REPRODUCTIVE-PERFORMANCE IN RELATION TO UTERINE AND EMBRYONIC TRAITS DURING EARLY GESTATION IN MEISHAN, LARGE WHITE AND CROSSBRED SOWS

Previous studies have shown that females of the Chinese Meishan breed and of their F1 cross with European Large White pigs are very prolific , producing about four more piglets per litter than control Large Whit e females. The main cause of this prolificacy is enhanced prenatal sur vival for a given ovulation rate in Meishan and F1 females and this is controlled by genes of the mother, not those of the conceptus. The ob jectives of this study were to determine whether genotypic differences in embryo survival were apparent in the period immediately after atta chment and to compare embryonic and uterine development at this time. Sows in their third parity (20 Large White, 14 Meishan, 25 Large White x Meishan F1 and 25 Meishan x Large White F1) were killed 20-22 days after mating and their reproductive tracts recovered for further study . There were significant differences between the purebred sows, and cr ossbred sows were approximately intermediate for the number of corpora lutea (20.7 +/- 0.9, 27.8 +/- 1.1, 22.4 +/- 0.8 and 23.3 +/- 0.8 for the four genotypes, respectively), the number of embryos (15.2 +/- 0.9 , 23.4 +/- 1.1, 17.2 +/- 0.8 and 18.8 +/- 0.8, respectively) and the p roportionate embryo survival (0.74 +/- 0.04, 0.84 +/- 0.04, 0.78 +/- 0 .03 and 0.82 +/- 0.03, respectively). There was a negative association within genotype between embryo survival and the number of corpora lut ea. Adjusting for the genotypic difference in the number of corpora lu tea increased the genotypic differences in embryo survival. Meishan ut eri were similar in weight to, although significantly shorter than, th ose from Large White females. Significant heterosis meant that uteri f rom F1 females were both heavier and longer than those from either pur e breed. Embryo attachment sites were significantly closer together in Meishan uteri than they were in uteri of the other genotypes. The hea d length and weight of embryos and the weight of the placentae did not differ significantly between the pure breeds, but were increased in F 1 sows, significantly so for head length and the weight of the placent ae. Within sow variation in distances between embryo attachment sites was less in Meishan and F1 females than in Large White females. Within sow variation in the head length and weight of embryos and in the wei ght of the placentae was always less in Meishan and F1 females than in Large White females, but this difference was only significant for the weight of the embryos in F1 females. Meishan females have higher ovul ation rates and a higher level of embryo survival after attachment tha n do Large White females. At this stage the number of embryos in F1 fe males is intermediate between the two pure breeds. F1 females must hav e very high levels of fetal survival, thus achieving similar litter si zes to purebred Meishan females by a different route. Heterosis in F1 females for increased uterine dimensions and embryonic growth and decr eased variability between embryos may be factors associated with high fetal survival.