Body composition was studied in 20 mature, non-pregnant, non-lactating cows from five breeds (Hereford, Aberdeen Angus, Dexter, British Friesian and Jersey) kept on four feeding levels until they attained equilibrium body weights that were proportionately 0·7,0·9,1·1 or 1·3 of their normal adult body weight.

Significant breed differences were found in the proportions of body tissues and organs and these were associated with breed differences in lactability (i.e. genetic milking potential adjusted for body size). As a proportion of body weight, intra-abdominal fat, liver, spleen and uterus increased significantly with lactability and hide decreased significantly. Empty gut and gut fill also increased with lactability but not significantly. Liver proportion in dairy breeds was 1·26 times the proportion in beef breeds. Corresponding values for intra-abdominal fat and hide were 1·43 and 0·83.

The most dramatic increases with feeding level were in the proportions of subcutaneous fat, both intra-abdominal fat depots, and the udder. All fat depots were completely depleted when body weight decreased to about 0·6 of its normal adult value. Strong decreases occurred in the proportion of muscle, carcass bone and offal. The proportion of empty gut decreased significantly with increased feeding level. Liver, tail, thymus and possibly gut fill were the only traits entirely unaffected by feeding level.

The near-constancy of liver proportion at equilibrium implies that the rapid response of the liver to a change in feeding level is eventually matched in magnitude by the slower responses in other tissues and organs, so that the original proportionality of about 1 kg body tissue for each 10 g liver is eventually restored.