Effect of Different Hormonal Treatments for Synchronizing Estrus on Fertility of Lactating Dairy Cows

This research was designed to study the effectiveness of applying three Timed Artificial Insemination (TAI) protocols on reproductive performance of Holstein Friesian lactating dairy cows. A group of 173 cows received an injection of GnRH on day 44 postpartum (pp) and PGF2α 7 d later, and then assigned into three treatments: (1) CO-72 (n = 65), received GnRH injection and TAI on day 54 pp; (2) MOVS (n = 42), received GnRH injection at 56 h after PGF2α followed by TAI 16 h later; (3) OVS-CO (n = 66), received two GnRH injections on day 54 and 61 followed by PGF2α injection on day 68 before another GnRH injection with TAI on day 71 pp. Pregnancy rates to first AI for cows in both MOVS and OVS-CO treatments tended to be greater (P 0.1) than those for cows in the CO-72 treatment while the overall pregnancy rates for cows in the MOVS treatment tended to be greater (P 0.1) than those for cows in the other two treatments. Pregnancy losses for cows in the OVS-CO treatment were lower (P 0.05) than those for cows in the CO-72 treatment. Number of services per conception for cows in the MOVS treatment were lower (P 0.05) than those for cows in the CO-72 treatment. Cows calved through November, December had greater (P 0.05) pregnancy rates to first AI, and lower (P 0.05) number of services per conception than those cows calved through September and October. Cows in the CO-72 treatment had higher (P 0.05) average daily milk yield during the first 5 months pp than for cows in both MOVS and OVS-CO treatments. In contrast, parity had no effect on any of reproductive performance measurements. In conclusion, cows in the MOVS treatment and those calved during November and December had better pregnancy rates than those cows in both CO-72 and OVS-CO treatments and those calved during September and October, respectively