THE EFFECT OF A GNRH ANALOG ON THE DYNAMICS OF FOLLICULAR DEVELOPMENTAND SYNCHRONIZATION OF ESTRUS IN LACTATING CYCLIC DAIRY-COWS

A GnRH analogue was used to synchronize ovarian follicular development prior to an injection of PGF(2 alpha) for the synchronization of estr us in lactating Holstein cows. On Day 12 (estrus = Day 0) of the exper imental cycle, cows (n = 8) were injected with 8 mu g Buserelin (BUS g roup), followed by 25 mg PGF(2 alpha) 7 d later (Day 19). Control cows (n = 7) received PGF(2 alpha) on Day 12 (PGF group). Ovaries were sca nned daily via ultrasonography, and plasma progesterone and estradiol concentrations were determined. Sizes of all visible follicles were re corded. Follicles were classified as small (3 to 5 mm), medium (6 to 9 mm), or large (greater than or equal to 10 mm). Between Days 12 and 1 6 of the cycle, the number of large follicles in PGF cows remained unc hanged (1.2), whereas in the BUS group, the number of large follicles decreased from 1.3 on Day 12 to 0.5 on Day 15. Only 4 of 7 PGF cows ov ulated a second-wave dominant follicle. In the BUS group, 7 of 8 cows ovulated a GnRH analogue induced dominant follicle that was first iden tified on Day 15. During the follicular phase (last 5 d prior to estru s), plasma progesterone declined in association with CL regression in both groups, and estradiol concentrations increased, reaching higher ( P<.0.05) preovulatory peak concentration in BUS cows than in PGF cows (14.0 +/- 1.0 vs 10.4 +/- 1.1 pg/ml). The number of medium-size follic les was smaller and the number of small-size follicles tended to be hi gher in BUS cows than in the PGF-treated group. On the day of estrus, the size of the ovulatory follicle (16.1 vs 13.3 mm) and the size diff erence between the ovulatory and second largest follicle (11.4 vs 6.2 mm) were both larger in BUS cows than in PGF-treated cows, suggesting a more potent dominance effect of the ovulatory follicle in the BUS co ws. This study suggests that a GnRH analogue can alter follicular deve lopment prior to synchronization of estrus with an injection of PGF(2 alpha) in lactating dairy cows.