SYNCHRONIZATION OF OVARIAN FOLLICULAR WAVES WITH A GONADOTROPIN-RELEASING-HORMONE AGONIST TO INCREASE THE PRECISION OF ESTRUS IN CATTLE - AREVIEW

Treatment with GnRH and PGF(2 alpha) is a practical method for control ling ovarian follicular and luteal functions and increasing the precis ion of estrus synchronization in cyclic and acyclic postpartum cows an d heifers. This method reduces considerably the period of time needed for estrus detection; it synchronizes the estrous cycle of 70 to 80% o f the cyclic cows to within a 4-d interval without any detrimental eff ect on the fertility rate (65 to 85%). Moreover, resumption of ovarian activity and normal fertility in acyclic cows is favored. Administrat ion of GnRH eliminates the large follicles by ovulation or atresia and induces emergence of a new follicular wave within 3 to 4 d after trea tment at any stage of the estrous cycle, but it Limits further growth of these emerging follicles by increasing atresia. The precision of es trus and the unaltered fertility rate is due to the synchronized selec tion of a new large growing follicle, which becomes the ovulatory foll icle after PGF2(alpha)-induced luteolysis 6 d after GnRH treatment. Al so, fixed-time Al programs without the need for estrus detection may b e possible using a second injection of GnRH in a GnRH-PGF(2 alpha)-GnR H protocol to ovulate the selected follicle at a precise time. We desc ribe a physiological model to explain how the precision of estrus is i mproved following PGF(2 alpha)-induced luteolysis, via the effect of p retreatment with GnRH on follicular development and luteal functions i n cattle. Application of this model to the development of reliable met hods of fixed-time insemination is also explored.