Investigation into the mechanisms of prolificacy of Meishan (Ms) pigs has lead to conclusions that differences among breeds involve several components of litter size. Components examined in Ms and occidental breeds have included follicular development, ovulation, time of ovulation, embryonic survival, and fetal survival .
Although many studies have examined Ms and Yorkshire (Y) females for embryonic survival (before 30 days of gestation), few laboratories have examined uterine and fetal interactions (after 30 days of gestation) in the Ms pig and their potential importance. We have reported that uterine and fetal interactions may have a role in the prolificacy of Ms females. Further, previous reports indicate that the number of fetuses does influence uterine length in occidental breeds of pigs. Evidence has indicated that ovulation rate of Ms and occidental pigs was similar at puberty and at early estrous cycles but diverged in favor of the Ms as they underwent more estrous cycles and later parities. Results from our laboratory indicated that ovulation rate was significantly higher for Ms females than for Y females at second parity but the reproductive age was not standardized.
The objectives were to determine the differences in ovulation rate, uterine length, space per fetus, fetal survival, and fetal development traits among cycle-matched (gilts bred at third estrus) Ms, Y, Ms x Y, and Y x Ms females at 50 days of gestation carrying 1/2 Ms and 1/2 Y fetuses. The objectives of Experiment 2 were to determine the differences in these traits for cycle-matched Ms and Y females at 50 days of gestation carrying purebred (Ms or Y) or crossbred (1/2 Ms and 1/2 Y) fetuses.