92 Seasonal variation in plasma vitamin A and E concentrations in horses with or without pasture access

Abstract

Vitamins A and E are essential nutrients for horses and often provided in the diet by fresh forage but become depleted when forage is cut and dried for hay and can lead to potentially fatal health problems in horses. This study evaluated seasonal changes in plasma vitamin A and E for a mature horse herd in the Upper Midwest to determine the impact of pasture access on the prevalence of vitamin deficiency. Researchers hypothesized horses on dry lot would be deficient all year while horses with pasture access would have increased vitamin A and E concentrations during the grazing season. The study took place between February and November 2022 in River Falls, Wisconsin. Twenty-seven mature horses were divided into 2 treatments based on management practices including 1) horses consuming a hay-only diet in a dry lot (n = 13) and 2) horses with pasture access in the summer (n = 14). All horses had ad libitum access to water and a vitamin and mineral mix (200 IU/kg Vitamin E; 176,320 IU/kg Vitamin A) for the duration of the study and received ad libitum cool-season grass hay round bales from September through May. Hay-only horses continued this diet from June through August while pastured horses continuously grazed cool-season grass pasture. Jugular venous blood samples were taken in February (winter), May (spring), August (summer), and November (fall) and plasma samples were sent theMichigan State Veterinary Diagnostic Lab for analysis of plasma vitamin A and E concentrations. Data were analyzed using PROC MIXED in SAS with significance set at P ≤ 0.05. Adequate vitamin A was classified as 175 to 300 ng/ml, and adequate vitamin E was classified as 2.0 to 4.0 µg/ml. Vitamin A deficiency was observed in 81% and 85% of horses in winter and spring, respectively, with an average concentration of 145 ng/ml. Pastured horses exhibited an increase in plasma vitamin A in the summer compared with dry lot horses (P ≤ 0.05), however 64% of pastured horses remained deficient. In the fall, vitamin A deficiency increased to 97% for all horses and plasma vitamin A averaged 139 ng/ml. When evaluating vitamin E, 70 to 74% of horses were deficient in the winter and spring, respectively, with an average concentration of 1.83 µg/ml. Plasma vitamin E concentrations did not change over time for dry lot horses and remained inadequate in the summer at 1.43 µg/ml (P > 0.05) while concentrations in pastured horses rose to 2.4 µg/ml (P ≤ 0.05). These results confirm recommendations to provide additional vitamin A and E to horses on a hay-only diet and demonstrates the short-lived increases in vitamin A and E for horses provided limited pasture access.