Vitamins

Vitamins are an essential aspect of balanced nutrition. They are involved in a plethora of body processes and are important for regular body function. There are two main categories of vitamins that horses require: fat-soluble vitamins which include E, D, A and K and water-soluble which include B and C. Please note that there are a variety of B vitamins.

In general, there tends to be more research into fat-soluble vitamins, such as vitamin E, as they are stored in the body and can result in toxicity issues when oversupplied. Water-soluble vitamins differ as they will be excreted in the urine as opposed to being stored in the body. Typically, toxicity issues are extremely rare with vitamin supplementation in horses, it is more common that there are deficiencies.

The NRC, 2007 (Nutrient Requirements of Horses) has established requirements for vitamin A, D, E, B1 and B2 but when last published, the research was insufficient to estimate a requirement for vitamin K, B3, B7, B9, B12, B6, B5 and vitamin C. Whether or not a horse’s vitamin needs are being met largely depends on what they are consuming daily as well as their internal health. For example, many B vitamins have endogenous production, which means that they are internally produced. The microbial fermentation in the hindgut produces B vitamins that the horse is then able to absorb. However, when this delicate ecosystem is disturbed, this can be compromised.

When reviewing the established requirements for vitamins, you will notice that they are listed in International Units (IUs). An IU is not a weight measurement, it is a means of tracking the effect or biological activity of the vitamin. When choosing a supplement, there are both powdered and liquid options. With production differences it would be ineffective to use a weight measurement. For example, 5 mg of a powdered supplement of vitamin E may not have the same biological effect as 5 ml of a liquid product. Therefore, IU is more effective when comparing the relative potency and biological effect.

Endogenous production of vitamins in the body is something that horse owners should be aware of. Most notably, the horse’s hindgut is a significant source of B-vitamins that they are then able to absorb. When maintained on a forage-based diet it is generally accepted that the animal’s requirements for those vitamins are met. When there are health issues related to the hindgut of the horse, supplementation of B vitamins may be necessary. A lesser-known fact is that the body also produces vitamin C! More commonly referred to as ascorbic acid, this vitamin does not need to be supplied in the equine diet as the liver produces adequate amounts.

Oftentimes, hay comprises the majority of what a horse consumes. In the summer, they may be maintained on pasture instead, however, healthy and robust pasture is rare in Ontario due to land constraints. Understanding the vitamin content of the forage you’re feeding can assist you in making educated decisions on vitamin supplementation.

There are numerous variables that influence the vitamin content of forages. Lush pasture is regarded as a good source of vitamin E and A, but once cut for hay the levels of these vitamins decrease quickly. Additionally, the vitamin content of the hay continues to decrease with storage.

So, how can a horse owner ensure that their vitamin requirements are met?

Well, with the insufficient amount of research, we cannot be 100% confident as the work has not yet been done to establish requirements for all vitamins. However, feeding a quality commercial product that is fortified with vitamins and minerals and fed at the recommended rate is a great start.

Horses that are fed fortified feeds in adequate amounts, whether it is a ration balancer or a performance feed, are likely consuming adequate vitamin amounts. Especially if this is paired with quality forage.

One question that I have been asked is if a horse owner should be changing their horse’s vitamin supplementation throughout the year to reflect the losses in the hay. This is not typically an issue that presents as if a diet is properly supplemented with a fortified vitamin and mineral source, the horse can be maintained on that amount year-round.

The one scenario that can warrant changes throughout the year is when the horse is maintained on hay during the colder months, and pasture during the summer. Lush pasture will be richer in vitamin content than cut hay, particularly vitamin E.

In this scenario, it may be ideal to keep your horse maintained on their regular ration balancer or performance feed year-round but add an additional vitamin E product when they are not on pasture.

With vitamin E metabolism, there is an individual aspect to it, as some horses will have a greater utilization of the vitamin. This means that on paper, you may be supplying enough vitamin E, but your horse could still be deficient. Some fortified products use natural sources, while others use synthetic which can also impact the bioavailability and amount the horse is able to absorb.

Best practice is to have your veterinarian run a blood test to determine vitamin E status, but this does not work well for all vitamins. For example, using blood testing to determine vitamin A status is not accurate as it does not account for the amount that is stored in the liver.

There is so much more we still have to discover and learn about vitamins for horses! Starting with a quality fortified product that uses bioavailable sources of vitamins and feeding it at the recommended amount is a great first step. Being aware of the research, or lack thereof, understanding the individuality of vitamin utilization, the endogenous production of vitamins within the body, and how vitamin content changes in forages is important to making educated decisions on vitamin supplementation.

By: Madeline Boast, MSc. Equine Nutrition

References:

Greiwe-Crandell, K. M., Kronfeld, D. S., Gay, L. A., & Sklan, D. (1995). Seasonal vitamin A depletion in grazing horses is assessed better by the relative dose response test than by serum retinol concentration. The Journal of nutrition, 125(10), 2711-2716.

Hymøller, L., & Jensen, S. K. (2015). We know next to nothing about vitamin D in horses!. Journal of Equine Veterinary Science, 35(10), 785-792.

National Research Council. 2007. Nutrient Requirements of Horses. Chapter 6: Vitamins.