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.
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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