Conclusions

The responses of growing poultry to individual amino acids may be determined empirically by the graded supplementation technique or by the diet-dilution method. Although initial evaluation procedures indicated considerable compatibility in growth responses derived by these methods, subsequent research revived the debate concerning the validity of the diet-dilution technique. It is a condition of this procedure that responses are not confounded by the unavoidable variation in protein contents of diluted diets. It is now known that this condition is not fulfilled since distinct and disparate responses occur to graded supplements of the limiting amino acid at each protein level in the diluted series. Indeed, it has now been concluded that the amino acid requirements of the growing chick are 'a simple linear function of the dietary protein content' (Morris et al., 1987). Their results also implied reduced utilization of the first-limiting amino acid as dietary crude

200 r

200 r

Dietary lysine concentration (g kg-1)

Fig. 14.19. The effects of dietary lysine concentration on lipid content of broiler chicks at 3 weeks of age. Experimental diets were fed from 7 or 8 days of age. (Source of data: Velu et at., 1972 (•); Gous and Morris 1985 (o).)

Dietary lysine concentration (g kg-1)

Fig. 14.19. The effects of dietary lysine concentration on lipid content of broiler chicks at 3 weeks of age. Experimental diets were fed from 7 or 8 days of age. (Source of data: Velu et at., 1972 (•); Gous and Morris 1985 (o).)

protein increased. This protein effect on amino acid utilization was attributed to amino acid imbalance. However, a study by D'Mello (1990) employing the supplementation technique indicated that moderate or severe imbalance exerted no effect on the utilization of the first-limiting amino acid. The biochemical basis of the protein effect on amino acid responses of growing chicks remains elusive. A limited analysis of data reveals that the diet-dilution and graded supplementation techniques may elicit contrasting effects on carcass fat content of chicks fed varying dietary concentrations of individual amino acids.

A critical review of factors affecting amino acid responses in growing poultry indicates that environmental temperature, immunological stress, sex, age, species, dietary energy content and dietary amino acid imbalance all exert their effects by altering food intake. However, in the case of deleterious antagonisms such as those involving BCAA or that between lysine and arginine, there are indications of genuine changes in the efficiency of amino acid utilization. In addition, there is limited evidence of genetic differences in amino acid utilization in growing chicks. The implications for other genetic models, including lean and fat lines and transgenic poultry are still unfolding.

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