Genetic and Environmental Aspects of Litter Size in Swine

Abstract

Experiment station litter records from Indiana, Oklahoma and Wisconsin were utilized to study the effects of environment and heredity on the number of pigs born dead and alive, number living through the first day, number alive at weaning (56 days), total weight of the litter at weaning and number of pigs at 154 days. The 3,781 litters were born during the years 1944 to 1958 and averaged 9.26 total pigs at birth. A least-squares analysis was made to evaluate the effects of station (location), breed, farrowing history, year, season, inbreeding of both dam and litter and age of dam. The mean squares for most effects were significant for all traits. All observations were corrected for these environment and breed effects by using the least-squares constants, and all further computations were made from corrected data. Estimates of heritability were obtained from the regression of daughter on dam, the record of the dam being repeated for each daughter record. These estimates were 0.09±.04, 0.08±0.04, 0.13±.O5, 0.19±.05 and 0.10±.04, respectively, for the five traits. Estimates of repeatability were computed from a one-way analysis of variance (dams and litters within dams), and the standard errors of these estimates were computed by using the formula of Fisher (1958). Estimates of repeatability were 0.17±.03, 0.13±.03, 0.06±.03, 0.05±.03 and 0.07± .03. The curve of the daughter means against the record of the dam was not linear for total litter size at birth, the slope being negative up to a litter size of six. This was considered to be an uncorrectable maternal effect of litter size of the dam.