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Limitations to the manipulation of growth

Many areas of science become associated with both a particular centre and with the scientists who staff it, and there is no more famous trilogy than growth, Cambridge and the Hammond School. It is therefore difficult to see how the contributions of this distinguished group of workers and their numerous disciples can be incorporated into an original review which substantially supplements the many excellent summaries of growth in mammals which already exist. Perhaps the only justification is that for each true disciple of Hammond there are imitators. In no other area of animal production is it so easy, or so frequently practised, for marginal adjustments to the conclusions arising from experimental studies to be made and then, as a result of many such modifications, for the over-all interpretation to be so different from those of the original author. The objective of this paper is to attempt to discuss those aspects of growth which can be generally accepted so that areas of ignorance, matters for debate or controversy can be identified. It is impossible to justify each statement in this paper with a comprehensive catalogue of references. The studies quoted should be considered merely as examples of specific features of growth rather than as either a comprehensive list of references or even the most important examples available since many of the experiments quoted are those in which the author had had some particular interest. It is accepted that this approach is subjective in terms of the material selected and others may reach very different conclusions by using different sources of information. The greatest achievements of the Hammond School, probably best summarized by Palsson (1955), were to provide data on the growth of farm livestock which had not previously been published, to provide a cadre of workers who were fascinated by growth and to provide a series of papers from which emerged principles concerned with growth and development. These papers resulted in a confidence that the manipulation of growth was possible and that such techniques could be usefully and profitably incorporated into the technology of animal production. The individual experiments, acting as catalysts for change, inaugurated a new interest in animal husbandry. In particular the experiments of Wallace (1948) and McMeekan (1940) provided evidence that the strategic use of nutrition could lead to the control of growth in early life and reduce mortality as well as enable the tailoring of the carcass to meet both the needs of the consumer and the farmer. It would be nake to believe that such concepts were new, they were at least as old as Virgil, but the Hammond School and the world-wide sub-