When preparing a horse for athletic events and sporting activities, whatever the discipline or level of difficulty, an important consideration is finding the horse's "ideal" body weight. This concept is well recognized in human athletics. For weight-bearing competitive sports like racewalking, running, and cross-country skiing, the amount of energy required to walk, run, or ski at any given speed is directly related to body weight. The higher the body weight, the greater the amount of energy required to move the body.

On the other hand, for individuals who are somewhat overweight, a reduction in body mass will provide a competitive advantage. For those athletes, a reasonable approach to enhance performance is a moderate reduction in body mass, particularly fat mass. Of course, it always is possible to get too much of a good thing--excessive loss of body weight actually can decrease performance ability because of loss of lean body mass (particularly muscle) and a lack of energy reserves. Thus, there is an "ideal" body weight and body composition (the relative quantities of lean and fat mass).

What about horses?

We first must recognize that it is the combined weight of the horse, rider, and tack that is important when considering the energy cost of the horse's movement. The effect of weight carriage is plainly evident in Thoroughbred racing--in handicapped races, lead weights are added to the saddle of the better-performing horses in an attempt to "even" the playing field. There is an old racing adage that says: "Weight will stop a train." This extra weight increases the amount of energy required to run and can make the difference between winning and losing.

Setting aside the issue of rider and tack weight, it also is likely that the weight of the horse can influence performance. One of the current controversies surrounding the use of Lasix (furosemide)--a drug given to racehorses to lessen the severity of exercise-induced pulmonary hemorrhage (EIPH or "bleeding")--is the potential for this drug to improve a horse's race performance independent of any effect on the severity of lung bleeding. Lasix is a diuretic drug, and there is a marked increase in urine production following its administration. In fact, if horses are denied access to food and water after receiving a typical dose of Lasix, there will be about a 2% reduction in body weight within two hours because of this increase in urinary losses.

A weight loss of this magnitude (about 20 pounds for a 1,000-pound horse) will reduce the energy cost of running, perhaps explaining why Lasix could enhance race performance.

For many other sporting disciplines, body weight also is an important issue. Horses which are grossly overweight (obese) will be at a disadvantage during exercise and might be at increased risk for some health problems. On the other hand, it is not desirable to have horses which are in poor body condition (underweight). What constitutes ideal body weight and condition will vary depending on the breed of horse, the discipline, and in some cases owner preference. For example, a properly conditioned endurance horse will be much leaner than a halter horse.

Most of you do not have access to a set of scales suitable for weighing horses. Instead, a system of body condition scoring provides a very useful means for ongoing assessment of body condition (see "Weighing In," October 2000 issue of The Horse). Use of this system will provide you with the best guide to your horse's body weight and help you identify any needs for adjustments to feeding and exercise programs.

The body condition scoring system relies on visual inspection and palpation of several conformation points, including the neck, withers, backbone, ribs, and tailhead. (The American Association of Equine Practitioners produces a brochure on the overweight horse that details this system; ask your veterinarian for a copy of this brochure.) The main criterion is the amount of flesh or fat covering these areas of the body. Scores range from 1 to 9; a condition score of 1 is applied to horses which are emaciated (extremely thin), while a score of 9 is indicative of a very fat horse (obese).

For horses not engaged in serious athletic pursuits, a condition score of 5 to 6 is ideal. These horses have moderate to good flesh coverage and the appearance of a well-nourished and well-kept animal. For show and dressage horses, a body condition score of 6 (moderately fleshy) might be more ideal.

On the other hand, the ideal condition score for a racehorse or an endurance horse is around 4 (moderately thin). For those horses, carrying excess condition is a definite disadvantage for athletic performance. However, particularly for endurance competitors, you must be careful not to allow the horse to become too thin.

Recent studies have shown that body condition score is an important factor for endurance performance. In one study of horses competing in the Tevis Cup 100-mile race, the average condition score for horses which successfully completed the course was 4.6, whereas the average score of non-finishers was 3.8 (Garlinghouse and Burrill, 1999). Furthermore, horses which were eliminated for "metabolic failure" (conditions such as colic, heat exhaustion, and muscle disorders) had even lower condition scores (less than 3). Those animals likely lacked the reserves in energy required for such prolonged exercise.

Overweight Horses

For humans, dogs, and cats, criteria are available to identify readily a person or animal which is overweight or obese. Generally speaking, a 10% increase in body weight over the standard is considered overweight, while a 20% increase in weight constitutes obesity. Unfortunately, no such standards exist for horses. As already mentioned, what is considered an ideal weight will vary between breeds and disciplines. Nonetheless, regardless of breed, horses with a body condition score of 7 or 8 are overweight, while horses with a score of 9 are obese.

For the average size light breed of horse (e.g., Thoroughbred, Quarter Horse, or Arabian weighing about 450 kg or about 1,000 pounds), each body condition score unit represents about 16 to 20 kg (35-44 pounds) of weight. Let's assume that this type of horse is performing moderate to vig-orous athletic activity and a body condition score of 4 to 5 is ideal. If, on the other hand, the actual condition score is 7, this horse is overweight by 32-40 kg (71-88 pounds), which is approximately 7-8% above the ideal body weight. At a condition score of 9, this horse would be overweight by up to 80 kg (176 pounds) or 16%. These figures will vary between horses and, to some extent, depend on the composition of the weight gain; lean (muscle) mass weighs more than fat tissue.

What causes horses to be overweight? By far the most common cause is overfeeding, particularly relative to activity level. Those of you who "count calories" will be very familiar with this concept--the energy balance equation relates energy input (calories in food) to energy output (calories expended in daily physical activities). If the horse receives too much energy in its diet, over time there will be a gradual increase in weight.

Ponies, in particular, are susceptible to obesity and require strict attention to diet and exercise for control of body weight. It has been proposed that this tendency toward obesity reflects their northern European heritage--in a cold, harsh climate with little in the way of quality feedstuffs, ponies became more efficient in utilizing nutrients in their food. With access to high-quality feedstuffs, this highly efficient metabolism translates into a tendency to readily gain weight.

Among breeds, Quarter Horses and Morgans appear to gain weight readily. In this context, most are familiar with the terms "easy keeper" and "hard keeper." Easy keeper horses readily gain weight when overfed and/or under-exercised (ponies being the classic example). The opposite is true of hard keepers. Even when lightly exercised, these horses tend to be on the thin side and maintaining reasonable body condition becomes difficult during periods of heavy training and competition.

What is the basis for this difference? Are genetics or problems with metabolism involved with a tendency to readily gain weight?

Unfortunately, we do not have the answers to these questions. But, based on recent studies in other species, it is very likely that genetics do play a role.

Several factors likely contribute to our tendency to overfeed horses. First, it is very easy to overestimate actual energy needs. Most literature concerning the maintenance (non-working horse) digestible energy requirements indicates that a 500 kg (1,100 pound) horse requires about 16 megacalories (Mcal) of energy (16,000 calories) per day. This figure is best applied to horses kept outdoors; horses which spend the majority of the day in stalls will have a much lower daily energy expenditure and thus lower energy needs. Therefore, some nutritionists advocate that the maintenance energy needs of the "couch potato" horse are about 30% lower (11-12 Mcal, or 11,000-12,000 calories per day for a 500-kg horse).

Another factor contributing to overfeeding is an overestimation of the amount of "work" the horse is performing. For example, it is common for pleasure horses to be ridden for one hour at a combination of walk and slow trot, perhaps completing two or three of these rides per week. Some owners might feel that this amount of exercise warrants an increase in calorie intake and will adjust their horse's feeding program accordingly. In reality, however, horses do not expend a great deal of energy during this type of exercise, perhaps no more than 1 Mcal per hour of walking/trotting. If these trail rides represent the horse's only form of exercise with no paddock turn-out, it is safe to assume that energy requirements, at most, approximate the National Research Council (NRC) recommendations for maintenance (i.e., 16 Mcal or 16,000 calories per day).

A basic lack of knowledge concerning the amount of energy in different feeds also can contribute to overfeeding. Beyond recognizing that grains contain more energy than hay (on an equivalent weight basis), it is important to understand that there can be wide variation in the energy content of different grains and hays. As examples, high-quality alfalfa hay has up to 30-40% more energy than average quality timothy hay, and cracked corn has about 10% more energy than oats because of the higher starch content.

Thyroid dysfunction (hypothyroidism) has been suggested as a cause of obesity and a "cresty neck" in horses. The hormones produced by the thyroid gland--thyroxine (T4) and tri-iodothyronine (T3)--play a critical role in body metabolism. A deficiency of these hormones will slow metabolism and can lead to weight gain. Therefore, hypothyroidism is an attractive explanation for obesity in horses. Indeed, it is common for overweight horses to be supplemented with L-thyroxine, a synthetic form of the natural hormone.

However, a word of caution here. Hypothyroidism is an over-diagnosed condition, which is perhaps a reflection of our tendency to overlook the obvious when seeking an explanation for obesity in horses. The bottom line is that overfeeding and a lack of exercise are by far the most common reasons for weight gain and obesity.

Consequences Of Obesity

Are there any health consequences related to obesity in horses? Once again, there is very little information available. However, it is possible that overweight horses are at greater risk for bone and joint injuries because of the added weight the limbs must bear. Overweight horses also might be more susceptible to certain diseases, including laminitis (founder). Ponies are particularly prone to obesity. When obese, ponies are very susceptible to founder and a serious disorder of fat metabolism called hyperlipemia. These problems, in part, are related to insulin resistance, similar to adult onset diabetes mellitus in humans. Most importantly, insulin resistance and the risk of developing founder and hyperlipemia are reduced after weight loss.

Overweight and obese horses are less tolerant to exercise, particularly in the warm summer months. With weight gain, fat is deposited underneath the skin--this layer of fat acts as an insulation barrier that is useful during the cold winter months, but it is a detriment during exercise and in hot weather because it impairs loss of heat from the body. Therefore, an overweight horse is more likely to overheat during exercise, a situation that is compounded in hot weather. In these horses, heart and respiratory rates will be abnormally high in relation to the amount of work performed, and recovery following exercise will be delayed. If you plan to ride your horse, particularly in competitions, you must deal with his weight problem.

Managing Overweight Horses

The first step in management of overweight or obese horses is to establish the current body condition score (and, if possible, body weight) and set a goal for weight loss. For example, let's assume that you have a horse with a body condition score of 8 and his ideal score is around 5. Based on height and breed, this horse has an ideal body weight of 450 kg (1,000 pounds). As one unit of body condition is about 20 kg (44 pounds), this horse is overweight by 60 kg (about 130 pounds).

Next, accurately assess the horse's current feeding and exercise program. Although it is common to feed using convenient units such as a "flake" or section of hay and a coffee can full of grain, this system is very inaccurate for assessment of actual feed intake. For example, alfalfa hay weighs more than grass hay per unit volume. Therefore, it is important to weigh amounts fed--you could use kitchen scales. If possible, send a sample of your hay for laboratory analysis. The data will give you a better idea regarding overall nutrient intake.

A few rules of thumb are useful here. Most horses will consume somewhere between 1.5-3% of their body weight per day in feed. For the overweight and "easy keeper" horse, about 1.5% of body weight is a reasonable ballpark figure. About two-thirds of this amount should be in the form of fiber--grass, hay, or other sources of forage and fiber (e.g., hay cubes or beet pulp). Regardless of body condition, fiber always should be the main component of a horse's diet. At the absolute minimum, a horse should eat 1% of his body weight per day in the form of forage and other fiber sources.

So, our fictitious fat horse for the past six months has been receiving 7 kg of grass hay and 3 kg of a typical sweet feed each day that contains added minerals and vitamins. He is ridden two or three times per week, usually 45 to 60 minutes of gentle trail exercise, and has some drylot turnout at other times. The amount of hay fed is appropriate--about 1.4% of his body weight (again, 1% is considered the safe minimum).

The diet is supplying 20.5 Mcal (or 20,000 calories) of digestible energy, which probably is 25% higher than this horse needs even considering activity level (see figures on page 90). This excess energy intake is certainly enough to explain weight gain. Assuming no change in feeding and activity level, we might expect this horse to gain a further 20 kg (or 44 pounds) over the next three months.

Clearly, this horse needs to go on a diet. One approach would be to set the energy intake at 70% of his maintenance needs (at the ideal weight of 450 kg or 1,000 pounds) or about 12 Mcal (12,000 calories) per day. However, we must be careful to maintain the intake of other nutrients at the maintenance level. For example, the simplest approach to a reduction in energy intake would be removal of sweet feed from the diet. However, as shown in the figure on page 90, a diet of only grass hay will not provide adequate protein, minerals, or vitamins. The lack of protein will encourage a loss of muscle mass rather than fat. This situation can be remedied by feeding a small amount of a protein/mineral/vitamin supplement.

The second important step in a weight loss program is an increase in energy expenditure through exercise. For several reasons, an increase in activity level will facilitate weight loss and an improvement in body condition. With physical conditioning, there are metabolic adaptations that encourage fat burning. As well, there will be conservation or even an increase in muscle mass. Over a period of weeks, the horse will become more "athletic" in appearance. As lean tissues such as muscle have a higher rate of metabolism than fat tissue, this change in body composition will raise the horse's resting metabolism.

There are two ways to increase activity level. First, you could provide more turnout time, thus allowing the horse more opportunity for voluntary exercise. Regardless of body condition, adequate turnout is important for the horse's overall well-being. Unfortunately, access to pasture is not an option for overweight horses as you will lose control of calorie intake. So, turnout must be restricted to drylots, at least until the weight problem is under control.

The second option is to increase the amount of formal exercise, such as longeing and riding. Any increase in exercise must be done slowly, probably no more than a 5% increase in duration or intensity every week or so. If the horse is already ridden two to three times per week, one option is to add one or two sessions per week. Alternatively, if time is a limitation, you might consider an increase in exercise intensity--more trotting and, with time, a little canter work.

As mentioned, overweight horses can be more prone to leg injuries, so close monitoring is required, particularly for the first few weeks of the conditioning program.

You also must be realistic in terms of how long it will take for the horse's ideal weight to be attained. Think months rather than weeks!

In the above example, a five- to six-month time frame is reasonable. Your horse will be unhappy for the lack of grain in his diet, and you might bemoan the extra time required for conditioning, but there is no "quick fix" for this problem. Periodic evaluations also are necessary; perhaps you should assess body condition score (and weight) monthly. Adjustments in feeding can be made once the ideal weight/body condition has been reached. For example, an increase in energy intake might be needed. In the long term, it always will be important to adjust feeding in relation to current activity level. If there are periods of reduced exercise (e.g., because of an injury), energy intake must be reduced accordingly.


Garlinghouse, S.E.; Burrill, M.J. Relationship of body condition score to completion rate during 160 km endurance races. Equine Veterinary Journal 1999; Supplement 30: 595.

Henneke, D.R.; Potter, G.D.; Kreider, J.L.; et al. Relationship between condition score, physical measurement, and body fat percentage in mares. Equine Veterinary Journal 1983; 15: 371.

About the Author

Ray Geor, BVSc, PhD, Dipl. ACVIM

