AE ratios and the ideal protein concept

Arai (1981) used A/E ratios ((essential amino acid content/total essential amino acid content including cystine and tyrosine) x 1000) of whole body coho salmon fry to formulate test diets for this fish. Fish fed casein diets supplemented with amino acids to simulate the A/E ratios of whole body tissue showed much improved growth and feed efficiency. Ogata et al. (1983) used A/E ratios based on amino acid composition data from cherry salmon (Oncorhynchus masou) to design test diets for cherry salmon and amago salmon (Oncorhynchus rhodurus) fry. A casein diet supplemented with amino acids to simulate the A/E ratios of cherry salmon resulted in better growth in both species than diets containing casein alone, casein plus amino acids to simulate the A/E ratio of eyed cherry-salmon eggs, or white fishmeal.

A/E ratios have been used as a means of estimating the requirements of all indispensable amino acids when only one is known by relating the A/E ratio of each indispensable amino acid to that of the A/E ratio of the known amino acid times the requirement value for the unknown amino acid (Moon and Gatlin, 1991). This technique has also been used by Forster and Ogata (1998) to estimate the amino acid requirements of Japanese flounder and red sea bream. This method has also been used successfully in silver perch (Ngamsnae et al., 1999).

The relationships between whole body amino acid patterns and amino acid requirement patterns discussed above are very similar to the ideal protein concept that has been advocated for use in expressing the amino acid requirements of pigs (Agricultural Research Council, 1981). The ideal protein concept is based on the idea that there should be a direct correlation between the whole body amino acid pattern of the animal and the dietary amino acid requirements of the animal. In addition, since lysine is normally the first-limiting amino acid in most feedstuffs, the requirements for the other indispensable amino acids are expressed relative to the lysine requirement. Thus, if one knows the dietary lysine requirement and the whole body amino acid composition of an animal, then one should be able to estimate the dietary requirement for the remaining indispensable amino acids relative to the lysine requirement. A comparison of the amino acid requirement values as determined by conventional means in our laboratory and as estimated based on the ideal protein concept for channel catfish is presented in Table 23.14. These data show excellent agreement. This procedure is essentially the same as the one discussed above for the use of A/E ratios.

Table 23.14. A comparison of amino acid requirement values (g kg-1 of protein) as determined by conventional means and as estimated based on the ideal protein concept for channel catfish.

Amino

Amino acid

Determined

Estimated

acid

ratio

requirement

requirement

Lysine

100

51

Arginine

78

43

40

Histidine

25

15

13

Isoleucine

50

26

26

Leucine

87

44

44

Met + Cys

44

23

22

Phe + Tyr

87

50

44

Threonine

52

20

27

Tryptophan

9

5

5

Valine

61

30

31

The ideal protein concept has been used as a means of estimating the amino acid requirements of a Clarias hybrid from Thailand (Unprasert, 1994). Requirement values were determined for lysine, methionine, threonine, tryptophan and leucine by growth studies. Requirement values for the remaining indispensable amino acids were then estimated relative to each of the amino acids determined by growth studies. Some variation was observed in these results which were attributed to difficulty in obtaining competent amino acid analysis of the whole body sample of the test fish.

To use this procedure, all that is needed to formulate diets based on the amino acid needs of a certain fish species is to determine the whole body amino acid composition and the lysine requirement of that species. This new procedure should be much less time consuming and less costly than determining amino acid requirements of the fish by conventional means.

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