Amino acid utilization by growing sheep is somewhat more difficult to assess than that by cattle because the impact of wool growth must be considered. Wool production represents an irreversible loss of amino acids which might otherwise be used for tissue deposition. Additionally, wool contains a high concentration of cysteine, and, as such, its production requires a large amount of sulphur amino acids, either as cysteine directly or as methionine for the production of cysteine.
In a classic experiment, Storm and 0rskov (1984) determined which amino acids were most limiting in ruminal microbial protein for growing lambs. The intragastrically maintained sheep were supplied with isolated ruminal microbial protein and an amino acid mixture supplying an additional 25% of all of the essential amino acids in the same profile as in microbial protein. Individual amino acids were subsequently removed from the supplement. The primary advantage of this deletion approach is that responses could be measured to individual amino acids under conditions where other essential amino acids were not limiting. The most limiting amino acid in microbial protein for growing lambs was clearly methionine, and lysine, histidine and arginine were also found to be among the limiting amino acids. Using a similar approach, Storm et al. (1983) reported that the efficiency with which truly digested rumen microbial protein was used for nitrogen retention was 66%. Clearly, this efficiency of use of microbial N for growth would have been increased by supplementation with those amino acids found to be limiting. However, there are concerns that amino acid use is greater for intragastrically maintained lambs than normally fed sheep due to changes in the gut as a result of the intragastric nutrition. In contrast to the efficiency of 66% observed by Storm et al. (1983) for intragastrically maintained lambs, MacRae et al. (1995) observed efficiencies of use of total essential amino acids of 50-59% in lambs fed forage or a forage and barley diet.
A number of earlier studies considered which amino acids were most limiting for growing lambs. Schelling and Hatfield (1968) observed that lambs fed a purified diet containing no true protein (580 g kg-1 maize-starch, 300 g kg-1 cellulose, 43 g kg-1 urea) were most limited by the supply of lysine. In contrast, subsequent work with lambs fed a similar diet indicated that methionine was the most limiting amino acid (Nimrick et al., 1970a). The conclusion that methionine was more limiting than lysine was supported by additional studies to quantify the methionine requirement (Nimrick et al., 1970b). Similarly, for lambs fed diets containing maize and hay, methionine was observed to be the most limiting amino acid (Schelling et al., 1973). Several points should be noted with regard to these studies. First, the lambs had relatively low growth rates; based on N retention, lambs probably had body weight gains of less than 100 g day-1. Second, the efficiency of utilization of the supplemental methionine (increase in methionine deposition/amount of methionine supplemented) by the lambs was low, being less than 20% based on the improvements in N retention and expected methionine content of the gain. Third, in some of these studies, improvements in N retention were observed when glutamate was supplemented, which suggests that non-specific N was limiting; therefore, glutamate was supplemented to lambs in some of the studies to ensure that its deficiency did not limit performance.
More recent work with growing lambs (Matras et al., 2000) determined if methionine and(or) lysine was limiting when lambs were fed barley-based diets or diets based on maize and maize gluten meal. Nitrogen retention was improved about equally by supplementation with lysine or methionine, with responses being greater for lambs fed the maize-based diets. However, responses to methionine and lysine in combination were not greater than those to either of the amino acids alone. Thus, it is not clear which amino acid was most limiting or why additive or synergistic responses were not observed if they were essentially co-limiting. As in the earlier studies, growth rates of lambs were low in this study.
Numerous studies have demonstrated that sulphur amino acids generally are most limiting for wool growth by mature sheep fed forage-based diets (Bird and Moir, 1972; Reis et al., 1973; Doyle, 1981; Liu et al., 2000). Wool contains much higher concentrations of cysteine (98 g kg-1 amino acids) than of methionine (6 g kg-1 amino acids) (MacRae et al, 1993), which suggests that cysteine would be physiologically more important than methionine for improving wool growth. However, methionine is capable of supplying cysteine via transsulphuration, which in sheep can occur in various tissues including the skin (Chapter 17; Liu et ah, 2000).
Recently, Liu and Masters (2000) developed a model to predict the methionine and cysteine requirements of sheep. Although the model focuses on wool growth, several important aspects relevant to growth are delineated. For methionine, requirements were set as the amount deposited in wool and in body proteins plus the amount oxidized. Methionine oxidation was predicted as a function of whole-body methionine flux, which was set as a function of metabolizable protein intake. Cysteine requirements were similarly estimated except production of cysteine from methionine via transsulphuration was subtracted from the amount of absorbed cysteine required. Transsulphuration of methionine to cysteine was estimated as a function of methionine flux. This model provides an excellent initial step in predicting animal requirements and does an excellent job of delineating some of the factors that will impact amino acid utilization. The model appeared to be quite sensitive to the estimation of oxidation, which reflects either the sensitivity of the model to this estimate or the inability of the model to predict oxidation with precision. The sensitivity of the model to estimates of oxidation would be expected because absorbed methionine would either be oxidized or deposited in either wool or the body. The partitioning of protein deposition between wool and fleece-free body also appeared to be an important consideration in the prediction of allowable wool growth. With regard to amino acid availability for growth, the model points out the issue of prioritization of use. The model calculates that for sheep experiencing no change in body protein, 0.42 g day-1 of wool will be produced, and that additional gains in protein deposition will be divided with 31% occurring in the fleece and 69% in the fleece-free body. If this partitioning truly occurs, it would set definite limits on the efficiency of amino acid use for tissue growth. The impact of this partitioning on efficiency of use of different amino acids would depend on the relative concentrations of the amino acid in wool and body proteins, but clearly would be important for methionine utilization due to the high cysteine concentrations in wool.
The model of Liu and Masters (2000) also addresses issues relevant to amino acid oxidation, which is the primary feature that impacts the efficiency of amino acid use. Oxidation is a function of the amino acid flux, which in turn is dependent on dietary protein intake. Thus, high producing animals (i.e. those absorbing large amounts of amino acids) will have a large portion of the amino acids oxidized and, consequently, low efficiencies of amino acid use. Thus, efficiency of amino acid use is not a constant value. One apparent flaw in the model is that all cysteine produced from transsulphura-tion is assumed to contribute to whole-body cysteine flux, and, consequently, increases in transsulphuration lead to increases in cysteine oxidation. In sheep, a large portion of transsulphuration appears to occur in the skin with utilization of the resultant cysteine prior to its entry into the whole-body cysteine pool. The authors suggest that future modifications to the model will address this issue.
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