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University of Nebraska and US Department of Agriculture, Lincoln, NE 68583-0908
Abstract
Direct and correlated responses were evaluated for nine generations of full-sib family selection for postweaning rate (LG) and efficiency (LE) of protein deposition in rats. Selection in LG was based on rate of protein gain (DP) calculated from mean live weight gain of standardized litters (4 
, 5 
) and composition of three sibs under ad libitum (B) feeding. Selection in LE was based on rate of protein gain/feed (PE) to a fixed age or weight under more limited (T) access to feed. There were two replicates of 20 males and 40 females each in selected LG and LE lines and three replicates of 20 single-pair matings each in unselected controls (C). Standardized selection differentials for family means of parents were about .9 a for generations 7 to 15 in both LG and LE. Inbreeding increased about 1%/generation until replicates were combined in generation 12, and reached 13% in selected and 9% in control lines at generation 20. In generations 5 and 15 to 20, each line was evaluated under both feeding environments. Responses per generation as percentages of controls in LG and LE lines were 2.2 and 1.6% for DP, 1.2 and .8% for PE, .5 and 1.2% for 3-wk weaning weight, 2.1 and 1.9% for daily 3- to 8-wk gain, 1.2 and .8% for daily feed, 1.1 and .7% for live weight gain/feed, 1.9 and 1.5% for female weight after parturition and 1.8 and 1.3% in female weight after weaning their first litter. Responses were nonsignificant in percentage skin, protein content of skinned empty body and litter size born. Realized heritability was .40 ± .07 for DP and .07 ± .04 for PE. During five subsequent generations of relaxed selection, linear decline in 3- to 8-wk growth rate/generation was greater in LE than in LG (–2.8 vs –1.9%) and under T than B feeding (–2.9 vs –1.8%). Decline in feed intake was –2% in both lines, but more in females than in males. Live weight gain/feed declined –1.8% in LE, but differed between males (–1.3%) and females (1.1%) in LG. Breeding female weights declined –1.7% in LE and –1.5% in LG. Litter size was unchanged. Selection for faster postweaning lean growth was as effective and responses were more stable under relaxed selection, than from direct selection for more efficient lean growth.
1 Published as Paper No. 7072, Journal Ser., Nebraska Agr. Exp. Sta., Lincoln. Partial publication from senior author's Ph.D. Dissertation.
2 Research Associate, Dept. of Anim. Sci., Univ. of Nebraska, Lincoln 68583-0908.
3 Roman L. Hruska U.S. Meat Anim. Res. Center, ARS, USDA, 225 Baker Hall, Univ. of Nebraska, Lincoln 68583-0908.
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