Selection for High and Low Fatness in Swine

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Abstract
Selection for both high and low backfat thickness was carried through 10 generations in two Duroc lines and through eight generations in two Yorkshire lines. An unselected control line derived from the same sources as the selected lines was maintained in each breed. After 10 generations of selection, the high-and low-fat Duroc lines differed by 2.6 cm. or 68% of the initial mean. The corresponding difference between the two selected Yorkshire lines after eight generations was 1.4 cm. or 44%. Least squares analyses revealed significant line differences in inbreeding effects in both breeds. In the high-fat Duroc and low-fat Yorkshire lines backfat thickness was significantly decreased by inbreeding, while in the low-fat Duroc and high-fat Yorkshire lines there was a small, although nonsignificant, increase with inbreeding. Realized heritabilities calculated from regressions of generation means on cumulative selection differentials were 0.47 and 0.48 for the high- and low-fat Duroc lines and 0.38 and 0.43 for the high- and low-fat Yorkshire lines. Selection response in the high- and low-fat Duroc lines and the low-fat Yorkshire line declined significantly as the experiment progressed, whereas the reverse was true for the high-fat Yorkshire line. Phenotypic variability, as measured by the coefficient of variation, decreased slightly in the high-fat lines, but showed no material change in the low-fat lines or the control lines. Adjustment of the data for differences in date of birth and pig's inbreeding reduced somewhat the asymmetry in selection response shown by the high- and low-fat Duroc lines during the first half of the experiment but failed to do so in the case of the selected Yorkshire lines. Estimates of heritability from regressions of offspring on mid-parent means were generally in good agreement with those based on selection responses. Thus, epistatic and/or maternal effects probably contributed little if anything to the variation of backfat thickness. It is concluded that the selection practiced was rather highly effective in both the up-ward and downward direction and that the heritability of backfat thickness is of about the same magnitude in Duroc and Yorkshire swine. Copyright © 1967. American Society of Animal Science . Copyright 1967 by American Society of Animal Science

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