Selection for rate and efficiency of lean gain in the rat.

Full-sib family selection for rate (WP) or efficiency (WP/F) of protein gain in rats from 3 to 9 weeks of age was applied for five generations. Three rats per litter were killed to estimate carcass protein. Standardized response/cumulative selection for WP was .19+/-.10 for WP, .28+/-.10 for 3- to 9-week gain .28+/-.08 for 9-week weight, .16+/-.08 for litter size, .22+/-.12 for skinning loss and -.07+/-.09 for fraction of protein in the live weight. Response from selection for WP/F was .18+/-.16 for WP/F, .20+/-.11 for WP, .21+/-.11 for weight gain, .16+/-.11 for 3-week weight, .21+/-.10 for 9-week weight, but negligible for skinning loss or body protein. Response to WP/F selection was extremely variable among generations, associated with generation differences in weight and composition at 9 weeks. Estimates of heritability from offspring-midparent regression were .20+/-.12 for WP and .24+/-.08 for WP/F. Estimates of genotype-generation environment interaction were large for growth, feed intake and skinning loss. Maternal effects were large for weaning weight, fraction of body protein and WP. Sire component genetic correlations were 1.08+/-.13 for WP with total gain, .92+/-.08 for WP/F with gross efficiency and .29+/-.25 for WP with WP/F. A partitional calorimeter was used to evaluate heat production of rats. Lines differed in average heat loss but not in heat loss per unit actual or metabolic weight. Response to selection has been steady for WP but probably could be improved by selecting for WP/F at a constant weight rather than a constant age.