Requirement estimation from doseresponse experiments an empirical approach

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The 'requirement' of an animal for a nutrient may be thought of as a point on a dose-response curve relating the level of intake of that nutrient and some measure of productivity of the animal or some indicator of metabolism. In the case of protein the 'requirement' is best considered as the requirement for individual amino acids rather than for the protein as a whole.

Although accurate estimates of the amino acid requirements for growth in the pig are necessary for efficient dietary formulation, considerable variation in the published experimentally determined recommendations is evident. The comprehensive reviews by Re rat and Lougnon (1968) and Rerat (1972) demonstrate the degree of this variation. The Agricultural Research Council (1981), in a technical review, emphasized the inadequacies of information concerning amino acid requirements of the growing pig. Estimates of requirements for amino acids were presented and some values are given in Table 11.1.

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  • CAB International 2003. Amino Acids in Animal Nutrition, 2nd edition (ed. J.P.F. D'Mello)
Table 11.1. Determined requirements of the growing pig for some amino acids. (Agricultural Research Council, 1981.)

Range of

requirement

(9 kg"1 air-dry

Amino acid

weight of diet)

Methionine + cystine

3.2-6.7

Tryptophan

1.2-2.2

Threonine

4.7-6.0

Isoleucine

2.7-6.7

Differences in factors such as the age, gender, breed and genetic strain of pig, physical environment, feeding level, dietary composition and experimental methods used are undoubtedly responsible for a great deal of the variation shown in Table 11.1. The methods adopted and the relative importance of experimental factors in the determination of amino acid requirements have been discussed in detail by RĂ©rat and Lougnon (1968) and Fuller (1978). Most estimates of amino acid requirements have been made from empirically derived dose-response relationships. The successful estimation of amino acid requirements using this approach is reliant on the formulation of a basal diet deficient in the amino acid being studied but adequate in all other nutrients. Graded additions of the limiting amino acid are made to the basal diet and responses are measured. Several problems are apparent. First, to ensure adequate levels of all nutrients excepting the amino acid being examined, implies that the requirements for all other nutrients are previously known. Unfortunately, this is not the case. To circumvent the problem, nutrients other than the one being examined may be supplied in generous excess of the best estimates of requirements. This practice, however, may lead to imbalance effects. The feeding of imbalanced mixtures of amino acids is likely to affect the growth rate of animals (Harper et ai, 1970). Imbalance may affect the utilization of the limiting amino acid (Chapter 7) and under conditions of ad libitum feeding a primary effect on growth due to decreased food intake may be observed. Secondly, there is no generally accepted single measurement of response and consequently the amino acid requirement determined will vary as to the type of response measured. Thirdly, assumptions need to be made concerning the shape of the response function. A linear plateau response may be appropriate for an individual animal, but will not be appropriate for the mean response of a group of animals (Fuller, 1994; Fisher, 1994). Further, some workers equate amino acid requirement with the input level corresponding to the point of maximum response whereas others choose the minimum input of the amino acid which produces a response not statistically significantly less than the maximum, as the required level. The latter approach is likely to lead to considerably lower estimates of amino acid requirements in cases whereby a large deviation of amino acid input from the input corresponding to maximum response, produces only a small alteration in response. Particularly where practical diets are used, comprising commonly used feedstuffs, there can be important differences between the available and gross amounts of a dietary amino acid, and this is a further significant source of variation.

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