Relation Between Ovulation Rate and Embryonic Survival and Development in Crossbred Gilts at 35 days of Pregnancy

In modern crossbred multiparous sows,an increase in ovulation rate (OR) isrelated witharelatively small increase in the number of vital embryos at 35 days ofpregnancy, which is mainly due to anincrease in early embryonic mortality (Da Silva et al.,2016). Earlyembryonic mortality,i.e. mortality before uterine implantation at 13 days of pregnancy,increaseswith the increase inOR probablydue to an increase inheterogeneity in the pool of ovulatory follicles and oocytes, increasing also embryonic heterogeneityand thereby embryonic mortality at the time of implantation(Popeet al, 1986; Geisert et al., 1982). Anincrease in OR was also related with adecrease in implantation and placental length ofthe vital embryos in multiparous sows at 35 days of pregnancy (Da Silva et al., 2016), indicating uterine crowdingalready at this stage of pregnancy.This might subsequently lead toahigherfoetalmortality, but might also compromisefoetalgrowth, leading to lower piglet birth weight and birth weight uniformity; associated withahigherpiglet mortality after birth (Milligan et al., 2002). Giltshave a lower OR (Belstra, 2003), which could imply different relationshipsbetween OR and embryonic survival and development. Thus, crossbred gilts available from aninsemination trialwith semen stored for different durationswere slaughtered at 35 daysof pregnancy andthe relationship between OR and embryonic survival and development was investigated.