A Genetic Study of Weight at Five Ages in Hampshire Swine2

Abstract

The results of a ten-year experiment involving ten generations of selection for heavy weights and eight generations of selection for light weights at 154 and 180 days of age in Hampshire swine are presented. A total of 2036 pigs from 288 litters in 124 sire groups are included. Rapid and Slow lines derived from a sample of the breed and maintained as closed herds became more and more distinct for 154- and 180-day weight as selection continued over the entire period. Heritability estimates for 154- and 180-day weights of 0.17 and 0.16 were obtained on the basis of line difference produced as compared to selection pressure exerted. Selection was as effective in separating the lines in later generations as in the first two, indicating the absence of important epistatic deviations from the additive scheme for these traits. Estimates based on regression of offspring on mean of parents and on intrasire regression of offspring on dam were combined and found to be 0.16 and 0.14 for 154- and 180-day weights, respectively. The less reliable method based on variance components gave heritability estimates of 0.18 and 0.44 for 154- and 180-day weights, respectively. Heritability estimates for birth-, 21- and 56-day weights of 0.07, 0.05 and 0.03, respectively, were obtained from combined regression values, while estimates of 0.28, 0.30 and 0.24 were obtained from variance components, respectively. Sex differences in weight were analyzed and boars were found to be significantly heavier than gilts by about 5 percent at birth and by about 3 percent at 21, 56, 154 and 180 days of age. Genetic correlations were estimated on the basis of weight differences produced between lines at birth, 21 and 56 days of age, by selection based solely on weights at 150, 154 or 180 days of age. It appeared from the size of the genetic correlations and the more reliable heritability estimates obtained in this study that indirect selection for heavy weights at 154 or 180 days of age on the basis of birth-, 21- or 56-day weights would be approximately 0, 20 or 50 percent as effective, respectively, as direct selection for heavy 154- or 180-day weights. Copyright © . .