Seasonal changes in systemic hormone profiles and their relationship to patterns of fibre growth and moulting in goats of contrasting genotypes.
40 Siberian and Icelandic × Scottish feral (IF) goats were fed on diets (grass hay, sugarbeet pulp, barley, wheat feed and grass pellets) containing 100 or 180 g crude protein in a 2 × 2 factorial experiment lasting 14 months. Seasonal changes in hormones and insulin-like growth factor-1 (IGF-1) were measured. Mean plasma concentrations of insulin, cortisol, triiodothyronine (T3) and thyroxine (T4) were highest in winter, and those of prolactin, somatotropin and IGF-1 were highest in summer. Siberian goats had higher plasma prolactin and lower plasma insulin concentrations than IF goats between January and March; Siberian goats had secondary fibre growth during this time whereas IF goats did not. The observed genotypic differences in times of onset and cessation of fibre growth were not associated with differences in the times of seasonal changes in any of the other hormones measured, and there was no effect of dietary protein level on hormone profiles. The higher mean growth rate and greater diameter of secondary fibre in Siberian goats was associated with higher mean concentrations of T4 than in IF goats. Four Siberian goats which exhibited secondary fibre growth during the summer had higher mean plasma insulin concentrations than other Siberian goats.