Ovarian morphology and follicular dynamics associated with ovarian aging in Bos indicus beef cows

Abstract

This study characterizes the ovarian volume, antral follicle count (AFC), and ovarian dynamics of cows at different ages. Nellore cows (Bos taurus indicus) were used in two experiments. In Experiment 1, 57 lactating cows ranging in age from 3 to 23 years had their estrous cycle synchronized based on progesterone/estradiol treatments. The ovaries were scanned by ultrasound to record ovarian volume and AFC prior to follicle aspiration (AFC1). A second AFC (AFC2) was performed 5 days later to count only the growing antral follicles. In Experiment 2, six long-lived (14- to 23-year-old) and three young (4- to 8-year-old) non-lactating cows were submitted to daily ovarian scanning ultrasound during an interovulatory interval. Blood samples were collected during the estrous cycle to assess serum progesterone concentration. Data were analyzed using Student's t-test, ANOVA, and regression analysis tests. In Experiment 1, there were more (P < 0.05) antral follicles in AFC1 (31.4 ± 3.5) than in AFC2 (22.6 ± 2.4). In AFC1, the volume of the right ovary (6.03 ± 0.5 cm³) was greater (P < 0.01) than that of the left one (4.53 ± 0.4 cm³), although the AFC did not differ between the two ovaries (15.3 ± 1.8 and 16.3 ± 1.8, respectively). In both AFC1 and AFC2, there was a decline in the number of antral follicles as the age of the cow increased (P < 0.01). Ovarian volume (average of both ovaries) was related (P < 0.01) with AFC (R² = 0.1499) and cow age (R² = 0.0911). In Experiment 2, young and old cows under the age of 20 had a pattern of follicular growth waves, while cows over 20 years old did not have waves of follicular growth. The progesterone profiles and corpus luteum size during the estrous cycle did not differ between the groups. In conclusion, although cows had increased ovarian volume and decreased follicular population as they age, the follicular growth pattern and corpus luteum functionality appear to be unaffected by age.

Keywords: AFC; Estrous cycle; Fertility; Ovarian reserve.