Genetic improvement of swine
Lentz, Wilbert Emil
An analysis of Canadian R.O.P. data for pigs born in 1964 and 1965 was undertaken with two main objectives in mind. The first was the identification of genetically superior litters and the second was the evaluation of heritability estimates for various performance traits and of the genetic and phenotypic correlations between them. In order to achieve the first objective it was necessary to investigate the various possible sources of environmental variation since these might have masked genetic differences. Both province and season (month or quarter) effects were statistically significant for most traits but the evidence favored a hypothesis that these differences were, for the most part, reflections of genetic differences between litters tested in the various provinces and periods. There was evidence of a season effect on per cent ham in the carcass and per cent lean in the ham face, but this cannot be considered to be conclusive since data from only one year were available. Carcass weight was found to have an important influence on all carcass traits including predicted yield and it was recommended that this trait be adjusted for carcass weight before it is utilized in a selection index. Sex of the pig also had a substantial effect on carcass characteristics, with gilt carcasses being superior to barrow carcasses. Sex differences in total fat were significantly smaller in the Lacombe breed than in the other breeds. Consequently, the R.O.P. sex corrections overcorrected this trait in this breed, but the sex corrections were effective in eliminating sex differences in predicted yield. Sexes also differed in growth rate with barrows growing faster than gilts, especially in the Yorkshire breed. Where sib or progeny testing is being employed and test groups are not balanced for sex it is advisable to apply a sex correction to age at slaughter (adjusted to a constant carcass weight) before it is included in a selection index. Heritability estimates were very high for all traits except growth rate (age at slaughter). The large sire components of variance which resulted in these high estimates were taken as evidence for the existence of strains of pigs which differ in average genetic merit for a given trait. On the basis of available information, both from the R.O.P. records studied and from the literature, recommendations for selection procedures were made. While the recommendations were formulated as guidelines for the establishment of a central swine breeding station in Saskatchewan, they should, for the most part, be applicable to R.O.P. and other swine improvement schemes.