A model which describes the response of groups of laying hens to different levels of amino acid intake is presented. The model is based on the assumption of simple linear relationships between amino acid intake and the output characteristics, egg production and maintenance, for individual birds. The response for a group of birds is then derived as the average of the individual responses. The shape of this flock-response curve depends on seven parameters, namely the mean maximum egg output (Ē max ), variation in E max, mean body weight (W), variation in W, the correlation between egg output and body weight, and two constants (a and b) representing respectively the quantities of amino acid required for unit egg output and for maintenance of unit body weight.Three methods of manipulating the model are described. These use (i) a computer simulation procedure; (it) the exact solution for the model and (iii) some reasonable approximations for practical application.The use of the model for describing experimental data and for predicting amino acid requirements is discussed, and the main limiting assumptions and some possible extensions of the principles involved are pointed out.

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This is a review of previously published data from experiments designed to measure the chicks' requirement for an essential amino acid in diets containing surplus protein (generally in the range 220-300 g crude protein (CP)/kg diet). The evidence shows that, within this range, the requirement for a first limiting amino acid increases nearly in direct proportion to the CP content of the diet. To explain this, the following possibilities are considered:(1) that energy supply was limiting the response to a critical amino acid, (2) that the experiments were conducted in environments which limited heat disposal by the chicks, (3) that results from the trials have been misinterpreted because growth rate rather than protein deposition was used as a response measure, (4) that the availability of amino acids in the diets was lower than had been assumed, (5) that nutrients other than the amino acid under study were limiting performance, and (6) that there was an imbalance of amino acids in the diet. It is concluded that only the last explanation fits the facts. The imbalances reported have led to a decline in the efficiency of the utilisation of the first limiting amino acid in some instances. The depressed growth observed with a high-protein diet limiting in one essential amino acid could not be attributed to lowered feed intake in most cases. The imbalance is, in these respects, different from classical imbalances which result from loading the diet with a mixture of essential amino acids. From the evidence reviewed, some doubt is cast on the validity of the dilution method for estimating amino acid requirements; nevertheless it its concluded that the dilution method remains the most trustworthy procedure for making estimates to be used in practical diet formulation. It is recommended that, in future, programmes for diet formulation should be modified to prevent Amino acid requirements for chicks: T.R. Morris et al. surplus protein being permitted in the solution, unless a proportional adjustment is made to the minima prescribed for amino acids likely to be present in limiting proportions.

1. The utilisation of fat and fatty acids supplied as maize oil, tallow or lard in the diet of 2-, 4- and 8-week old turkeys has been investigated: a mixture of 10% fat and 15% cellulose replaced glucose monohydrate in a low-fat basal diet. 2. At 2 weeks of age the absorbabilities of the fats were 96, 57 and 91% for maize oil, tallow and lard respectively; at 4 and 8 weekds of age the corresponding figures were 97, 70 and 92% and 98, 74 and 90%. 3. The unsaturated fatty acids (18:1, 18:2) were highly absorbed from all fats; this was true also of the saturated acids (16:0, 18:0) in the maize oil, but in lard and, especially, tallow the absorbability of these fatty acids was lower. 4. The effect of age on the utilisation of tallow could be explained by the improvement in the absorption of these fatty acids. 5. The determined and calculated ME values of the fats are given.

Because feed intake is controlled in broiler breeders, amino acid supply is determined by the composition of the feed and the level of feed intake. Controlling amino acid supply during the laying cycle can be facilitated by the use of a model for calculating requirements. A possible model is outlined and the various components discussed. Typical calculations suggest that the model can provide a useful basis for practical feeding decisions. Model elements include: levels of animal performance; utilization of amino acids for egg production, maintenance, and tissue growth; population structure; and the variation of feed intake and the covariance between feed intake and requirements.

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