There is no doubt that empirical methods have attracted considerable criticism due to their perceived inflexibility, and recent efforts have become focused on modelling (Chapters 11 and 16) as the way forward. It is unfortunate that two camps have emerged on issues relating to amino acid nutrition of animals, as future developments are most likely to be dependent on a combination of empirical and modelling approaches. Thus, it is difficult to see how maintenance requirements might be established from first principles. For example, it is yet not possible to collate aspects such as immune and signalling functions into models for the determination of maintenance requirements and we will, therefore, continue to rely on empirical estimates of these requirements for the foreseeable future. Advances relating to NO, glutamine and signalling roles have all arisen from empirical investigations and not from model-driven pursuit. Clearly, there is much scope for interdependent progress on issues in amino acid metabolism and nutrition and the extent of polarization in current approaches to problem-solving cannot be justified, even in respect of the development of economic models (Chapter 25).
Hitherto, amino acids such as methionine and lysine were viewed purely as performance enhancers to supplement the deficiencies of cereals. However, an additional criterion in the future is likely to be efficacy for reduction of N pollution from animal wastes (Chapter 24). Any real progress on this issue will clearly depend on future regional and global environmental policies. As legislation becomes more stringent, there are prospects for the regular use of an additional range of amino acid supplements, including threonine and tryptophan.
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