8/ OCTOBER 2024 THE RIDER Food for Thought: Vitamins in the Equine Diet When reviewing the established requirements


An Equine Nutrition Series by Madeline Boast,


MSc. Equine Nutrition Vitamins are an essential aspect of balanced


nutrition. They are involved in a plethora of body processes and are important for regular body func- tion. 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 vita- mins.


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 ex- tremely 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 re- search 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 vi- tamins that the horse is then able to absorb. How- ever, when this delicate ecosystem is disturbed, this can be compromised.


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 op- tions. With production differences it would be inef- fective 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 liq- uid 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 ab- sorb. 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 com- monly 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 main- tained on pasture instead, however, healthy and ro- bust pasture is rare in Ontario due to land constraints. Understanding the vitamin content of the forage you’re feeding can assist you in making edu- cated 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 contin- ues to decrease with storage.


So, how can a horse owner ensure that their vi-


tamin 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 vita- mins. However, feeding a quality commercial prod- uct 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 perform- ance feed, are likely consuming adequate vitamin amounts. Especially if this is paired with quality for- age.


One question that I have been asked is if a


horse owner should be changing their horse’s vita- min 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 individ-


ual 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 prod- ucts 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 recom- mended amount is a great first step. Being aware of the research, or lack thereof, understanding the in- dividuality of vitamin utilization, the endogenous production of vitamins within the body, and how vi- tamin content changes in forages is important to making educated decisions on vitamin supplemen- tation.


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 Require- ments of Horses. Chapter 6: Vitamins.


By: Madeline Boast, MSc. Equine Nutrition About the author: Madeline Boast completed


her master’s in Equine Nutrition at the University of Guelph and started an independent nutrition com- pany known as Balanced Bay. She has worked with a variety of equids – from miniature ponies to com- peting thoroughbreds. Through Balanced Bay she designs customized balanced nutrition plans that prioritize equine well-being. This includes diets for optimal performance as well as solving complex nu- tritional issues and everything in between. For ad- ditional information see www.balancedbay.ca


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