1. Milk-substitute diets, in which 330-360 (L) or 610-700 (H) g/kg milk protein was replaced by protein from a thermo-alkali-treated soya-bean flour (SF) or from fish-protein concentrate (FPC), were compared in an experiment involving fifty Friesian calves, of which half were supplemented with a growth promoter, Grofas, known to have bacteriostatic properties. The liquid diets were fed ad lib. from 48 h of age until 136 kg live weight.
2. Seven calves, given non-milk protein, died or were removed from the experiment. There was little difference between treatments in the incidence of diarrhoea or in other observations on health of surviving calves, but those given non-milk protein maintained a lower mean rectal temperature.
3. Weight gain was reduced, especially during the first 3 weeks of life, by inclusion of non-milk protein. The reduction was greater for SF than for FPC, and greater at the H level. Supplementation with Grofas improved weight gain for calves given non-milk protein, but tended to reduce that for calves given milk protein.
4. Apparent digestibility of dry matter and protein was reduced when non-milk protein was used. The reduction was greatest at a young age, greater for SF than for FPC and greater at the H level. Apparent digestibility of fat was most markedly reduced with FPC, especially at 1 week of age, and was increased by Grofas supplementation. Digestibility of the carbohydrate in SF was low at 1 week but increased with age. Grofas supplementation caused a marked reduction in the disappearance of SF-carbohydrate in the alimentary tract. Apparent absorption of ash and calcium were reduced by non-milk protein, especially by FPC. The reduction in absorption of ash from SF was moderated by Grofas supplementation. Although Ca retention for calves given SF at the H level was very low, no bone defects were observed.
5. Nitrogen retention was slightly lower for calves given non-milk protein, but the efficiency of retention of apparently digested N was greater for calves given non-milk protein because of the much reduced urinary N excretion associated with a lower apparently digested N intake.
6. Dressed-carcass weight and ‘killing out’ percentage were lower for calves given non-milk protein, especially SF at the H level. Adrenal weights were markedly increased by feeding SF especially at the H level. Pancreas weight was greater for calves given SF than for those given FPC.
7. Weight of abomasal contents at slaughter was much lower for calves given non-milk protein. Weight of intestinal tissue and of total tissue in the alimentary tract were markedly increased by non-milk protein, especially by SF at the H level, and reduced by Grofas supplementation. The increased weight was associated with increased thickening of the walls of both the small and large intestine, which was possibly associated with hypertrophy of muscle cells.
8. It is concluded that up to 360 g/kg milk protein could be replaced by protein from this thermo-alkalitreated SF, and up to 610 g/kg milk protein from this FPC without markedly affecting performance, especially if an effective growth promoter is included in the diet. The beneficial effect of Grofas appeared to result from the reduction in the fermentation of the oligosaccharides of SF, which was reflected in higher digestibility particularly of fat and absorption of Ca and in reduced thickness of the intestines.