During the summer season, plasma concentrations of insulin-like growth factor I (IGF-I) in Arabian, Anglo-Arabian and Hucul horses as well as colts and fillies were higher compared to the winter season. Significant differences in IGF-I concentration between the seasons were found in Hucul horses (P≤0.01) and in colts (P≤0.05). Season had no significant effect on 1.25(OH)2-D3 concentration in the horse breeds studied. However, plasma concentrations of this hormone were always higher in the summer than in the winter season regardless of the breed (except Anglo-Arabians) and sex. Knowledge and monitoring of the physiological concentrations of IGF-1 and vitamin D3 metabolites in different seasons of the year, combined with analysis of growth rate during these periods may contribute to the improvement of the horse management and feeding system. Because of the association between these hormones and the frequency of developmental orthopaedic diseases, this information could be potentially used in veterinary practice.

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