Written by: Shannon Pratt-Phillips, PhD.

Shannon Pratt-Phillips, PhD, explains how to help your horse shed unwanted pounds.

Thumbnail for Formulating a Feed Program for: An Overweight Horse

Pam Mackenzie photo

Obesity and overweight horses are becoming increasingly common, with some studies reporting that more than 50 per cent of horses today are overweight, with upwards of 20 per cent being obese. Obesity is usually defined as a body condition score greater than 7, with a horse being considered overweight if it is greater than 6, on the nine-point Henneke scale.

Similar to humans, accumulation of adipose tissue (fat) in horses results in an inflammatory process that is associated with several adverse health conditions – such as insulin resistance, equine metabolic syndrome, laminitis, reproductive problems, joint and orthopedic issues and work or exercise intolerance.

Also similar to humans, weight (fat) loss in horses is achieved when the horse expends more calories than he consumes, and this can be accomplished by decreasing calorie (energy) intake, increasing energy expenditure through work or exercise or, ideally, a combination of both.

A 500kg horse in ideal body condition requires approximately 16.7 mcal of energy per day. Horses with different metabolism types, however, have different requirements, with so-called easy keepers requiring fewer calories to maintain body weight (15.15 mcal/day) and hard keepers requiring more calories (18.15 mcal/day).

Most overweight horses do tend to be easy keepers, and, therefore, nutritionists usually use the lower energy requirement for such horses (15.15 mcal/day). Exercise increases caloric expenditure, though the National Research Council (NRC) does not report specific values for an easy keeper in work. There are, however, equations to determine requirements for such horses, or to even get estimates of actual workload to add in the calories burned per day (by adding up the minutes worked at each gait).

There are several studies that have achieved weight loss in horses, by decreasing energy (calorie) intake to approximately 60-80 per cent of their required energy needs, which often requires a restriction in forage intake to 1.25-1.5 per cent of body weight, down to even 1 per cent of body weight. The latter restriction would be for horses that may be resistant to weight loss, and should only be done under veterinary supervision.

Overweight horses should also be fed a lower quality hay in terms of nutritional density, because this allows them to eat more of it (by weight) for a given amount of calories. For example, a horse could eat 5kg of a hay with 2mcal/kg energy density to result in 10 mcal, or a horse could eat 5.9kg of a hay that is 1.7 mcal/kg for about the same amount of energy – and more hay consumed means less boredom.

Such hays typically have lower protein contents also, however. These horses, therefore, often require a protein and vitamin/mineral supplement, which may be achieved through a “ration balancer” type of product or through a mix of soybean meal and a vitamin/mineral supplement. I have personally found better results through soybean meal (usually 50-200 grams per day is sufficient to make up for the lack of protein provided by the restricted amounts of hay) and then a concentrated pelleted vitamin mineral supplement (fed in the amounts of about 50 grams per day). Such a small amount of feed (250 grams max) contributes less in terms of calories than a ration balancer, which is often fed in higher amounts (0.5-1kg per day) and, therefore, contributes more calories to the diet. Because of the additional calories in such a diet, hay would need to be restricted further.

Finally, overweight horses shouldn’t have access to any grass. It is too hard to regulate and determine actual intake, so it is impossible to count calories. They should only get access to a dry lot. Their weighed out hay could be provided there and/or in their stall.

Research conducted by my graduate student (presented in part at the 2013 Equine Science Society conference in New Mexico) found that some of the biggest challenges of feeding overweight horses include owner commitment, given that it does require work and dedication by the owner and boarding facility employees if applicable, and regulating pasture or hay intake (grazing muzzles proved to be very effective in our group of horses).

In Example Diet 1, we will use a 550kg easy keeper horse that is 50kg overweight and should be 500kg. I usually recommend starting a diet at 80 per cent of the horse’s current energy intake (that is maintaining the 550kg body weight), but meeting the horse’s full protein/vitamin/mineral requirements, so the only nutrient that is restricted is energy. I usually recommend revising the diet monthly to meet the new (ideally lower!) body weight’s needs.

This diet meets 80 per cent of this horse’s energy requirements to facilitate weight loss, but easily meets its other nutritional requirements as well. If the horse didn’t lose weight at a reasonable rate, forage intake could be reduced further (with a small increase in soybean meal if needed). Current forage intake at 7kg for 550kg body weight horse is 1.27 per cent of body weight, and could be decreased a little more as needed. Ideally, exercise would be included in the horse’s management strategy to increase calories burned and speed the weight loss process.

Another option, example diet 2 also meets 80 per cent of the horse’s energy requirements to facilitate weight loss. Because of the amount of balancer product that is required and its energy (calorie) contribution, however, the hay fed needs to be decreased to 6kg (1.09 per cent body weight). Ideally, exercise would be included in the horse’s management strategy to increase calories burned and speed the weight loss process.

Example Diet 1
– 7kg of grass hay with 8% CP and 1.8 mcal/kg DE
– 100g of soybean meal
– 50g of a vitamin/mineral supplement formulated for easy keepers with: 4.3% Ca, 1.7% P, 2,000mg/kg Cu, 800mg/kg Zn, 340,000 IU/kg vitamin A, 65,000 IU/kg vitamin D and 15,000 IU/kg vitamin E.
– 50g of salt (approximate free choice consumption)

Example Diet 2
– 6kg of grass hay with 8% CP and 1.8mcal/kg DE
– 0.75kg of equine balancer product with 20% CP, 3% Ca, 2.5% P, 250 mg/kg Cu, 700 mg/kg Zn, 38,000 IU/kg vitamin A, 7,000 IU/kg vitamin D and 1,000 IU/kg vitamin E (note this is very similar to several commercial products I looked at, and fed at rates based on manufacturer’s recommendations)
– 50g of salt (approximate free choice consumption)


There are several products available that claim to increase metabolic rate and increase rate of weight loss for overweight horses. If only there were such a product – it would be used extensively in human nutrition! The only product available that truly increases metabolic rate and helps a horse burn more calories and increases weight loss is thyroid hormone – Thyro-L® – and this should only be used as a last resort and under veterinary supervision.

Doctor’s Notes
In this year’s series, equine nutritionist, Shannon Pratt-Phillips, Ph.D., is providing some example diets for horses in different classes, but remember these are examples, as each horse is different. In each example, an average 500kg mature body weight is used, all noted requirements are taken from the NRC’s 2007 Nutrient Requirements of Horses, and all forage and feed information is from www.equi-analytical.com and Pratt-Phillips’ own databases.
Obviously, a horse’s diet will differ depending on the quality of hay or feeds offered, and hay analyses are suggested to accurately evaluate your own horse’s diet. If commercial feeds or supplements are warranted in a diet program, Pratt-Phillips presents a fictional formula (and provides goal percentages for several nutrients), though these may or may not be very similar to commercially available brands. It is suggested that you look for similar feeds, and work with a qualified equine nutritionist to help you select a feed or supplement that compliments your hay to meet the nutrient needs of your horse.