Nutritive value of mixed proteins

Abstract

1. Two series of protein feeding-stuffs each consisting of a fish meal, meat-and-bone meal, soya-bean meal, groundnut meal and sunflower-seed meal were analysed for total amino acid composition and evaluated, both individually and combined in all possible pairs, as supplements to cereal-based diets for growing chicks by the total protein efficiency (g weight gain/g protein consumed; TPE) procedure. Each pair of feeding-stuffs provided 120 g supplementary protein/kg diet and the diet was made up so that the relative amounts of protein provided by each of the pair of constituents were (w/w): 120:0, 100:20, 80:40, 60:60, 40:80, 20:100 and 0:120 respectively, in addition to 60 g protein/kg provided by cereals.2. In all but one of the twenty pairs of feeding-stuffs studied the mixtures exhibited a marked synergistic effect in that the TPE value was higher than the appropriately weighted mean of the TPE values obtained with the individual components.3. Neither chemical score ([amount of limiting amino acid/the chick's requirement for the same amino acid] × 100) nor essential amino acid index; geometric mean for the ratio, amount of essential amino acid: the chick's requirement for that amino acid, for all ten essential amino acids) calculated from the amino acid composition of the dietary constituents could be used routinely to predict the results of the chick growth test, although chemical score did parallel the TPE values in some instances. In a number of instances, mixtures containing an apparently less favourable amino acid composition than one of the components of the mixture gave a higher TPE value.4. It seems likely that the relative proportions of a number of amino acids determine the optimum combination of a mixture of proteins. The removal of amino acid deficiencies alone is not sufficient to ensure that a given mixture of proteins produces optimum performance in growing chickens.