Abstract: This experiment was designed to investigate the application of estrus synchronization and Ovsynch-timed artificial insemination(TAI) in dairy heifers.1 046 Holstein dairy heifers imported from Australia were randomly assigned into two groups.The heifers in the control group were checked for spontaneous heat, and the heifers in experimental group were treated by prostaglandin F_2α(PGF_2α) to induce heat synchronization or treated by different TAI programs (Ovsynch or Ovsynch+CIDR program).The estrus synchronization rate, 24 d-non-return rate after AI, first AI conception rate and 21-day pregnancy rate were analyzed in each group.The results showed that there was no significant difference in 24 d-non-return rate and the first AI conception rate between the experimental group heifers and control group (P>0.05), but the 21 d pregnancy rate of the experimental group was significantly higher than that of control group (53.1% and 35.3%;P<0.05).There was no significant difference of the estrus synchronization rate between different PG treatments (PG, PG-7-PG and PG-11-PG) (76.1%, 81.7% and 84.6%;P>0.05).There was no significant difference in 24 d-non-return rate, the first AI conception rate and 21 d pregnancy rate in Ovsynch (GPG) and Ovsynch+CIDR (GPG+CIDR) programs (P>0.05).The first AI conception rate of dairy heifers was significantly affected by different AI technicians (P<0.05), but there was no effect of frozen semen from different bulls on the first AI conception rate (P>0.05).Estrus synchronization and TAI programs could increase the submission rate as a result increasing the 21 d pregnancy rate by inducing dairy heifers to heat in a short time, and accelerate the efficiency of AI to reduce the feeding cost effectively.

Key words: dairy heifers; estrus synchronization; ovsynch-timed artificial insemination; 21 d pregnancy rate