The body composition of 18 each of Mashona, Afrikaner and Hereford heifers was measured at the beginning and after 16 and 32 weeks of the experiment. The heifers not slaughtered at the beginning of the experiment were fed a complete diet containing 132 g crude protein and 12·0 MJ metabolizable energy/kg dry matter. Before slaughter, the animals were deprived of food and water for 24 h. Each animal was infused with 1 mCi of tritiated water (TOH) in order to measure total body water (TBW) and to estimate body fat.

The growth rate of the three breeds of heifers was similar despite differences in age and initial live weight. Both TBW and fat proportions, however, differed significantly (P < 0·01) between slaughter stages for each breed and between breeds at each slaughter stage. At the first, second and final slaughter stages the proportions of TBW were: 68·0, 59·4 and 54·5% for Mashona; 70·;5, 64·3 and 58·3% for Afrikaner and 65·3, 57·6 and 46·2% for Hereford heifers respectively. The corresponding proportions of body fat were: 10·2, 18·4 and 24·2% for Mashona; 6·6, 12·0 and 20·0% for Afrikaner and 13·7, 20·8 and 25·8% for Hereford heifers respectively.

There was a close relation between empty body weight and live weight at slaughter which was not influenced by breed. Both TBW and fat were estimated more accurately when TOH space and live weight were used jointly. However, the slopes of the prediction equations for each breed were significantly different (P < 0·05) in the case of both total body water and fat. It was necessary to use separate equations for each breed in order to predict either body water or fat. The significance of these findings for the estimation of body fat in live cattle is discussed.

The effects of reducing the feed intake of 12 winter-calving (Hereford male × British Friesian female) cows to the maintenance level for 6 weeks from week 10 of lactation were investigated during two consecutive lactation periods. Initially, six cows were in their first and six in their third lactation. In each year the period of ‘underfeeding’ was preceded by a period of ‘adequate feeding’ (maintenance plus milk production requirements) and succeeded by a period of ‘refeeding’ (2 × maintenance). Changes in body water, estimated by the dilution of tritiated water and deuterium oxide at the end of each feeding period, were used to calculate changes in body tissue. Milk yield during the period of ‘adequate feeding’ indicated that individual cows differed markedly in milk production potential. During ‘underfeeding’ milk yield was related to lactation potential. Cows of higher potential tended to maintain higher yields than those of lower potential but in doing so they incurred higher body-weight losses. The ‘refeeding’ period did not result in significant increases in milk yield. The weight losses during ‘underfeeding’ consisted mainly of fat but some cows apparently mobilized relatively large amounts of protein. More fat per unit body weight loss was mobilized by the heifers than by the cows, which, on the other hand appeared to mobilize relatively more protein. On ‘refeeding’, only a few cows were able to achieve complete tissue repletion.

Populations of domestic pigs, very different in appearance and size to recently imported exotic breeds, are found in many countries in Africa. Although often referred to as indigenous or native, in East and Southern Africa at least, they may be descended from stock originally introduced by Europeans, probably in the nineteenth century (Mason and Maule, 1960). Diverse phenotypes have developed in different parts of the continent (e.g. Jollans, 1959; Joubert, Combrinck and Bonsma, 1960), and reports generally refer to their poor conformation, slow growth rates and low productivity. To some extent this is probably a reflection of their environment and diet, since they largely subsist by foraging around human habitations and cultivations (Epstein, 1971).