Factors that influence follicle recruitment, growth and ovulation during ovarian superstimulation in heifers: opportunities to increase ovulation rate and embryo recovery by delaying the exposure of follicles to LH.

摘要

A review. The outcome of ovarian follicle superstimulation protocols in heifers is influenced by the number of follicles that are stimulated to grow and the number induced to ovulate. Only a proportion of the follicles that are stimulated to grow progress to ovulation. The failure of some of these follicles to ovulate may be due not to an intrinsic deficiency but rather to their relatively small size when the FSH treatment is initiated. Consequently, these follicles do not have the opportunity to undergo full maturation within the time-frame of a conventional superstimulation protocol. It is suggested that delaying the LH surge would allow the completion of maturation by a greater number of follicles, resulting in an increased ovulation rate and in the recovery of a greater number of viable embryos. Initial support for this hypothesis was provided by the relatively high ovulation rate and embryo recovery in cows stimulated with FSH for 6 days, commencing on day 2 or day 10 of the oestrous cycle.An increase in ovulation rate was also observed when a norgestomet implant was used to delay the occurrence of the endogenous LH surge by up to 16 h relative to when a spontaneous LH surge would typically occur after superstimulation with gonadotropin. Recently, a GnRH agonist/LH protocol has been developed in which the pituitary is desensitized to GnRH and ovulation is controlled by injection of LH. Preliminary evidence using this protocol indicates that a delay in occurrence of the LH surge by approximately 12-15 h may increase the numbers of ovulations and transferable embryos.