The effects of bovine somatotropin on hair follicle activity and cashmere fibre growth in goats

The aim of this study was to determine the role of circulating growth hormone (GH) profiles in the cessation of secondary hair follicle activity and secondary fibre growth in goats during autumn/winter and to determine whether it is possible to extend the period of growth of this fibre by artificially increasing circulating concentrations of this hormone. Nine cashmere goats were each injected, weekly, with 50 mg of bovine somatotropin (bST; treated) in carrier oil, for a period of 5 months between late August and late January. Goats of a second group of 9 animals were injected with sesame oil and served as controls. Treated goats had significantly higher circulating concentrations of growth hormone (P < 0.001), insulin (P < 0.01), thyroxine (P < 0.001), and insulin-like growth factor-I (IGF-I; P < 0.001). Secondary fibre growth rate and liveweight gains were significantly (P < 0.05) higher in treated than control goats. The proportion of active secondary follicles was positively associated with circulating IGF-I concentrations in treated but not control goats. Follicle activity was not related to the profiles of any of the other hormones measured. The mean date of cessation of secondary fibre growth was not affected by treatment. It is concluded that treatment with bST stimulated the rate of secondary fibre growth but did not delay the time of cessation of this growth or the decline in follicle activity. It is further concluded that circulating hormone profiles do not directly control follicle activity and secondary fibre growth and moult and that their effects probably involve changes within the skin and/or hair follicle, possibly in hormone receptor populations, deiodinase enzyme activity, or growth factor synthesis.