USE OF A GONADOTROPIN-RELEASING-HORMONE AGONIST OR HUMAN CHORIONIC-GONADOTROPIN FOR TIMED INSEMINATION IN CATTLE

Three experiments were conducted to evaluate a synchronization protoco l with AI at a predetermined time. In Exp. 1, 169 dairy heifers were a ssigned randomly to two groups: 1) timed AI (TAI), consisting of GnRH agonist injection (d 0, 1700), PGF(2 alpha) injection (d 7, 1700), GnR H agonist injection (d 8, 1700), and AI (d 9, 0800); and 2) Al at estr us (AIE), consisting of GnRH agonist injection (d 0, 1700), PGF(2 alph a) injection (d 7, 1700), and AI at detected estrus. Pregnancy rate wa s 25.8% for TAI (n = 89) compared with 48.7% for AIE (n = 80; P < .001 ). Experiment 2 was comparable to Exp. 1, but the second GnRH agonist injection in TAI was given 48 h after injection of PGF(2 alpha). Heife rs in TAI (n = 187) were inseminated at detected estrus if estrus occu rred within 39 h after administration of PGF(2 alpha) (n = 47). Pregna ncy rates were 45.5% for TAI and 48.0% for AIE (n = 177). Conception r ate was reduced for TAI (45.5 [85/187] < 61.2% [185/139]; P < .005). I n Exp. 3, the second injection of GnRH agonist, given at 48 h after in jection of PGF(2 alpha), was replaced with hCG; (3,000 IU, i.m.). No d ifferences in pregnancy rate were detected for TAI (52.9% [54/102]) vs AIE (56.1% [55/98]). Conception rate was reduced for TAI (52.9 [54/10 2] < 72.3% [55/76]; P < .005). Delaying the second GnRH agonist inject ion by 24 h improved pregnancy rate, but replacing the second injectio n of GnRH agonist with an injection of hCG; did not prevent a reductio n in conception rate.