SUMMARYStarting in 1959 from a crossbred foundation, a specialized synthetic sire line has been developed with the aim of making rapid genetic improvement in lean meat production. Within-line selection, firston ultrasonic backfat and then on a performance index, was supplemented over 11 generations by a process of competitive immigration, in which individuals of any breed from outside were incorporated into the line on merit. By Generation 11 the Sire Line contained genes from 9 pure breeds and 2 hybrid strains.After a rapid initial phenotypic decline in backfat, there was little apparent response to index selection. In Generation 11, a centrally tested sample of the Sire Line showed significant improvements over contemporary Large Whites in growth rate, feed efficiency, eye-muscle area, lean percentage and some fat depths, but was significantly worse in killing-out percentage. With the exception of teat number there was no evidence of a decline in reproductive performance, to which no attention was paid during selection. Estimated heritabilities of growth rate and backfat were not sufficiently different from those reported for purebred pig populations to indicate that additive genetic variance had been changed by mixing breeds. Some of the advantages and disadvantages of the ‘open synthetic’ method of genetic improvement are discussed.
Genetic improvement of feed efficiency of grazing livestock
