Abstract

The objective of the present study was to evaluate the ovarian response of cows to gonadotropin-releasing hormone (GnRH) injection on day 6 of the estrous cycle. The estrous cycles of 10 cows were synchronized with 2 intramuscular (im) injections of prostaglandin F2α given 11 d 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 d 6 of the estrous cycle (estrus = d 0). Daily, from estrus d 0 to the next estrus d 23, cows had their ovaries scanned by ultrasound. Profiles of the mean number of follicles (that is, 4 - 6 mm and ≥ 7 mm) and the diameter of the largest follicle (F1) were compared by least squares analysis of variance using the general linear model (GLM) procedure of SAS. In response to an injection of GnRH, the number of small follicles in group GnRH6 on day 6 (1.5 ± 0.6 follicles) increased (P ≤ 0.05) on day 8 (5.0 ± 0.7 follicles) and the number of medium and large follicles (≥ 7 mm) on day 6 (2.0 ± 0.5 follicle) decreased (P ≤ 0.05) on day 9 (0.8 ± 0.6 follicles). In conclusion, an injection of GnRH on day 6 of the estrous cycle could promote the emergence of a new follicular wave in cows.

Key words: Ultrasonography, follicle, GnRH, Iranian Holstein cows.