1. The yeast grown on normal paraffins (British Petroleum Proteins Ltd, London) contained
62% crude protein and, except for lower content of methionine, its amino acid composition
was similar to that of white fish meal.
2. The value to growing pigs of yeast + methionine as a protein supplement to diets based on barley and fine wheat offal was compared with that of white fish meal. The two protein supplements supplied the same amount of total nitrogen and were compared at a ‘standard’ level, commonly used in practice, and at a ‘low’ level. Two experiments were conducted: a feeding trial covering the live-weight range from 20 to 90 kg, and a metabolic trial (20–60 kg live weight) in which N retention and digestibility were measured.
3. There was a small but significant difference in favour of the yeast treatment for growth rate and feed conversion ratio, but there were no consistent differences in the linear measurements of the carcasses due to protein source.
4. In the metabolic trial there was no significant difference in performance, N retention,
apparent N digestibility or linear carcass measurements and no consistent difference in tissue
components, between the diets supplemented with yeast or fish meal.
5. In pigs given the ‘low’-protein diets, performance and most of the other variables measured were significantly poorer than in those given the ‘standard’ protein diets, irrespective of whether yeast or fish meal was the source of supplementary protein.
6. It is concluded that yeast (+ methionine) may be closely equated with high-quality fish
meal as a protein supplement in diets for growing pigs. The small differences found are discussed in relation to possible differences in availability of amino acids and energy values in the diets.