Follicular development and steroid concentrations in cows with different levels of fertility raised under nutritional stress

The aim of the present study was to characterize ovarian follicular development and steroid concentrations during postpartum and the estrous cycle of Brangus Ibagé cows (3/8 Nelore+5/8 Aberdeen Angus) with different levels of fertility. Cows were classified as having high or low fertility according to the calving interval (CI). The average CI of the herd from which cows used in this study were selected was 404.6±5.44 and 711.2±20.89 days for the high and low fertility groups, respectively. Four cows of high fertility and five cows of low fertility had calves removed between 70 and 100 days after parturition. Ovarian activity was monitored daily by ultrasound for 16 days after calf removal. Days to emergency of the first follicular wave after calf removal, number of follicles with diameter >9 mm, growth rate of largest follicle, maximum diameter of largest follicle, length (days) and number of follicular waves were recorded. During this period, blood was collected daily for measurements of serum progesterone (P4) and estradiol (E2) concentrations. In another experiment, ovarian activity and P4 and E2 concentrations were examined during estrous cycle in five cows of high fertility and four cows of low fertility. Ovarian activity and steroid concentrations were assessed from the day prior to estrus to the 15th day of the estrous cycle (estrus=day 0). In postpartum cows of high fertility, the total number of follicles >5 mm and the maximum diameter of the largest follicle were higher than in cows of low fertility (P<0.05). Concentrations of P4 and E2 did not differ between groups in the postpartum cows. However, E2 increased 5 days after calf removal (around 90 days of postpartum) in the high fertility group, followed by an increase in P4 with average values indicating ovulation around 100 days postpartum. In cycling cows, the profile of follicular development was similar between cows of high and low fertility. There was no difference between groups for number of follicles >5 mm, but the day effect was significant (P<0.01). Plasma concentrations of P4 and E2 were similar in both groups. These data suggest that cows, from a population raised in the same environment have different fertility as a consequence of individual physiological characteristics.