Effect of Presence of Corpus Luteum at the Beginning of Ovsynch Protocol on Pregnancy Rates in Lactating Dairy Cows

The aim of the study was to investigate the effect of presence of CL at the beginning of Ovsynch protocol on pregnancy rates in lactating dairy cows. A total of 218 lactating Holstein cows that were at least 45 days postpartum with no reproductive disorders were used in the present study. Cows were divided into two experimental groups according to the presence and absence of corpus luteum (CL) on the first day of transrectal ultrasonography (day 0). Cows having CL (CL (+)) were randomly allocated to two subgroups as CL (+)/A and CL (+)/B. Cows in CL (+)/A group (n = 74) were treated with Ovsynch protocol, while cows in CL (+)/B group (n = 78) were treated with PGF(2 alpha) on the first day of examination and seven days later Ovsynch protocol was started. Cows having no CL (CL (-)) (n = 66) were left untreated on the first day of examination and Ovsynch protocol was started seven days later. Pregnancy rates were found 23%, 25.6% and 40.9% in CL (+)/A, CL (+)/B and CL (-) groups, respectively. The pregnancy rate in CL (-) group was significantly higher than those of CL (+)/A, and CL (+)/B groups (P<0.05). It is suggested that the success of Ovsynch synchronization protocol may be related to starting time of the protocol and to ovarian physiology in cyclic dairy cows.