175 METABOLIC STRESS IMPAIRS FOLLICULAR GROWTH IN SUPEROVULATED HEIFERS

Abstract

Metabolic stress during the early postpartum period has been postulated to be one of the major causes of prolonged calving intervals. To study the impact of metabolic stress on ovarian follicular function via hormonal analysis of follicular fluid and follicular growth, 12 cycling heifers were randomly allocated to a group (n = 6) that received a control diet (ad libitum grass silage) or to an experimental group (n = 6) that was fasted during the superovulation (SO) treatment for 4 days until ovariectomy (Jorritsma et al. 2003 Theriogenology 60, 151–161). Heifers were synchronized with a CIDR^® intravaginal device (Pfizer AH) during 7 days, and a prostaglandin injection (5 mL Enzaprost^® CEVA AH) was administered 1 day before CIDR^® removal. On day 9 of the synchronized cycle, the dominant follicle of the first follicular wave was removed and the standard SO protocol was started 2 days later (twice daily decreasing doses for 4 days, total 200 mg of Folltropin-V^®, Bioniche AH). Cows received a CIDR^® device during SO to suppress a spontaneous LH surge. To collect ovarian tissue at 22 h after the LH peak by ovariectomy, a controlled LH surge was induced by a GnRH injection (1 mg of Fertagyl^® Intervet SP AH) at the time of CIDR^® removal (Vos et al. 1994 J. Reprod. Fertil. 100, 387–393). Ovaries were transported to the laboratory at 37°C directly after ovariectomy. For each animal, follicular growth was determined by counting the number of follicles >8 mm in diameter (defined as presumptive follicles). The size category of follicles was based on the volume collected after puncturing (8 to 10 mm, 10 to 12 mm, >12 mm). To determine the quality of the follicle, follicular fluid of each individual follicle was analyzed for estradiol (E2) and progesterone (P4) concentrations. Healthy follicles at 22 h after the LH peak were defined as follicles showing low E2 and high P4 >0.5 μmol L^–1 (E2/P4<1; Dieleman et al. 1983 J. Endocrinol. 97, 31–42). Statistical analysis was performed by logistic regression for grouped data (P < 0.05; mean ± SEM). The total number of follicles per animal (follicles >8 mm in diameter) did not differ between the control (18 ± 2.8) and fasted group (19.8 ± 2.9). However, the number of large follicles (>12mm) that developed in the fasted heifers was significantly lower (2.7 ± 0.9) compared with that in the control heifers (6.0 ± 1.7). No difference was observed for the percentage of healthy follicles per animal between fasted (32 ± 9.2%) and control (56 ± 13%) heifers. These data show that follicular growth was impaired during SO treatment in fasted heifers. Although not significantly different, a numerically lower number of healthy follicles was observed in the fasted group of heifers, possibly indicating a negative influence of metabolic stress on follicular function. Whether metabolic stress affects the metabolic composition of the follicular fluid and as a consequence influences oocyte and embryo quality is under current investigation.