Effect of Different Time of Starting Ovsynch Protocol After Artificial Insemination on Synchrony Parameters and Fertility in Holstein Cows

The objective of this study was to consider effect of three different time of starting Ovsynch after Artificial Insemination, on synchrony parameters, pregnancy loss and fertility in Holstein cows. Multiparous cows (> 30 Kg/d, n = 450) randomly assigned to one of three groups. Groups according to time of starting Ovysnch were as follows: 1- Starting Ovsynch on day 25 after AI, 2- Starting Ovsynch on day 28 after AI and 3- Starting Ovsynch on day 32 after AI. All cows received an injection of GnRH at time of starting Ovsynch regardless of their pregnancy status, and cow’s diagnosed not pregnant using transrectal palpation received an injection of PGF2alpha seven days later and continued the protocol to receive an injection of GnRH 56 h after PGF and TAI 16 h later. Ultrasonography used to determine response to three injection of Ovsynch, pregnancy diagnosis at first check (32±3 after AI) and second check (65±4 after AI). Results showed response to first GnRH of Ovsynch and ovulatory response to second GnRH of Ovsynch affected by time of starting Ovsynch (P = 0.03). Proportion of cows responded to first GnRH of Ovsynch in group started Ovysnch on day 32 after AI (74.0%) was higher compared to groups started Ovsynch on day 25 (46.0%) or 28 (50.0%) after AI (P = 0.03). Ovulatory response to second GnRH of Ovsynch was greater in groups 32 compared to other groups (P = 0.02). Conception rate at day 32 (P = 0.02) and 65 (P = 0.01) after AI was greater in cows started Ovysnch on day 32 after AI (28.0 and 25.3%) compared to groups started Ovsynch on day 25 (20.6 and 19.3%) or 28 (22.0 and 20.0%) after AI. Cows starting the Ovsynch protocol 32 d after previous AI increased fertility by improving synchrony to the protocol.