Induced ovulation of the first <i>postpartum</i> dominant follicle in beef suckler cows using a GnRH analogue

Abstract

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 × 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 μg 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 &amp;gt; 0.05). The proportion of cows ovulating the first dominant follicle was higher (P &amp;lt; 0.05) following GnRH treatment (12 of 12 and 7 of 10; GnRH-G and GnRH-D, respectively) than with controls (2 of 12). The mean interval from parturition to first ovulation in all cows in the GnRH-G treatment was reduced (16.1 ± 0.8 days; P &amp;lt; 0.01) compared with the interval for cows in the GnRH-D treatment (24.5 ± 3.6 days) or controls (27.1 ± 2.5 days). There was no difference in the duration of the first luteal phase post partum in control (9.3 ± 3.0 days) and GnRH-treated cows (8.0 ± 1.0 and 9.6 ± 3.7 days; GnRH-G and GnRH-D, respectively) or in the proportion of cows with short cycles (4 of 7, 5 of 6 and 4 of 5; control, GnRH-G and GnRH-D, respectively). In conclusion, a single injection of GnRH analogue during the growing–plateau or declining phase of the first postpartum dominant follicle of beef suckler cows induced ovulation in most cows but did not alter the proportion of cows with short cycles compared with controls.