Biochemical indicators of bone formation in foals after transfer from pasture to stables for the winter months.

Serum alkaline phosphatase activity and serum concentrations of osteocalcin, free thyroxine, protein, and cholesterol were measured in 14 foals monthly from birth to 1 year of age. A transient decrease (P less than 0.05) in alkaline phosphatase activity and osteocalcin, free thyroxine, and cholesterol concentrations was found in November after transferring the horses from pasture to stables for the winter months. During the 2 subsequent months, serum alkaline phosphatase activity and osteocalcin concentration increased slightly. Protein concentration was highest in late fall and decreased (P less than 0.001) in April and May, suggesting inadequate protein nutrition during the winter months. The data were interpreted to indicate a substantial slowdown of bone formation in foals after transfer from pasture to stables. Factors influencing bone formation may include a decrease in physical activity and in serum free thyroxine concentration.