The response of laying hens to dietary amino acids

The level of a single essential amino acid that is either deficient or in excess of requirement may result in a diet that does not optimise the economic efficiency of a laying hen production system. The objectives of this study were, first, to quantitatively describe the effect of increasing the dietary levels of a single limiting amino acid on the egg production characteristics of laying hens by a statistical analysis and assessment of the published literature. Second, to compare three methods of describing the dietary amino acid concentration; as a proportion of the diet (g/kg of feed), as a proportion of the crude protein (g/kg crude protein) or as a proportion of the ideal crude protein (g/kg ideal crude protein). Sufficient published experiments were available to give statistically valid comparisons of lysine (42 experiments), methionine (77 experiments), methionine plus cystine (77 experiments) and tryptophan (21 experiments). Amino acid concentration was described by three different methods; concentration in diet (g/kg of feed), concentration in the crude protein (g/kg crude protein) or concentration in the ideal crude protein (g/kg ideal crude protein). An exponential curve gave the best fit to these data sets for almost all variables. The exceptions were egg weights with tryptophan (no relationship (p>0.05)) and egg weights and egg mass output with methionine plus cystine (linear relationship only). Expressing the egg production responses as a proportion of the crude protein, as compared to a proportion of the diet, gave a reduction in the residual standard deviation and increased the proportion of explained variation for all variables examined. The results indicated that expressing amino acid supply as a proportion of crude protein is preferable in laying hen nutrition.