Factors affecting the body composition of growing and adult animals.

The body composition of any animal is the consequence of the effect of nutrition over a period of time on the genetic and physiological factors that, together with nutrition, determine its phenotype. This paper will consider only those aspects of body composition that are of major importance, first in the conversion of animal feed to saleable product in meat animals and, secondly, in the quality of the meat as perceived by the consumer. These are (a) killing-out ratio, i.e. empty carcass weight:total body-weight at the time of slaughter; (b) the proportions of dissectable bone, lean and fat in the carcass; (c) the chemical composition of the lean and adipose tissue. The quantity of published information on the body composition of meat animals (and laboratory species used as models for meat animals) is enormous. This paper deals mainly with beef cattle. The Agriculture and Food Research Council (Agricultural Research Council, 1980) has produced a very comprehensive review of the literature relating to the body composition of ruminants. This brief paper is not intended to compete with this review nor does it attempt to bring it up to date in a bibliographic sense. Its purpose is only to outline the range of genetic, physiological and nutritional factors that affect body composition and explore the interactions between them. Let us suppose it was possible to make a precise estimate, in vivo, of the body composition of all Charolais and Hereford pedigree bulls reared on their farms of birth to 400 d. Meat and Livestock Commission figures (Allen & Kilkenny, 1980) indicate that the weights of Hereford and Charolais bulls range between 319-591 and 436779 kg respectively. Clearly some of the difference between bulls in weight-for-age has been genetic, some environmental. Representative equations given by the Agricultural Research Council (1980, Tables 1.21 and 1.22) to predict the fat content of the empty carcass after correction for breed and rate of gain/d,