Protein quality assessment by the use of weanling rats. 1.—Some factors affecting growth measurements

Abstract

AbstractInterrelationships amongst the factors—protein source, breed group, sex and level of food intake, have been investigated with weanling rats housed in cages with solid or screen floors for a three‐week period during which they were fed diets limiting in protein content. The responses were measured in terms of weight gain, FCE (food conversion efficiency), apparent NPU (net protein utilisation) and carcass fatness.The patterns of responses as measured by bodyweight gain, FCE and app. NPU (carcass) were similar but not identical. Higher values for bodyweight gain, FCE and app. NPU (carcass) were associated with greater carcass fat content. The best diet (that containing Danish herring) led to the most carcass fat and the F1 crossbreds were the fattest breed group.Rats on solid floors consumed less food ad lib. It is suggested that this may result from more coprophagy in solid floor cages or greater thermal insulation from the bedding.The evidence indicated that the Norwegian Hooded (N.H.) rats possessed a relatively lower dietary requirement for lysine and methionine than either of the other two breed groups, so that phenylalanine, histidine, leucine and isoleucine may have limited their growth, whilst lysine and methionine were the most limiting amino acids in the three diets for the Albino and F1 crossbred groups. A valid biological evaluation of proteins for farm livestock, therefore, requires an accurate knowledge of the relative amino acid requirements of the rat strains used. The F1 crossbred groups gave higher app. NPU (carcass) values than the other two groups and showed slightly greater sensitivity to dietary differences as measured by FCE and app. NPU (carcass). The omission of any adjustment for bodyweight maintenance partly accounted for these higher values.The interactions of level of food intake and breed with diet as measured by FCE throw doubt on the reproducibility and validity of protein efficiency ratio values. Covariance adjustment of response for variations in ad lib. food consumption was inappropriate as the regression may have been influenced by other factors, such as vigour and maintenance requirement of the rat. The use of two dietary protein levels for each test protein allowing an adjustment for bodyweight maintenance, would have overcome only some of the defects mentioned.