The effects of varying dietary concentrations of amino acids on body composition of growing poultry are imperfectly documented. However, the results of three studies indicated striking effects of dietary isoleucine and lysine on fat content of 3-week-old broiler chicks. Contrasting effects were observed, depending on the degree of deficiency of either amino acid (Figs 14.18 and 14.19). At very low levels of isoleucine or lysine, fat content was relatively low but this increased progressively with graded additions of either amino acid.
300 350 400 450 500 550 600
Valine intake (mg day-1)
Fig. 14.17. Daily weight gain and valine intake of turkey poults fed 14.2 (•) and 20.2 (o) g leucine kg-1 diet. (From D'Mello, 1988; source of data: D'Mello, 1975. Reproduced with permission of The World's Poultry Sci ence Association.)
This, presumably, is a reflection of extremely low food intakes which are a characteristic feature of severe amino acid deficiency (D'Mello and Lewis, 1978; Chapter 7). However, a point was reached for each amino acid when further dietary additions reduced carcass fat content. Thus the effect of an amino acid on carcass fat concentrations depends upon the extent of deficiency, a severe inadequacy eliciting much lower fat concentrations than moderate deficiency. It may be argued that it would be more instructive to consider lipid gain in relation to amino acid intake. However, the lack of requisite data in the paper by Velu et al. (1972) does not permit such an approach without employing untenable assumptions. In any event, it is unlikely that any manipulation of data would substantially alter the pattern of contrasting responses shown in Figs 14.18 and 14.19.
It should be noted that the responses relating to severe deficiency (Velu et al., 1972) were derived by the graded supplementation procedure with purified diets. Responses referring to moderate deficiency were obtained by the diet-dilution technique using diets based largely on conventional protein sources (Gous and Morris, 1985; Burnham et al., 1992). The carcass fat data shown in Fig. 14.19 obtained by the diet-dilution method are markedly different from those observed by Seaton et al. (1978) with the graded supplementation technique. Their data indicated no effect of lysine, within the range 7.2-16.8 g kg-1 diet, on carcass fat content of 3-week-old chicks despite continuing growth increments up to 10.4 g lysine kg-1 diet. Thus methodological aspects should be considered in any interpretation of carcass fat responses to dietary amino acid concentrations.
Dietary isoleucine concentration (g kg-1)
Fig. 14.18. The effects of dietary isoleucine concentration on lipid content of broiler chicks at 3 weeks of age. Experimental diets were fed from 8 days of age. (Source of data: Velu et a/., 1972 (•); Burnham et a!., 1992(G).)
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