The Hereditary and Environmental Portions of the Variance in Weaning Weights of Poland-China Pigs.

1930 a study of the effects of inbreeding on Poland-China pigs was IN begun at the Iowa Agricultural Experiment Station. The foundation animals, 27 sows and three boars, were purchased from various breeders. Most of them were only about six months old when purchased but a few were older gilts which had already been bred. In this way some offspring of three other boars were also added to the herd. Since the summer of 1930 no breeding stock has been purchased, all replacements being selected each spring from the more promising individuals raised. The first selections were made at weaning time and were based on the individuality of the pig and the production records of its dam. At examinations later in the summer or in the fall some of the pigs selected at first were rejected and were not actually used for breeding. Generally four boars were used each season. Each boar was mated to a representative group of sows in so far as concerned age, production, typescore and relationship to their mates. Thus from each boar there came some closely inbred and some outbred pigs. Fall farrowing duplicated spring farrowing as nearly as possible except that few gilts farrowed in the fall following their first litters. Only 12 of the foundation animals were inbred (pedigrees being traced to 1923 as a base date), and they were less than four percent inbred. The highest relationship between any two of these 44 animals was 32 percent in the case of non-litter mates and 51 percent between three individuals which were litter mates. The average inbreeding of the herd has been rising slowly but even in 1935 (the last year included in this study) the average inbreeding of all pigs weaned (11.8 percent) was less than that produced by mating half brother and sister in a random breeding population. The average inbreeding (8.5 percent) of the 1633 pigs weaned in the entire six years was only a little above that (6.2 percent) which would result from mating ordinary first cousins. Assortive mating was expressly avoided, care being taken to mate each boar to a typical sample of sows.