Comparative Protein Evaluation Studies by Carcass Retention and Nitrogen Balance Methods

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Abstract
Protein evaluations were carried out with rats and chicks to elucidate certain problems encountered in earlier studies. The following observations were recorded: 1) Linseed and cottonseed meals when supplemented with the limiting amino acids promoted much improved growth in both rats and chickens. Net protein utilization (NPU) in the chicken, in contrast with results in the rat, was only slightly improved. 2) Significant differences were observed in protein utilization when the carbohydrate source was varied. For both the rat and the chicken a combination of corn starch, glucose, and a small amount of dextrin was superior to the single components of the mixture. 3) A cod fillet flour was noted to have an inferior NPU for chicks compared with commercial menhaden fish meal. Subsequent study showed the fillet flour to be deficient in arginine for the chicken. For the rat this flour was noted to be deficient in methionine. 4) Generally good agreement was observed between NPU values obtained by either the nitrogen balance or the carcass retention method in the rat. The former method consistently gave higher values than the latter, and it is suggested that these differences result primarily from a somewhat lower protein intake (and therefore better utilization) due to the equalized feeding practice with the nitrogen balance method.