Dietary factors affecting the utilisation of protein by laying hens and their effects on the concentrations of certain nitrogenous constituents in the excreta

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Abstract
New techniques involving extraction with organic solvents and lithium carbonate solution have been used to fractionate nitrogenous components of the excreta from laying hens fed diets in which the supplementary protein was groundnut meal, poor‐quality fish meal or good‐quality fish meal. The effect of adding methionine or lysine or both to the groundnut diet was also studied. The proportion of ingested nitrogen excreted as uric acid or urate was significantly higher when the dietary protein supplement was groundnut meal than when it was good‐quality fish meal or groundnut meal supplemented with both lysine and methionine. Nitrogen retention, egg production and the apparent digestibility of nitrogen were higher on diets containing good‐quality fish meal than on diets containing groundnut meal alone. Intermediate effects were obtained when feeding diets supplemented with poor‐quality fish meal of groundnut meal containing either lysine or methionine. The implications of these findings are discussed. The main differences in protein utilisation associated with variations in dietary protein quality were reflected in the proportions of ingested nitrogen excreted as urates and as protein. Other nitrogenous components of excreta examined showed little variation with dietary protein quality. Benefits derived from the addition of methionine and lysine to the groundnut diet were obtained mainly by a more efficient use of absorbed amino acids.