ESTRUS SYNCHRONIZATION AND FERTILITY AFTER THE CONTROL OF FORMATION AND REGRESSION OF THE CORPUS-LUTEUM, AND EMERGENCE OF THE OVARIAN DOMINANT FOLLICLE IN CATTLE

Three experiments were conducted with the aim of improving estrus sync hronization and conception rates to Al after progestogen-based estrus synchronization. In Experiment 1, stage of the estrous cycle was synch ronized in heifers, and on Day 6 of the ensuing cycle (Day 0 of treatm ent) heifers (n=14) received a norgestomet implant, with prostaglandin (PG) administered on Days 0 and 1. This resulted in an increase in pu lsatile secretion of LH and continued growth of the dominant follicle. On Day 4, groups of heifers received 500 (n=4), 1,000 (n=4), 1,500 (n =3) or 3,000 (n=3) IU hCG, which induced the dominant follicle to ovul ate. This was followed within 48 h by the emergence of a newly-recruit ed dominant follicle that ovulated after the removal of norgestomet an d injection of PG on Day 10. The dose of 500 IU hCG was sufficient to ovulate the dominant follicle on Day 4, and this dose was used in subs equent experiments. In Experiment 2, stage of the estrous cycle was sy nchronized in cows, and on Day 6 of the ensuing cycle (Day 0 of treatm ent) the cows were given PG and a single norgestomet implant On Day 4 of treatment cows were divided into 2 groups: Group 2A (n=17), injecti on of PG; Group 2B (n=13), injection of PG plus 500 IU hCG. Norgestome t implants were removed on Day 10, coinciding with an additional injec tion of PG. Cows were inseminated 12 h after estrus. Cows that receive d hCG (ovulated a newly-recruited dominant follicle) had a higher (P<0 .05) pregnancy rate than control cows (ovulated a persistent dominant follicle; 61 and 23%, respectively). In Experiment 3, heifers at diffe rent stages of the estrous cycle were given PG and a norgestomet impla nt on Day 0, and a second injection of PG plus 500 IU hCG on Day 4. Th e heifers were divided into 2 groups and those in Group 3A (n=9) were given PG on Day 10, with the norgestomet implant removed on Day 11. He ifers in Group 38 (n=9) were given PG at implant removal on Day 11. In jection of PG 24 h before norgestomet implant removal induced an earli er onset of estrus and tended to improve the closeness of estrus synch rony, but pregnancy rates (89%) were the same for both groups of heife rs. The norgestomet/hCG regimen allows for control of the development and regression of the corpus luteum (CL), together with ovulation of a newly-recruited dominant follicle. This protocol can be further devel oped for estrus synchronization programs to consistently achieve close synchrony of estrus and high fertility in cattle.