The effect of GnRH on the pregnancy ratio in low-yielding local race cows: comparison of different injection times.

The objective of this article was to investigate the efficiency of GnRH administrations at different time points after induced luteolysis on pregnancy rates in low-yielding subfertile cows. One thousand six hundred and ten healthy and subfertile dairy cows of different ages and races were used in this study. Cows were randomly divided into 4 groups. Estrus cycles were synchronized by two, with 11-day intervals, injections of the prostaglandin F2α-analogue (PG). The artificial inseminations (AIs) of all animals were achieved at the 72nd and 96th hours following the last PG injection. The animals in groups I (n 257), II (n 337), and III (n 675) were used for the administration of a single dose of GnRH at different time points. Accordingly, GnRH was applied at 48th, 64th, and 72nd hours following the last PG injection in groups I, II, and III, respectively. Group IV was accepted as a control without GnRH injection (n 341). The pregnancy rates in groups I, II, III, and IV after transrectal pregnancy examinations were found to be 89.88%, 91.09%, 83.25%, and 77.12%, respectively. In our study, maximal pregnancy rates could be obtained with GnRH injections performed at 48th and 64th hours following luteolysis induction (P < 0.001). There was a 6-8% decrease in pregnancy rates due to the injection of GnRH in the 72nd hour (P < 0.001). These dramatic losses and gains in pregnancy rates in our study emphasized the necessity of taking the time of injection into account when using GnRH to stimulate ovulation. It can be said that the success of GnRH stimulation of ovulation is directly related to the follicle wave dynamics at the time of injection point and the character of a dominant follicle.