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Seasonal Vitamin A Depletion in Grazing Horses Is Assessed Better by the Relative Dose Response Test than by Serum Retinol Concentration^1,2,3,

Kathleen M. Greiwe-Crandell^*,4, D. S. Kronfeld^*, L. A. Gay^* and D. Sklan

^* Department of Animal and Poultry Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 Department of Animal Science, Hebrew University, Rehovot 76-100, Israel

Vitamin A influences growth and reproduction in horses. A retinol dose response (RDR) test for retinol has been shown to be better than serum retinol concentration for assessing vitamin A status in other species, so we have compared these two methods in the horse. Forty-five Thoroughbred broodmares were assigned randomly to three groups fed pasture and hay (PH), pasture, hay and vitamin A-free concentrate (PHC), or hay and concentrate (HC) in early summer (May 1991). Mares in pasture groups 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 concentration. In the weanlings (PH and PHC only), no differences between groups were found for serum retinol, but the RDR was significantly higher in the PH group, which had suffered a respiratory infection, than in the PHC group. These findings indicated that vitamin A depletion was detected more readily by the RDR test than by serum retinol concentration, that consumption of pasture delayed depletion in the late fall, and that infection was associated with lower vitamin A status.

KEY WORDS: • vitamin A • relative dose response • horses • forage • infection

¹ Part of this study was presented in a preliminary report (Greiwe-Crandell, K.M., Kronfeld, D.S., Morrow, G.A. & Teigs, W. (1993) Vitamin A depletion in Thoroughbreds: a comparison of pasture and non-pasture feeding regimes. Proceedings of the Thirteenth Equine Nutrition and Physiology Symposium, Gainesville, FL, pp. 1–2.).

² Supported in part by Paul Mellon, Upperville, VA, Mr George Ohrstrom, The Plains, VA, and the Waltham Centre for Equine Nutrition and Care, Verden, Germany.

³ The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

⁴ To whom correspondence should be addressed: Virginia Tech MARE Center, 5527 Sullivans Mill Road, Middleburg, VA 22117-9701.

Manuscript received 31 December 1994. Revision accepted 8 June 1995.