Importance of dietary protein level in the carcass analysis method for determining net protein utilisation in the chick

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Abstract
Two series of studies concerning chick NPU (net protein utilisation) determinations were carried out. In the first, two levels of dietary protein were compared with diets deficient in methionine or lysine. While the 10% protein level gave higher absolute NPU values than the 13% level, the former may lead to erroneous conclusions if the dietary protein is deficient in lysine. Since lysine is required in very low amounts for maintenance and the 10% protein level provides little more than the maintenance needs of the chick, the 13% level is to be preferred; this level provides sufficient excess for growth purposes so that a lysine deficiency can be more effectively recognised by the NPU value obtained.In the second series of experiments, three levels of fibre and maize oil were included factorially in a 13% protein diet composed of isolated soya protein supplemented with methionine. The combination of 3% fibre and 6% maize oil gave considerably higher NPU values than were obtained with any of the other combinations, although there was a general trend for higher NPU values with increased dietary energy. No adequate explanation for this finding is available at present.