GENETIC AND PHENOTYPIC RELATIONSHIPS BETWEEN FOOD-INTAKE, GROWTH, EFFICIENCY AND BODY-COMPOSITION OF MICE POST-WEANING AND AT MATURITY

Genetic and phenotypic variation in post-weaning growth, food intake, efficiency and body composition of mice post weaning and at maturity, were examined to determine whether genetic variation in efficiency exi sts and to predict likely responses to selection for increased food ef ficiency in post-weaning animals. Genetic variation was found for aver age daily gain, mid-weight, daily food intake and proportion of body f at both post weaning and at maturity. Residual food intake calculated from phenotypic regression had a heritability of 0.27 (s.e. 0.06) past weaning and 0.24 (s.e. 0.08) at maturity, and was very similar to res idual food intake calculated using genetic (co)variances, indicating g enetic variation in efficiency exists in post-weaning and mature mice. Although the phenotypic correlation between residual food intake post weaning and at maturity was low (0.29), the genetic correlation was m oderate (0.60). This suggests that selection for efficiency in young a nimals will lead to a correlated improvement in maintenance efficiency of mature animals. Genetic correlation estimates suggest that correla ted responses in other traits would include a concomitant decrease in post-weaning food intake, a slight increase in weight at weaning, a sl ight increase in post-weaning fat proportion and little or no change i n post-weaning growth. In mature animals there will be an associated d ecrease in daily food intake and a slight decrease in mature size and body fat proportion. The results suggest that residual food intake of young animals might be a suitable selection criteria for use in livest ock species to improve efficiency in young animals and also in the bre eding herd.