Abstract
The correlated responses of 11 traits related to reproductive development and productivity of breeding females in two lines of Durocs and two lines of Yorkshires selected for a single trait, backfat thickness, are described. Both the Durocs and Yorkshires consisted of a high-fat line (Lines D-H and Y-H) and a low-fat line (Lines D-L and Y-L) with selection in the two breeds carried through 13 and 11 generations, respectively. In addition, an unselected control line (Lines D-C and Y-C) derived from the same sources as the selected lines was maintained in each breed. The reproductive traits studied were conception rate, number of services required per conception, length of gestation period, dam's weight on day bred, on 109th day of gestation, and on day litter was weaned, weight changes during the gestation and suckling periods and litter size at birth and at 21 and 56 days. Compared with controls, conception rate declined in all lines except the D-H line, although none of the time trends for this trait was significant. Number of services per conception showed essentially no change in any of the lines. Gestation length also snowed no appreciable change in Durocs but increased significantly in both the Y-H and Y-L line. In both Durocs and Yorkshires, upward as well as downward selection was generally accompanied by non-significant time trends in dam's weights and dam's weight changes. Only for dam's breeding weight in Durocs was there fairly conclusive evidence of a divergence in trends between the high and low lines. Although none of the time trends in litter size were significant, there was a tendency for litter size to increase in the D-L line and to decrease in the D-H line, while the converse was true for the Y-H and Y-L lines. Genetic correlations between backfat thickness and the several individually recorded reproductive traits were estimated using both daughter-dam variances and covariances and observed selection responses. While the signs of the correlations based on selection responses were generally in accord with those of the parameter estimates, any conclusions concerning the predictive value of the parameter estimates are at best tentative. As a whole, the results give no clear indication of a consistent decline in reproductive fitness due to the selection for backfat thickness. Thus, the experiment indicates that breeders may select for decreased fatness without lowering reproductive performance. Copyright © 1970. American Society of Animal Science. Copyright 1970 by American Society of Animal Science