Selection for Fertility in Two Inbred Lines of Swine

Abstract

The amount of selection as measured by the average deviation of selected parents from their generation averages in number of pigs farrowed alive and number of pigs weaned was determined in two inbred lines of swine (Minnesota No. 1 and Minnesota No. 2) in the Minnesota swine breeding project. The average annual selection differentials for the Minnesota No. 1 and No. 2, respectively, in number of pigs per litter farrowed alive was .73 pig and .48 pig; for number of pigs weaned per litter the selection differentials were .81 pig and 1.14 pig. Theoretical rates of change in each trait were computed, using the selection differentials, the heritabilities of the traits, and estimates of the effect of inbreeding. The theoretical rate was compared with the estimate of the “actual” rate determined by the regression of the trait on a time scale in years. In the Minnesota No. 1, actual and predicted rates were in rather close agreement; in the No. 2 there was only agreement as to direction of change. Selection appeared to accomplish most in the line where most selection was practiced.