SUMMARY
The objective of this study was to show the risk factors affecting the conception rate in Holstein heifers after synchronization of estrus. A total of 275 Holstein heifers housed in a free barn were used for the experiment. The herd was visited regularly at four week intervals for synchronization of estrus using Heatsynch and CIDR-Heatsynch protocols. A group of four to 14 animals, depending on the availability, were referred to the experiment at each visit. Estrus induction rates in the two protocols were 93.9% and 94.9%, respectively. There was no difference in the conception rate between the two protocols. Conception rate after artificial insemination (AI) or embryo transfer (ET) were 46.3% and 51.4%, respectively. The risk factors affecting conception rate in heifers were daily weight gain (odds ratio [OR]= 4.673; P= 0.036) and body condition score (BCS) (OR= 3.642; P= 0.018). Furthermore, estrus synchronization protocol (OR= 1.774; P= 0.083) and the absence of corpus luteum (CL) at the initiation of treatment (OR= 0.512; P= 0.061) had a tendency to affect the conception rate, while age (OR= 0.715; P= 0.008) was a protective factor to conception rate. In conclusion, positive daily weight gain before AI or ET, higher BCS, younger age, and the presence of CL at the initiation of estrus synchronization in dairy heifers increased the likelihood to conceive.