Ovarian follicular development in prepubertal heifers is influenced by level of dietary energy intake

Objectives were twofold: 1) to determine the chronology of development of dominant ovarian follicles during the peripubertal period in heifers and 2) to determine whether feeding a diet with low energy content that delays onset of puberty alters chronology of dominant ovarian follicular development in peripubertal heifers. Ten heifers of composite breeding (1/4 Angus, 1/4 Hereford, 1/4 Red Poll, 1/4 Pinzgauer) were randomly assigned, at 8 mo of age, to receive a diet designed to produce 0.9 (n = 5) or 0.3 (n = 5) kg body weight gain per day for the duration of the experiment. To characterize changes in size of ovarian follicles, real-time linear ultrasonography of ovaries was conducted in all heifers every other day until puberty occurred. Blood samples were collected weekly to determine concentrations of progesterone and 17β-estradiol. Determination of time of puberty was based on increased concentrations of progesterone, ultrasound depiction of ovulation, and subsequent presence of a corpus luteum. Size of the dominant ovarian follicles differed prior to puberty (p < 0.03); diameter of the dominant ovarian follicle was greater in all heifers as the first ovulation approached as compared to earlier in prepuberty. Heifers fed the greater amount of energy exhibited larger dominant ovarian follicles at a younger age in comparison to heifers fed the lower amount of energy. Age and weight at puberty differed among heifers receiving diets with greater energy content compared to those receiving diets with lower energy content (372 ± 7 and 435 ± 9 days of age at puberty, respectively, p < 0.003; 263 ± 7 vs. 221 ± 3 kg, respectively, p < 0.01). Persistence of dominant ovarian follicles did not differ among heifers fed the diet with greater energy compared to heifers fed the lower-energy diet. Neither growth rate of dominant ovarian follicles nor rate of regression differed consistently between treatment groups. Additionally, size of the ovulatory follicle at puberty did not differ between treatment groups; however, 4 of the 5 heifers fed the diet with greater energy content had a luteal phase of abnormal length subsequent to the first ovulation, whereas all of the heifers fed the low-energy diet had luteal phases of normal length subsequent to first ovulation. We conclude that the chronology of development of dominant ovarian follicles differs in heifers fed diets of different energy content, with maturational events being delayed in heifers fed a lower-energy diet.