Com pelling evidence fo r the genetic regulation of som e com ponents of the som atotropic axis is presented using inform ation gathered from a variety of anim al m odels. C urrently, a simple m easure of circulating ST that shows a consistent association with genetic m erit fo r growth is not available. F urtherm ore, there is a need to elaborate on the various physiological and m etabolic functions of som atotropin (ST) to ensure that m odification of circulating ST levels by selection does not have undesirable effects on o th er production traits. Insulin-like grow th factor-1 (IG F-1) appears to be a useful physiological predictor of genetic m erit for growth. W hile selection for higher plasm a levels of IGF-1 in mice has resulted in higher grow th rates, particularly around puberty, it has also caused an increase in m ature body size. This may not be desirable in an anim al production system due to increased feeding costs to support the breeding population. H ow ever, initial evidence would suggest that an associated increase in m aternal reproductive capacity m ore than offsets the increase in m ature body size. W e conclude that plasm a levels of IGF-1 w arrant further exam ination as an aid to increasing the rate of genetic gain in grow th.

Hugh T Blair, S. N McCutcheon, D. DS Mackenzie

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume XVI. Poultry, fish and horse genetics and breeding, growth and reproduction, immune response and disease resistance., , 246–255, 1990
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