Serum Profiles of Vitamins A, E and D in Mares and Foals During Different Seasons

Twenty mares and 15 foals of the Finnhorse breed were used to determine serum levels of vitamins A, E and D during different seasons. Monthly serum samples were analyzed from mares for 22 mo and from foals for 13 mo after birth. The horses were kept on pasture from early June until early October and in stables during the winter months. Seasonal differences (P < .001) were found in serum retinol, retinol to protein ratio, α-tocopherol and α-tocopherol to cholesterol ratio. Values were lowest from February through May and highest from June through August. Serum levels of retinol and α-tocopherol of the foals were much lower (P < .001) than in mares during the first 4 mo of life. As in the mares, serum levels of retinol and α-tocopherol decreased (P < .001) in winter. At 1 yr of age, serum levels of these vitamins increased rapidly on pasture to values that were close to those found in adult horses. Serum 25-hydroxyvitamin D levels were very low throughout the year. However, a small seasonal variation also was found in 25-hydroxyvitamin D concentrations in the mares. Serum 1,25-dihydroxyvitamin D levels in pregnant mares in winter were similar to those found in lactating mares in summer. These levels also were low compared with those in animals of other species. The results are interpreted to indicate that dried hay and stored oats are not sufficient to ensure adequate circulating levels of vitamins A and E in pregnant mares and weanlings in winter and that a deficiency may develop within a few months if the horses are kept in stables for long periods of time without fresh green forage or vitamin supplementation.