SEASONAL VITAMIN-A DEPLETION IN GRAZING HORSES IS ASSESSED BETTER BY THE RELATIVE DOSE-RESPONSE TEST THAN BY SERUM RETINOL CONCENTRATION

Vitamin A influences growth and reproduction in horses. A retinol dose response (RDR) test for retinol has been shown to be better than seru m retinol concentration for assessing vitamin A status in other specie s, so we have compared these two methods in the horse. Forty-five Thor oughbred broodmares were assigned randomly to three groups fed pasture and hay (PH), pasture, hay and vitamin A-free concentrate (PHC), or h ay and concentrate (HC) in early summer (May 1991). Mares in pasture g roups produced 23 foals (March through June) that had access to their dam's diets and were also studied. In the mares, significant vitamin A depletion developed in 2 mo in the nonpasture group (HC) and in 8 mo in the two pasture groups (PH and PHC) according to the RDR test, and in all three groups at 8 mo as shown by a decrease in serum retinol co ncentration. In the weanlings (PH and PHC only), no differences betwee n groups were found for serum retinol, but the RDR was significantly h igher in the PH group, which had suffered a respiratory infection, tha n in the PHC group. These findings indicated that vitamin A depletion was detected more readily by the RDR test than by serum retinol concen tration, that consumption of pasture delayed depletion in the late fal l, and that infection was associated with lower vitamin A status.