DIFFERENTIAL RESPONSE OF THE LUTEAL-PHASE AND FERTILITY IN CATTLE FOLLOWING OVULATION OF THE FIRST-WAVE FOLLICLE WITH HUMAN CHORIONIC-GONADOTROPIN OR AN AGONIST OF GONADOTROPIN-RELEASING-HORMONE

A series of experiments with Holstein heifers was conducted to develop the capability of inducing accessory corpus luteum (CL) with a GnRH a gonist (Buserelin, 8 mu g; GnRHa) or hCG; (3,000 IU) to increase plasm a progesterone concentrations (Exp. 1, 2, and 3) and to test whether i nduction of accessory CL with hCG will increase conception rates in he ifers (Exp. 4) and lactating cows (Exp. 5). In Exp. 1, heifers were tr eated on d 5 after estrus with GnRHa (n = 8) or saline (n = 7); heifer s in Exp. 2 received hCG (n = 5) or saline (n = 4) on d 5. Experiment 3 allowed a contemporary evaluation of heifers treated on d 5 with GnR Ha (n = 6), hCG (n = 6), saline (n = 6), or GnRHa at d 5 and hCG at th e time of the induced ovulation (n = 5). The GnRHa and hCG were equall y effective in inducing an accessory CL (93% induction rate), but the subsequent increase in progesterone concentrations was greater in hCG- treated heifers. A greater half life of hCG may provide longer LH-like stimulation of the first-wave follicle and subsequent developing acce ssory CL or a greater luteotropic effect on the original CL. Induction of an accessory CL with hCG on d 5 or 6 after insemination did not in crease pregnancy rates in fertile heifers (Exp. 4: hCG = 64.8% vs cont rol = 62.9%; n = 243) or lactating dairy cows during summer heat stres s (Exp. 5: hCG = 24.2% vs control = 23.5%; n = 201).