Question

I have an aged Arabian broodmare that weighs about 950 lb (435 kg). She’s fed both alfalfa/orchardgrass and teff hays. While she has 24/7 turnout, she has no access to green grass. Because of an injury sustained during endurance competition, I decided to breed her, and she’s now four months pregnant. Blood test results suggest a vitamin A deficiency, but when I look at the diet, she should be consuming plenty of vitamin A. Should I ignore serum vitamin A levels, or should I try a supplement in an attempt to raise this?

Answer

Vitamin A research found that serum is not a useful indicator of vitamin A status until levels are precipitously low. A relative dose-response test was found to be a more accurate measure of vitamin A status. In addition to the type of test used, handling of blood samples may also affect results, as vitamins are organic compounds that are susceptible to oxidation. If the vet that drew the blood let the sample get too warm or sit around too long, it may have affected the accuracy of the test.

Nevertheless, if the results are accurate, your mare may have a vitamin A deficiency. If she had been on green pasture I would question the lab results as green pasture contains an abundance of the beta-carotene. However, the amount of vitamin A in hay fluctuates tremendously. To calculate the amount of vitamin A in hay, conversion from beta-carotene to vitamin A must be recognized. This conversion happens with help of a specific enzyme found in the digestive tract. Because individual horses produce varying quantities of the enzyme, some horses can test low in vitamin A even if a thorough ration evaluation indicates sufficient intake of the nutrient. Also, the amount of carotene in hay can be variable, depending on how the hay was handled during harvesting and how long it was in storage.

I would not ignore the low test result. You may have the lab run another test to double-check the results. If the result is the same, a supplement is appropriate, as inadequate vitamin A can affect the mare’s pregnancy. Vitamin A is important for the health of all the reproductive tract endothelial tissues. Because your mare is on an all-forage diet, it would be best to add a high-quality vitamin and mineral supplement that includes an appropriate level of vitamin A. Avoid supplementation with a source of vitamin A by itself, as excessive intake can be toxic.