Some Factors Affecting Litter Size and Fetal Weight in Purebred and Reciprocal-Cross Matings of Chester White and Poland China Swine

Abstract

Factors affecting litter size and fetal development as determined at 25 and 70 days of gestation were studied in matings of 55 gilts and 6 boars including purebred Chester White (CW), purebred Poland China (PC) and their reciprocal crosses (CW-PC and PC-CW). The two breeds of gilts differed in ovulation rate (CW 13.3; PC 11.9; P<0.05). Litters sired by CW boars showed a greater difference in number of pigs between the 25th and 70th days of gestation than did litters sired by PC boars. When averaged over both purebred and crossbred matings, the difference was between 10.0 and 6.0 for CW sired litters on the respective days and between 8.8 and 8.3 for PC. A difference between these two breeds of sires in their embryo mortality is suggested. Purebred fetuses were significantly heavier than crossbred fetuses at the 25th day, and a similar but insignificant trend was found at the 70th day. Approximately three-fourths of the variance in the probabilities of embryo survival in different pregnant gilts was nonbinomial in character indicating individual differences in the maternal contribution to survival, genetic plus environmental. Embryo survival rate appeared to have greater influence than ovulation rate in determining litter size. Membrane weight in terms of standard partial regression was significantly less important than fluid weight in the determination of fetus weight at the 25th day. It became significantly more important by the 70th day and fluid weight significantly less. Fetus weight was positively but insignificantly associated with both number of fetuses in the litter and numbers of fetuses in the uterine horn at the 25th day. By the 70th day, the standard partial regression on numbers of fetuses in the horn was significant and negative and differed significantly from the standard partial regression on litter size (−0.45 vs. 0.31). There was a trend between the 25th and 70th days toward the development of an effect of number of fetuses in the uterine horn upon fetus weight which could not be accounted for by their association through membrane weight. Copyright © . .