145 EFFECT OF TRANSFER OF CATTLE ELONGATING EMBRYO TO A REPEAT BREEDER COW ON PREGNANCY RATE AND INCIDENCE OF A RETURN TO ESTRUS
In cattle, a low pregnancy rate directly affects calf and milk production and brings about economic loss to livestock producers. The number of repeat breeder cows in herds decreases the overall pregnancy rate. Therefore, many studies have focused on improving the cows’ reproductive performance. On the other hand, it has been demonstrated that the major embryonic loss occurs in the first or second week of pregnancy, after which mother cows recognize the presence of their embryos via a signal protein secreted by the embryos, interferon-τ. Therefore, to bypass the period with a high incidence of pregnancy loss in cattle, we used elongating embryos for embryo transfer (ET) to repeat breeder cows. Elongating embryos on Day 14 postoestrus were used for the present study because embryos of this stage have the ability to produce interferon-τ and embryos of later stages are easily damaged when they are collected from the uterus by flushing. In this study, Japanese Black cows were superovulated and inseminated when they showed standing oestrus. Elongating embryos were collected by nonsurgical uterine flushing. Collected embryos that had evident embryonic discs and intact trophectoderms were used for ET. These embryos were transferred, using plastic catheters or ET guns to ipsilateral uterine horns, to the corpus luteum of synchronized crossbred (Japanese Black × Holstein-Friesian) recipient cows. Recipient cows were divided into 2 groups according to the previous reproductive history of each cow: reproductively normal cows that were pregnant with fewer than 3 times of AI or regular ET (using Day 7 embryos) after every parturition (n = 33) and repeat breeder (RB) cows (n = 42). Elongating embryos were transfer on Day 14 after oestrus, and all cows were monitored for signs of standing oestrus twice a day until Day 80. Pregnancy was diagnosed with ultrasonography every 10 days from Day 30 to 80 postoestrus. The data were analysed by Fisher’s exact test. The pregnancy rate of reproductively normal cows was 54.5%. However, in repeat breeder cows, the pregnancy rate was significantly decreased (2.4%; P &lt; 0.01). When the repeat breeder cows did not become pregnant, transfer of the elongating embryos was repeated another 5 times continuously and the incidence of a return of oestrus was monitored daily. At the first ET period, the average interestrus interval in nonpregnant repeat breeder cows was 50 ± 7.2 days. As the ET period proceeded, the interestrus intervals were gradually decreased, and at a sixth ET period, it approximated the normal oestrus cycle length (22.8 ± 0.8). These results indicated that although transfer of elongating embryos could bypass the critical period of high incidence of embryo loss, it did not improve the pregnancy rate of repeat breeder cows. Moreover, it suggests that the uterine environment of repeat breeder cows was gradually deteriorating in a time-dependent manner.