Nutritionally induced anovulation in beef heifers: Ovarian and endocrine function preceding cessation of ovulation

Angus x Hereford heifers were used to determine endocrine and ovarian funct ion preceding nutritionally induced anovulation. Six heifers were fed to ma intain body condition score (M), and 12 heifers were fed a restricted diet (R) until they became anovulatory. Starting on d 13 of an estrous cycle, he ifers were given PGF(2 alpha) every 16 d thereafter to synchronize and main tain 16 d estrous cycles. Ovarian structures of M and R heifers were monito red by ultrasonography daily from d 8 to ovulation (d 1 of the subsequent c ycle) until R heifers became anovulatory. Concentrations of LH and FSH were quantified in serum samples collected every 10 min for 8 h on d 2 and 15 ( 48 h after PGF(2 alpha)), and estradiol and IGF-I were quantified in daily plasma samples from d 8 to 16 during the last ovulatory cycle (Cycle -2) an d the subsequent anovulatory cycle (Cycle -1). During the last two cycles b efore anovulation, M heifers had 50% larger (P < .0001) ovulatory follicles than R heifers and 61% greater (P < .0001) growth rate of the ovulatory fo llicles. There was a treatment x cycle x day effect (P < .001)for concentra tions of estradiol. The preovulatory increase in estradiol occurred in the R and M heifers during Cycle -2 but only in M heifers during Cycle -1. A tr eatment x cycle x day effect (P < .05) influenced LH concentrations. During Cycle -2, LH concentrations were similar for M and R heifers, but during C ycle -1, M heifers had greater LH concentrations than did R heifers. Concen trations of FSH were greater (P < .05) in R than M heifers after induced lu teolysis when R heifers failed to ovulate. There was a treatment x cycle in teraction (P < .05) for IGF-I concentrations, and M heifers had 4.7- and 8. 6-fold greater IGF-I concentrations than did R heifers during Cycle -2 and -1, respectively. We conclude that growth rate and diameter of the ovulator y follicle, and concentrations of LH, estradiol, and IGF-I are reduced befo re the onset of nutritionally induced anovulation in beef heifers.