Abstract
Data on 10 carcass traits were analyzed for correlated responses to selection for high and low backfat thickness in two lines of Durocs and two lines of Yorkshires. Included were comparisons of the selected lines with their respective unselected controls based on data collected through 16 generations in the Duroc lines and 14 in the Yorkshire lines. Beginning with ninth generation Durocs and seventh generation Yorkshires, vertebra and rib numbers were also studied for correlated responses. When compared with the changes in backfat thickness, the relative divergences between the high and low Duroc and Yorkshire lines averaged, respectively, 76 and 83% for depth of carcass backfat, 73 and 82% for % fat cuts, 59 and 60% for % lean cuts, 51 and 26% for carcass length, 51 and 54% for % fat in ham, 48 and 54% for % lean meat in ham, 38 and 46% , for loin eye area and 37 and 50% for ham weight. Of the remaining traits, only dressing percentage and % bone in ham in Durocs showed a divergence, averaging about 30 and 39%, respectively, of that of backfat thickness. Vertebra and rib numbers showed no change in the low lines, but decreased somewhat in the high lines. Regressions of the carcass traits on both years and live pig backfat thickness indicated that depth of carcass backfat, dressing percentage, % fat cuts and % fat in ham are positively correlated genetically with backfat thickness. Genetic correlations of the remaining traits with backfat thickness appear to be negative. Relative to changes in live pig backfat thickness, there was a tendency for most carcass traits to change faster in low-fat Yorkshires than in low-fat Durocs. The reverse was the case for high-fat Yorkshires when compared with high-fat Durocs. Therefore, if the observed breed differences are at all typical, selection for less backfat may be expected to result in relatively greater increases in meatiness and correspondingly greater decreases in over all fatness in Yorkshires than in Durocs. Copyright © 1973. American Society of Animal Science. Copyright 1973 by American Society of Animal Science.