Page 78 - Speedhorse June 2019

Deficiencies can occur if horses are never on pasture and don’t get enough green forage, or if the hay being fed is
old and doesn’t have much carotene left. Crandell’s PhD work involved looking at whether recommended level in the National Research Council (and a horse’s require- ment for vitamin A) was too low. “I found that the traditional levels recommended were a bit low – right at the point where you might see subclinical signs of vitamin A deficiency,” she says.
It’s not always easy to check vitamin A levels in the blood to determine whether a horse is deficient or has too much of this vitamin. “Because the vitamin A level is maintained in the blood by the liver, taking a blood sample doesn’t always give you an accurate indication. If a horse is deficient, you may get lower serum levels of vitamin A, but with toxicity you might not see an increased level of vitamin A in the blood.”
Instead of taking a blood sample, it’s more effective to use a function test for vitamin A. “This is called relative dose response (RDR). It is done by giving a dose of vitamin A, then looking at how the body reacts to it. In my PhD work, I did a series of RDRs by taking blood samples from the test horses and then giving them a dose of vitamin A, and 15 hours later when it had peaked taking another blood sample – to see a rise and fall, or lack of rise, in this vitamin. If there was a large rise in blood levels, this could be an indication that the horse was deficient,” says Crandell.
She did this study over a three-year period. “The first year, we were trying
to induce deficiency in vitamin A. We
were not able to make horses on pasture deficient. But in winter when the grass was no longer green, the relative dose response showed that they were becoming deficient. Just as soon as there started to be a little bit of green in the grass in the spring, however, the deficiency resolved,” she says.
The amount of vitamin A in hay can lose up to 80% of the carotenes within the first 24 hours of being cut, but green, leafy alfalfa has a lot of vitamin A and the remaining carotene levels are adequate for many months.
In her study she had some horses in a dry-lot, being fed 2-year-old hay. There isn’t much vitamin A left in hay by the time it’s that old. “As soon as hay is cut, the amount of vitamin A starts dropping. In earlier studies, researchers measured the loss that occurs within the first 24 hours of cutting alfalfa hay, and found that 80% of the carotenes were gone by that time.” Green, leafy alfalfa hay has
a lot of vitamin A, however, because the remaining carotene levels are still very adequate for many months.
“Loss of vitamin A in hay is estimated
at about 10% per month as it continues to decrease, so you can see that after 2 years the hay will be quite low in vitamin A, with very little carotene left. The horses in my study that were fed 2-year-old hay in a dry lot were fairly deficient after 6 months.
The liver stores vitamin A, and this is how horses can go through winter without becoming too deficient; a fully-loaded liver can provide the needed vitamin A, but if horses are on old hay for a longer period, they may become deficient. If you are just feeding hay (no pasture), this is some- thing you need to pay attention to,” says Crandell. The hay should be relatively fresh and not too old.
“Concentrate feeds supply vitamin A because all manufacturers add a standard amount. This level may drop, however, over time.” If you are feeding a bag of feed that’s a year old, for instance, the vitamin A level won’t be what it was originally.
“Some of this vitamin is lost in the man- ufacturing or pelleting process (especially
if there is heat involved), but today the
form of vitamin A that feed manufacturers use has a protective coating that enables more of it to make it through the process- ing. The protective coating also slows the degradation that normally takes place over time. Thus, if a bag of feed was just a few months old it would still have good levels of vitamin A – especially if it’s been stored in a cool dry place.”
Heat affects vitamin A, so if you have bags of feed stored in the loft of your barn where it might be more than 100 degrees in the summer, the protective coating won’t be adequate. You need to store feed in a cool, dark place. “Light also increases oxidation, so if you have the bag open for a long time, you wouldn’t have much vitamin A left in that feed,” she explains. Shelf life is limited, and you can usually tell when feed is get- ting old and going bad by the smell.
VITAMIN A DEFICIENCY
76 SPEEDHORSE, June 2019
“Vitamin A passes from the dam to the fetus, but not enough to have much stores in the foal’s liver when it is born. Foals are not born deficient, but they need to have vitamin A in their diet from birth onward.”
growing horse. This is one reason we see slow growth/poor growth in young horses that are deficient in vitamin A,” Crandell explains.
Vitamin A is important for proper embryonic development. “A deficiency can cause embryonic loss if the embryo is unable to develop properly. Vitamin A also affects reproduction because it is crucial for a healthy lining in the reproductive tract. It also helps with production of sperm cells (spermatogen- esis) and eggs, and is important for the cells in the placenta, as well as fetal growth,” she says.
If a mare doesn’t have enough vitamin A, or has too much, the unhealthy lining of the reproductive tract also affects estrus. “This vitamin is also important in production of steroid hormones, in both males and females – such as progesterone and estrogen. Vitamin A also passes from the dam to the fetus, but not enough to have much stores in the foal’s liver when it is born. Foals are not born deficient, but they need to have vitamin A in their diet from birth onward. The mare’s milk is a good source of vitamin A. There is a lot of vitamin A in milk,” she explains.
INTERACTIONS WITH OTHER NUTRIENTS
A well-balanced diet is essential for good health in order to provide all the necessary nutrients, including vitamins. Deficiencies (or excess) of some nutrients can affect absorption and function of others. “If the
EQUINE HEALTH