Abstract:
Proc. Aust. Soc. Anim. Prod. (1978) 12: 194 GROWTH AND BODY COMPOSITION OF MICE SELECTED FOR GROWTH RATE UNDER AD LIBITUM OR RESTRICTED FEEDING D. J. S. HETZEL* and F. W. NICHOLAS* Growth rate is an important character in many animal breeding programs. However, response to selection for increased growth rate is often associated with undesirable correlated responses such as larger mature size and increased proportion of fat in the carcass. An experiment was conducted in which mice were selected for postweaning weight gain from 3 to.6 weeks of age. There were two treatments: line SF was Selected under Full feeding (ad libitum) and line SR was Selected under a Restricted feeding regime, in which all mice were fed about 82% of ad libitum intake. Each treatment had a separate control line, CF and CR respectively. Both lines showed significant responses to selection and after . four generations of .selection, all lines were compared on ad libitum feeding. Measurements were made of growth, food intake and body composition between birth, and 20 weeks of age which approaches mature age. Results are given as deviations of a selected line from its control with the appropriate standard error. TABLE 1: Comparisons of selection lines for several growth characters There were no significant differences between control lines for all characters, nor between any of the lines in birth weight or protein percentage at 6 or 20 weeks of age. Clearly selection for growth rate has acted on different component characters at the two feeding levels. On ad libitum -feeding, selection has been for higher food intake with a correlated increase in mature weight and fatness at maturity. On restricted feeding, selection has reduced 3 week weight, (weaning), with an associated reduction in food intake, but tiith no significant improvement in growth rate from 3 to 6 weeks on ad libitum feeding. However, mature weight and fatness at maturity have been reduced. Gross efficiency measured as weight gain per unit of food intake between 3 an&6 weeks was improved in both selection lines but the SR line had none of the undesirable correlated responses at maturity. Therefore selection under restricted feeding, i.e. where appetite is not expressed, may be a means of avoiding these unfavourable characteristics. * Department of Animal Husbandry, University of Sydney, N.S.W. 2006. 194.
