Life cycle biological efficiency of mice divergently selected for heat loss1

Abstract

Divergent selection in mice for heat loss was conducted in 3 independent replicates creating a high maintenance, high heat loss (MH) and low maintenance, low heat loss (ML) line and unselected control (MC). Improvement in feed efficiency was observed in ML mice due to a reduced maintenance energy requirement but there was also a slight decline in reproductive performance, survivability, and lean content, particularly when compared to MC animals. The objective of this study was to model a life cycle scenario similar to a livestock production system and calculate total inputs and outputs to estimate overall biological efficiency of these lines and determine if reduced feed intake resulted in improved life cycle efficiency. Feed intake, reproductive performance, growth, and body composition were recorded on 21 mating pairs from each line × replicate combination, cohabitated at 7 wk of age and maintained for up to 1 yr unless culled. Proportion of animals at each parity was calculated from survival rates estimated from previous research when enforcing a maximum of 4, 8, or 12 allowed parities. This parity distribution was then combined with values from previous studies to calculate inputs and outputs of mating pairs and offspring produced in a single cycle at equilibrium. Offspring output was defined as kilograms of lean output of offspring at 49 d. Offspring input was defined as megacalories of energy intake for growing offspring from 21 to 49 d. Parent output was defined as kilograms of lean output of culled parents. Parent input was defined as megacalories of energy intake for mating pairs from weaning of one parity to weaning of the next. Offspring output was greatest in MC mice due to superior BW and numbers weaned, while output was lowest in ML mice due to smaller litter sizes and lean content. Parent output did not differ substantially between lines but was greatest in MH mice due to poorer survival rates resulting in more culled animals. Input was greatest in MH and lowest for ML mice for both offspring and parent pairs, consistent with previous results in these lines. Life cycle efficiency was similar in MC and ML mice, while MH mice were least efficient. Ultimately, superior output in MC mice slightly outweighed the lower inputs in ML animals resulting from decreased maintenance energy requirements. Therefore, selection to reduce maintenance energy requirements may be more useful in terminal crosses or in a selection index to reduce possible negative effects on output, especially reproductive performance.

Feed and reproductive efficiency differences between divergently selected lines for birthweight environmental variability in mice

“…Williams and Jenkins () argued that decreasing feed intake without affecting growth or production would reduce maintenance energy thus improving feed efficiency. Reducing maintenance energy requirements in livestock is an appealing goal since up to 70% of feed intake may be consumed to meet these requirements (Bhatnagar & Nielsen, ,b). It was interesting to see that animal weights in the low line were 15% lower, but only consumed 6% less than those in the high line showing that the maintenance cost is expensive in terms of feed intake.…”

“…It was interesting to see that animal weights in the low line were 15% lower, but only consumed 6% less than those in the high line showing that the maintenance cost is expensive in terms of feed intake. According to the Resource Allocation Theory developed by Beilharz, Luxford, and Wilkinson (1993), fitness components, number of parturitions, litter size and survival of progeny were connected because the required metabolic resources for their function are interrelated, as resources used for one function are no longer available for any other function. There may be some exceptions for this assumption, which are termed "resource association" by Glazier (1999) and Rauw (2009).…”