Effect of a High Energy Diet on Steroidogenic Capacity of Dominant Follicles, Secretion of LH and Estradiol, and Precocious Puberty in Beef Heifers.

Abstract

The objectives of the present study were to evaluate the impact of a high energy diet in early weaned heifers on estrogenic capacity of the dominant follicle and related changes in systemic hormone concentrations. Twenty heifers were weaned at 86 ± 2 d of age and 137 ± 3.3 kg body weight. Heifers were randomized to receive isonitrogenous diets (14.1% crude protein) that contained either a high energy content (HIGH; 2.2 Mcal/kg; n = 10) or a lesser energy content (control diet; CONT; 1.70 Mcal/kg; n = 10) from weaning until approximately 300 d of age. Target rates of gain were 1.50 kg/d for the HIGH and 0.75 kg/d for the CONT treatment. Within treatments, heifers were designated to be ovariectomized (OVX) to assess follicular function, or to remain intact (INT) for assessment of secretion of LH and precocious (≤ 300 days of age) puberty. In the CONT-OVX and HIGH-OVX treatments, daily ultrasonographic evaluation of ovarian follicles was initiated at 130 days of age and continued through two complete follicular waves. In the subsequent (3rd wave), the ovary bearing the dominant follicle was removed the day after it achieved a diameter of 9 mm to assess intrafollicluar estradiol concentrations. Daily blood samples were collected to evaluate systemic estradiol concentrations. In the CONT-INT and HIGH-INT treatments, serial blood samples were collected at 15 min intervals for 12 hr at 145, 170, 208 and 236 days of age to assess secretion of LH. Weekly blood samples were collected and analyzed for progesterone concentrations to determine age at puberty. Average daily gain was greater (P < 0.05) in the HIGH than CONT treatment (1.57 ± 0.16 vs. 1.21 ± 0.13 kg/d respectively) and body weight was greater (P < 0.05) in the HIGH than CONT treatment from 186 d of age (treatment x age, P < 0.05). Precocious puberty occurred in 3/6 heifers in the HIGH-INT and 4/5 heifers in the CONT-INT treatment. For heifers that reached puberty, age at puberty was greater (P < 0.05) in the CONT-INT (253 ± 1 d of age) than HIGH-INT (194 ± 9 d of age) treatment. Body weight at puberty was also greater (P < 0.05) in the CONT-INT than HIGH-INT treatment. Mean LH concentration was greater (P < 0.05) in the HIGH-INT than CONT-INT treatment at 170 and 208 days of age but did not differ on other days of serial blood sample collection. Neither frequency or amplitude of LH pulses differed between treatments. Systemic estradiol concentrations in the CONT-OVX were greater (P < 0.05) than in the HIGH-OVX treatment on the day of ovariectomy (1.68 ± 0.21 and 0.94 ± 0.25 pg/ml, respectively) and the day preceding ovariectomy (2.16 ± 0.37 and 0.86 ± 0.32 pg/ml, respectively). Follicular fluid concentrations of estradiol did not differ between the HIGH-OVX (91.4 ± 36.4 ng/ml) and CONT-OVX (110.0 ± 88.49 ng/ml) treatments. In conclusion, the HIGH dietary treatment increased body weight gain and LH secretion and resulted in an earlier age at puberty, however, the incidence of precocious puberty did not differ between treatments. Intrafollicular concentrations of estradiol did not differ between the HIGH-OVX and CONT-OVX treatments. It is unknown whether the lack of difference due to diet is due to the unexpectedly high incidence of precocious puberty in the CONT treatment, or if the mechanism of action of the HIGH treatment is exerted at alternative sites within the reproductive axis. (poster)