Induced ovulation of the first postpartum dominant follicle in beef suckler cows using a GnRH analogue.

There is a low incidence of ovulation of the first dominant follicle that develops in the early postpartum period of beef suckler cows, which prolongs the interval from calving to first ovulation. The objective of this study was to determine whether a single injection of a GnRH analogue would ovulate the first postpartum dominant follicle. Limousin x Friesian beef suckler cows were assigned at parturition, over two years (16 cows in year 1; 19 cows in year 2), to one of three treatments: (1) untreated (control; n = 12), (2) GnRH analogue (20 micrograms buserelin i.m.) administered in the growing-plateau phase of the first postpartum dominant follicle (GnRH-G; n = 12) and (3) GnRH analogue administered in the declining phase of the first postpartum dominant follicle (GnRH-D; n = 11). From day 8 or 9 post partum, the ovaries of each cow were examined daily by ultrasound to determine the time of GnRH injection and ovulation. Blood samples were collected daily for progesterone measurement to confirm ovulation and in year 2 to determine the duration of the first oestrous cycle. The mean (+/- SEM) number of days from parturition to development of the first dominant follicle was 11.0 +/- 0.3, 10.3 +/- 0.5 and 10.1 +/- 0.7 for cows assigned to treatments 1-3, respectively (P > 0.05). The proportion of cows ovulating the first dominant follicle was higher (P < 0.05) following GnRH treatment (12 of 12 and 7 of 10; GnRH-G and GnRH-D, respectively) than with controls (2 of 12).(ABSTRACT TRUNCATED AT 250 WORDS)