Ovulation synchronization and timed artificial insemination（Timed AI; TAI）programs are essential protocols for the reproductive management of the dairy cows. Since the development of the Ovsynch protocol, a wide variety of programs are improved to gain higher fertility. To minimize costs and labor, short-term ovulation synchronization protocols（Short-Synch protocol）that consists of the administration of the luteolytic agents and ovulation induction agents have been developed. The conventional Short-Synch protocol have disadvantages in terms of time and labor, because either administration of the drugs or TAI is performed late at night or early in the morning. Moreover, pregnancy rates of conventional Short-Synch protocols were similar to that of Ovsynch. Thus, the advantage of these Short-Synch protocols was only the reduction in hormone treatment costs and numbers of the treatments. To obtain a much higher pregnancy rate, we developed a Short-Synch protocol in which estradiol benzoate（EB）was administered as an ovulation inducer 24 hours after PGF_2α treatment, and TAI was performed 24-28 hours after EB injection（EB-Short-Synch）. We evaluated the reproductive efficiency of the simple short-term ovulation synchronization protocol for TAI using EB as an ovulation stimulus in lactating dairy cows under field conditions and to compare its effects with that of GnRH. The conception rate of EB-Short-Synch was higher than that of the conventional protocol, in which GnRH was administered 56 hours after PGF_2α administration and TAI was performed 16-20 hours after GnRH injection（GnRH-Short-Synch）.

　Short-Synch protocols are particularly useful if the cows have a functional corpus luteum（CL）because PGF_2α is the most effective luteolytic drug in the presence of a functional CL. For the accurate detection of a functional CL, ultrasonography has been used and studies have reported that the ultrasonographic appearance and/or diameter of the CL were key factors. In this study, cows with at least one CL with a diameter ≥ 20 mm and one follicle with a diameter between 8 and 20 mm were defined to have a functional CL. In addition, if a cow showed any estrus signs, it was enrolled for examination after a ≥ 10-day wait period. If a cow had a bloody mucus discharge, it was enrolled for examination after a ≥ 7-day wait period. As the conception rate of these two Short-Synch protocols was equal to or significantly higher than that of the conventional protocols, these criteria were considered to be useful to diagnose existence of the functional CL. To focus on the numbers of follicles with a diameter ≥ 10 mm coexisting with a corpus luteum with a diameter ≥ 20 mm, conception rate of EBShort-Synch were equal to that of GnRH-Short-Synch when two or more follicles with a diameter ≥ 10 mm were present. However, when there was only one follicle with a diameter ≥ 10 mm, the conception rate of EB-Short-Synch was significantly higher than that of the GnRH-Short-Synch. As the numbers of follicle with a diameter ≥ 10 mm can be confirmed using ultrasonography, it is suggested that ultrasonographic diagnosis will help in the effective use of Short-Synch protocols.