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

The outcome of ovarian follicular superstimulation protocols in heifers is influenecd by the number of follicles that are stimulated to grow and the number induced to ovulate. At present, only a proportion of the follicles that are stimulated to grow progress to ovulation. The argument is developed in this review that 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 Based on this argument, we propose that delaying the LH surge would allow for completion of maturation by a greater number of follicles, resulting in an increased ovulation rate and in 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 d, commencing on Day 2 or Day 10 of the estrous cycle. An increase in ovulation rate was also observed when a norgestomet (NOR) 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 was developed in which the pituitary in heifers 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 to 15 h may increase the number of ovulations and transferable embryos. (C) 1998 by Elsevier Science Inc.