Lifecycle efficiency of mice divergently selected for heat loss

Divergent selection for heat loss as an indication of maintenance energy requirements in mice resulted in a low (ML) and high maintenance line (MH), and an intermediate control line (MC). Improved feed efficiency has been observed in ML mice, which would be economically beneficial in livestock species. However, previous work showed evidence of unfavorable correlated responses to selection in ML mice compared to MH mice. Therefore, the objective of this study was to model a lifecycle similar to a livestock production system using these lines and determine if these correlated responses diminish the benefit of improved feed efficiency by calculating lifecycle efficiency. Feed intake, reproductive performance, productivity, and body composition were recorded on 63 mating pairs and sampled offspring from each line. Values from mixed-model analysis of these data were used to calculate total lean output of offspring and culled mating pairs and total energy intake of offspring and mating pairs. Lifecycle efficiency was calculated as the sum of offspring and parent output divided by the sum of offspring and parent input. Pairs were cohabitated at 7 wk of age and maintained for up to 1 yr unless culled and offspring were maintained from 21 to 49 d of age. Survival was measured as number of parities achieved by a mating pair. Hypothetical parity distribution of 100 mice at equilibrium was calculated using survival analysis results. High maintenance mice consumed 7 to 20% more feed for maintenance than ML mice. Control mice had greater body weights, total numbers weaned and weaning weights compared to selected animals, while there were no significant differences between MH and ML mice for any traits. Control mice had greater survival rates overall, though ML mice had greatest survival rates up 5 parities, while MH mice had greater survival rates in later parities. Although ML mice were more efficient than MH mice, they were nearly identical to MC mice due to poorer productivity. Differences in body weight and numbers weaned had the greatest effect on lifecycle efficiency, therefore selection for reduced maintenance energy requirement should be utilized as part of selection indices or in terminal crosses