Conclusions

The categories of imbalances, antagonisms and toxicities elucidated in studies with the rat constitute a satisfactory basis for classifying the adverse effects of amino acids in farm livestock. Amino acid imbalances are ubiquitous in conventional diets fed to non-ruminants, causing reductions in growth and N utilization. In pigs fed once daily, imbalances may also occur at the tissue level due to differential rates of absorption of amino acids from crystalline and intact protein sources. With poultry, high-protein diets based on poor-quality ingredients may depress utilization of the first-limiting amino acid, an effect attributed to amino acid imbalance. Antagonisms occur widely in livestock nutrition due to adverse ratios of lysine and arginine and of the branched-chain amino acids in some common feedstuffs. Adverse effects may also arise from the presence of non-protein amino acid analogues such as mimosine in Leucaena and SMCO in brassica forages. Although ruminants are normally less susceptible to dietary amino acid imbalances and toxicities, there are notable instances when ruminal metabolism serves to precipitate deleterious effects through the production of reactive metabolites from mimosine, SMCO, and tryptophan.

The adverse effects of amino acid imbalances are precipitated via reductions in food intake and alterations in the brain uptake and metabolism of amino acids, but the precise mechanisms await elucidation. In contrast, antagonisms are mediated via a diverse array of mechanisms including modulation of enzyme activities and competition with specific indispensable amino acids for transport and protein synthesis.

Animals are able to adapt to disproportionate intakes of certain amino acids by enhancing disposition. However, the most striking example of degradation is that achieved by some rumen bacteria enabling Leucaena-fed cattle and goats in many regions of the tropics to metabolize mimosine to innocuous residues.

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Gaining Weight 101

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