Protein Excess & Deficiency in Horses

In the previous blog post, protein requirements and protein quality was discussed. However, there can be complications when protein requirements are greatly exceeded. This installment of Fridays with Finn will cover both protein deficiency and protein overload in horses.

Protein Deficiency

Research suggests that protein deficiency in the horse is less common when compared to the excess protein being provided. However, there are still negative health consequences associated. Symptoms of protein deficiency in mature horses are muscle loss, weight loss, inadequate feed intake, and poor growth of hooves and hair. In addition to the previously listed symptoms, pregnant mares may experience fetal loss as well as poor milk production if they are deficient in protein. In young horses, a decrease in growth is seen when protein is lacking in the diet. If you have a nutritionist complete a hay analysis, they will be able to determine if an additional protein source should be added to the diet.

Excess Protein

When a horse is supplied with excess dietary protein that protein will be broken down and excreted from the body as urea in the urine. When there is additional urea to be excreted, the horse may experience an increase in water loss. When more water is lost, the water intake requirement of that horse will increase. This could be problematic for horses under intense exercise who may already struggle with hydration.

Research has shown that when high protein diets are given to exercising horses it interferes with the acid-base balanced in their body. Additionally, when horses urinate more and have a higher urea content in the urine it negatively impacts the air quality in the barn and can lead to respiratory problems down the road.

A study using Arabian horses under training has suggested that improving protein quality (read the last blog post if you missed it!) but decreasing protein quantity in the diet is a good option. This way you can ensure a working horse is not deficient in important amino acids such as lysine, but without the undesired effects of oversupplying protein.

Other problematic consequences of oversupplying protein are that the increased urea can lead to earlier fatigue during exercise. Another concern related to exercise is heat. When a horse is using protein for energy, they produce more body heat which increases sweating and therefore increases the risk of dehydration in working horses.

A final area that is important to highlight is the environmental impact of providing excess protein in livestock nutrition. Unfortunately, the equine sector does not have the same published parameters on nitrogen and urea excretion as other livestock sectors, but it is well documented that when too much protein is fed, there is an increase in nitrogen released in the urine. This negatively impacts both air quality and groundwater. Plus, protein shouldn’t be wasted! It is an expensive feed ingredient – so really there is just no good reason to oversupply protein.

All of this highlights the need for precision in ration formation. Stop guessing with your horse’s nutrition! Have a hay analysis done and get an equine nutritionist to balance the diet to your forage analysis.

Client Story

I was recently consulting with a veterinarian who was having some nutritional challenges with his client’s polo horses. These horses were located in Nigeria, so a much hotter climate than Canada! Therefore, when curating improved diets for these animals, protein was an important aspect. Due to the heat and hydration complications that excess protein can cause, it was crucial to ensure that these horses were not being supplied with a significant amount of excess protein.


Bott, R. C., Greene, E. A., Trottier, N. L., Williams, C. A., Westendorf, M. L., Swinker, A. M., … & Martinson, K. L. (2016). Environmental implications of nitrogen output on horse operations: a review. Journal of Equine Veterinary Science45, 98-106.

Graham-Thiers, P. M., Kronfeld, D. S., Kline, K. A., Sklan, D. J., & Harris, P. A. (2003). Dietary protein and fat effects on protein status in Arabian horses during interval training and repeated sprints. Journal of Equine Veterinary Science23(12), 554-559.

Graham-Thiers, P. M., Kronfeld, D. S., Kline, K. A., Sklan, D. J., & Harris, P. A. (2003). Dietary protein and fat effects on protein status in Arabian horses during interval training and repeated sprints. Journal of Equine Veterinary Science23(12), 554-559.

Loos, C. M., McLeod, K. R., Stratton, S. C., van Doorn, D. A., Kalmar, I. D., Vanzant, E. S., & Urschel, K. L. (2020). Pathways regulating equine skeletal muscle protein synthesis respond in a dose-dependent manner to graded levels of protein intake. Journal of Animal Science98(9), skaa268.

Miller, P. A., & Lawrence, L. M. (1988). The effect of dietary protein level on exercising horses. Journal of Animal Science66(9), 2185-2192.

National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. https://doi.org/10.17226/11653.

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