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Selection for Maintenance Energy Requirements in Mice: Lifecycle Biological Efficiency
Divergent selection for heat loss as an indication of maintenance energy requirements in mice resulted in low (ML) and high (MH) maintenance lines, and an intermediate control line (MC) in 3 replicates. Improved individual 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 in reproductive performance and body composition of ML mice compared to MH mice. Therefore, the objective of this study was to use these lines to model a lifecycle similar to a livestock production system and determine if these correlated responses diminish the benefit of improved individual feed efficiency by calculating lifecycle efficiency. Feed intake, reproductive performance, productivity, and body composition were recorded on 21 mating pairs and sampled offspring from each line by replicate combination. 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. Results from mixed-model analysis of recorded 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. Survival was measured as number of parities achieved by a mating pair. Hypothetical parity distribution of 100 pairs at equilibrium was calculated using survival analysis results assuming a maximum of 4, 8, or 12 parities, and efficiency was calculated separately for each scenario. High maintenance mice consumed 7 to 20% more feed for maintenance per unit body mass than ML mice. Control mice had greater body weights, total numbers weaned, and weaning weights compared to MH and ML animals, while there were no significant differences between MH and ML mice for these traits. Control mice had greater survival rates overall, although 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 over a lifecyle 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, which also include positive emphasis on output, or in terminal sire lines, to minimize negative responses.
Keywords: heat loss, maintenance energy, feed efficiency