The somatotropic axis of the dairy cow revisited

The greater understanding of the somatotropic axis in the lactating dairy cow that has been achieved in recent years is briefly reviewed in this article. Specifically discussed are: the significance of multiple forms of insulin‐like growth factor messenger RNA (ribonucleic acid) caused by alternative splicing, and the possible functions of the various translated extension proteins; the possible roles of multiple sites of growth hormone secretion other than in the pituitary, and the implications for possible autocrine or paracrine growth hormone secretion; the consequences for increased circulating growth hormone half‐life, and possibly biological effectiveness, caused by growth hormone binding proteins; and the increasing evidence for the presence of growth hormone receptors in the bovine mammary gland. The need for further research is highlighted in the present review; published information regarding the bovine, in many of these areas, is still largely insufficient.