, from estrus (Day 0) to the subsequent estrus (Day 23). Profiles of the mean number of follicles (i.e. 4- to 6-mm and ≥7 mm), the diameter of the largest follicle (F1), and comparison of 2- and 3-wave cows were compared by least squares analysis of variance using the general linear model (GLM) procedure of SAS. The number of 4 to 6 mm follicles had a tendency to decline until Day 6 (3.0 ± 2.1 v. 0.8 ± 1.6). In response to an injection of GnRH, the number of small follicles in group GnRH6 on Day 6 (1.5 ± 1.6 follicles) increased (P < 0.05) on Day 8 (5.0 ± 1.7 follicles) and the number of medium and large follicles (≥7 mm) on Day 6 (1.8 ± 0.5 follicles) decreased (P < 0.05) on Day 9 (0.8 ± 0.6 follicles). Over the 4-day period before GnRH treatment, the diameter of F1 was increasing (from 4.7 ± 0.9 mm to 10.0 ± 0.6 mm) in cows treated with GnRH on Day 6 of the estrous cycle; thereafter, the diameter of the F1 decreased (P < 0.05) within 2 days of GnRH treatment. In the control group the diameter of the F1 increased over the 4-day period before GnRH treatment (from 5.0 ± 0.9 mm to 11.2 ± 0.6 mm), whereas it remained constant (11.2 ± 1.2 mm to 11.2 ± 1.2 mm) between Days 6 and 9 of the estrous cycle (P < 0.003). Results showed that 4 out of 5 cows in control group had 2 follicular waves, whereas all of the cows (n = 5) in GnRH6 group had 3 follicular waves. The first follicular wave begins at Day 1.1 ± 0.50 and 0.70 ± 0.50 in the control and GnRH groups, respectively. The second follicular waves appeared at Day 11.00 ± 0.80 and 8.30 ± 1.50 (P ≤ 0.05) for the 2- and 3-wave animals, respectively. The third follicular waves (n = 6) initiated at Day 16.30 ± 1.50. Cows with 2 follicular waves had shorter estrous cycles (P ≤ 0.05) than 3-wave cows (20.50 ± 0.60 v. 22.40 ± 0.90). The ovulatory follicle in 2-wave cows grew larger (14.00 ± 1.80 v. 12.50 ± 0.90 mm; P ≤ 0.05), and maintained for a longer period of time (9.50 ± 1.80 v. 6.1 ± 0.90; P ≤ 0.05) than in 3-wave cows. In conclusion, an injection of GnRH on Day 6 of the estrous cycle could promote the emergence of a new follicular wave earlier in cows and alter the estrous cycles with 2 waves into 3 follicular waves.