Selection for Rate and for Efficiency of Lean Growth in Rats: Net Life-Cycle Efficiency1

Effects on components of net life-cycle input costs/gram of lean tissue output were modelled for responses from 14 generations of family selection for rate (LG) or for efficiency (LE) of postweaning lean gain. Compared with unselected controls (C), LG and LE rats were heavier by 15 and 3% at 3-wk weaning, 23 and 12% at 8 wk and 23 and 13% at 15-wk breeding age. The LG and LE rats ate 12 and 5% more feed than C rats, gained protein 27 and 17% faster and thus 13 and 11% more efficiently from 3 wk to a mean 250 g final weight. The LG rats were fatter and LE rats less fat than C rats (6.7 and 6.0 vs 6.4%). Fasting heat loss was 6% below C for both LG and LE rats. The LG rats were slightly below C rats in fertility, but were above C in litter size and preweaning viability; LE rats had slightly smaller litters and poorer survival than C rats. The production system assumed “marketing” of females after weaning first litters and of surplus young at 6, 7, 8 or 9 wk of age. Total feed input/protein output (g/g) for 6-, 7-, 8- and 9-wk marketing age relative to C was −5, −2, −3 and −3% in LG, but −3, −2, −4 and −6% in LE, mostly from reduced breeder feed in LG, but from feed for both breeders and young in LE. Compared with C, nonfeed costs for breeders and young were −19 and −16% in LG but were only −7 and −10% in LE. Total costs at 6- to 9-wk end points relative to C were −12 to −8% in LG and −4 to −8% in LE. Optimum end points were below 6 wk for LG, 7 to 8 wk for LE and 7 wk for C, and would be still younger for lower ratios of nonfeed to feed costs. At optimum age end points for each, total costs/g protein relative to C were −9% for LG, but only -5% for LE, because of the lower LE reproductive rate. These results question direct selection for the postweaning lean tissue feed conversion ratio.