Studies on the Protein Efficiency Ratio Method for the Evaluation of Poultry Feed Supplements. Modifications Associated with Choice of Dietary Protein Level for Assay

Abstract

Four chick growth experiments were conducted to compare the protein efficiency ratio (PER) and the total protein efficiency ratio (TPE) methods for evaluating five protein concentrates; soybean meal, peanut meal, normal fish meal, fish meal subjected to either dry or moist heating; and gelatin-supplemented casein, the latter serving as the standard. The concentrates substituted for 40% of the total protein contents of basal corn-soybean rations varying widely in protein levels. There were no significant differences between the PER (based on average weight gain and protein intake) and the TPE (calculated as the total weight gain, including weight of dead chicks, divided by the total protein intake) at the protein levels studied. A new index, the maximum protein efficiency ratio (PERmax), was defined as the PER value estimated at the optimum dietary protein level. It was estimated at the point of inflection on the graph of feed efficiency vs. percentage protein content. Reasons and the procedure for the use of feed efficiency in the estimation are discussed. The three methods were further evaluated by comparing differences or separations among PER values determined at the conventional 10% protein level, PER at 18.5%, TPE at 18.5%, and the PERmax. For most paired comparisons, the conventional PER gave the widest separations, but in the case of dry- and moist-heated fish meals, separation by the PERmax method (at a higher protein level) was the widest. Based on the amino acid contents of the whole diets and literature information, it was concluded that the conventional PER gives greater separations where diets or proteins have greater relative deficiencies in sulfur amino acids. Where there are greater relative deficiencies in lysine, the PERmax or PER at higher but suboptimal protein levels gives greater separations. The importance of cystine/methionine ratio is also discussed.