Direct and correlated responses to selection for efficiency of lean gain in mice

Improvement in feed efficiency when selection is based on gain:feed ratio h as often been accompanied by a reduction in feed intake. The following four criteria were used in mass selection for improved lean gain efficiency in mice with an objective of evaluating changes in lean gain and intake: 1) ga in deviation, animals selected had the greatest gain in fat-free mass (FFM) after adjustment to a constant intake; 2) intake deviation, mice selected had the least feed intake after adjustment to a constant gain in FFM; 3) in trinsic efficiency, similar to the second criterion except that adjustment was also made for average weight maintained during the period; and 4) a pos itive control that used the ratio of gain in FFM: feed intake as the select ion criterion. A fifth line, in which a male and a female were selected at random from each litter, served as a negative control. Experimental animals were outbred mice of the CF1 strain. Two replicates of the five lines were included in the study. Twelve males and females were pair-mated within eac h line-replicate combination each generation. Feed disappearance was measur ed from 25 to 42 d. Mice were scanned to obtain an electrical conductivity measurement for prediction of FFM. After six generations of selection, real ized heritabilities for gain:feed, gain deviation, intake deviation, and in trinsic efficiency were .00 +/- .04, .04 +/- .29, .35 +/- .08, and .28 +/- .06, respectively. There were no differences among lines for gain:feed rati o. The correlated response in feed intake reduction was significant in the intake deviation and intrinsic efficiency lines (-.17 +/- .05 and -.21 +/- .04 g.d(-1).generation(-1), respectively). The realized genetic correlation s between the ratio and gain deviation, intake deviation, and intrinsic eff iciency were .83 +/- .15, .01 +/- .04, and .21 +/- .12, respectively. Litte r size was depressed in all selected lines.