To determine your horse’s nutrient requirements—particularly with respect to energy intake—you need to first calculate its body weight and body condition. Body weight is measured in kilograms or pounds. Body condition refers to how much or how little fat coverage an animal has, and it can be measured through both subjective visual inspection to obtain a “score,” or through more objective and quantitative body measurements.

Determining Body Weight

The most accurate way to determine a horse’s weight is by weighing the horse. Scales clearly give the most accurate reading and can detect minor changes in body weight (though it should be pointed out that weighing a horse daily will likely reveal fluctuations due to fecal loss rather than true weight). Although most horse owners do not have access to scales, they are fairly common at equine veterinary clinics and research facilities. Also, many feed companies own portable scales and can bring them to your facility to accurately assess your horse’s weight and to find out how much of their product you need. If you run a relatively large boarding facility or have mares and foals whose weights you’d like to keep track of (because growth of foals should be monitored closely), a scale is a wise investment. An equine scale can be purchased for approximately $3,000.

Because of the need for some kind of weight estimate in horses (for determining the amount of feed, de-wormer medication, etc.), several research groups have identified equations to estimate body weight using body measurements such as heart girth and body length.

Body length is measured from the point of the shoulder to the point of the buttock. Heart girth is taken around the midsection, behind the elbow, and beyond the highest part of the withers.

Weight (kg) = [(heart girth) x (heart girth) x (body length)] / 11,990

where heart girth and body length are measured in centimeters

or

Weight (lbs) = [(heart girth) x (heart girth) x (body length)] / 330

where heart girth and body length are measured in inches.

It is also possible to get a reasonable estimate of body weight from heart girth alone, which has given rise to the popularity of “weigh tapes.” Readily available for purchase at most tack and feed stores, these tapes are placed around the heart girth area. Incremental weights have been printed directly on the tape to give a rapid estimate.

The calculation of body weight (from both body length and heart girth) tends to be a little more accurate than using the heart girth alone (from a weigh tape), simply because differences in body length can greatly affect weight. For example, the weight of a horse with a long body would be underestimated if determined by heart girth alone.

Calculations specific for estimating the weight of growing horses are more complicated, and a scale is recommended to monitor growth rates in these horses. Weight determination in growing horses is more important for ensuring smooth, consistent growth patterns than for ideal “body weight” with respect to fat coverage. Rapidly growing foals can be prone to developmental problems and orthopedic diseases such as epiphysitis or osteochondritis dissecans.

Measuring “Condition”

While important in determining your horse’s overall nutrient requirements, knowing how much your horse weighs still doesn’t tell you if it is too fat, too thin, or in good overall condition with respect to fat coverage. There are a few ways to estimate condition or overall adiposity (amount of fat cover) in horses. One way is through physical examination of the horse and assigning it a subjective score of adiposity. Other more quantitative means include those derived from direct measurement of the animal.

Henneke Scoring

Since the 1980s, veterinarians and horse owners alike have been using theHenneke Body Condition Scoring (BCS) system to estimate fat coverage of horses. This system uses a scale of 1 through 9, determining body weight and ideal condition where 1 represents an extremely emaciated horse and 9 represents a grossly obese one. Areas of the body that are examined include the shoulder and elbow region, the ribs, the withers, the loin and tailhead region, and the crest of the neck.

Occasionally some people will use a 0–5 or 1–5 scale instead of the 1–9 Henneke scale. These scales would follow the sample principles in which the higher number refers to a fatter animal. The 1–5 scale is commonly used to evaluate dogs and cats as well as dairy cattle and swine.

Using the Henneke scale, a horse in good general condition whose ribs cannot be seen but can be easily felt is assessed as a 5. Table 1 shows the correlation between the condition score numbers and the expected condition of the various assessed areas:

The scale is useful because it is easy to learn, but there are several drawbacks. For instance, it is very subjective; one horse owner might label a horse a 3 while another might label it a 2 or 2.5. This discrepancy generally isn’t an issue unless different people are keeping records. For example, if it wasn’t known that different people were scoring the horses, one might assume a horse has suddenly lost or gained weight. It is also difficult to use the scale when tracking a horse over time to observe small changes in body condition score. In this case, careful record keeping and photos can be helpful to monitor body condition score changes with dietary management.

Another problem with the Henneke system is that not all horses follow the chart smoothly. For example, some horses may carry more weight around their ribs but won’t have much coverage along the hind end. Therefore, it is possible for a horse to be a 6 at the ribs but only a 4.5 or 5 in another region. In these kinds of situations, owners and clinicians must average the entire body’s scores to obtain the horse’s true overall score.

Cresty Neck Scores

Researchers in Virginia have established a similar subjective scoring system for the neck region, as horses with a so-called “cresty neck” appear to be more prone to metabolic dysfunctions such as insulin resistance. The cresty neck scores (CNS) range from 0 to 5, where 0 indicates no visible appearance of a crest (no fleshy region along the mane) and 5 signifies a crest so large it droops to one side. Most horses will have some tissue coverage along the crest (in the 1–2 range), and horses with more fat coverage will be in the 3–4 range. A score of 3 or higher would be considered a “cresty neck” (Carter et al., 2009a). It is rare to see horses with a score of 5 and a drooping neck, though it is fairly common in miniature donkeys. Ideally, horses should have a cresty neck score of less than 3.

Quantitative Measures of Adiposity

Because estimates of fat coverage are subjective, researchers have been investigating more objective and quantitative measures of adiposity. For example, in humans the body mass index (BMI) is well-established to compare body weight with height to give an indication for overall fat mass.

Human BMI = [Weight (lbs) x 703] / [Height (inches) x Height (inches)]

A human BMI less than 18.5 is considered underweight, 18.5–24.9 is normal weight, 25–29.9 is overweight, and a score over 30 is considered obese. The waist-to-hip ratio (waist circumference divided by hip circumference) is another measure of body fat in humans and is particularly important as abdominal fat is believed to be more associated with health concerns than general subcutaneous fat (located just below the skin).

Similar ratios have been derived in horses. The girth-to-height ratio (G:H or girth divided by height) has been shown to be useful to estimate overall adiposity in horses and is well correlated to body condition scores. One study that examined these measurements (Carter et al., 2009a) found that horses and ponies had different ranges of girth-to-height ratios associated with obesity.

Another quantitative means of measuring fat coverage is with an ultrasound. A veterinarian or trained technician can measure “rump fat” on the horse’s hind end using the ultrasound probe at a point 5 cm lateral from the midline of the animal at the midpoint of the pelvic bone (Kearns et al., 2002). The amount of rump fat is measured in centimeters. Then, the percentage of body fat in the animal can be calculated (Kane et al., 1987):

% Body Fat = 2.47 + [5.47 (Rump fat in centimeters)]

For example, if the rump fat is 3 cm, then the total percentage of body fat is 18.88% (3 cm x 5.47 = 16.41, + 2.47 = 18.88). Most lean horses would have body fat percentages in the range of 8–14%, while horses with excess body fat would likely have values in the 16–30% range. It should be noted that this equation to determine body fat was only assessed in a couple of breeds of horses and may not be applicable to all body types. Nonetheless, rump fat (measured in cm) is still a useful measure of adiposity and can be monitored over time.

Ideal Weight?

So what is the ideal body score or level of adiposity in horses? At this point, veterinarians and nutritionists aren’t certain. Most horse owners, veterinarians, and nutritionists agree that, in general, a leaner animal is healthier (within reason). In this sense, a horse with a body condition of 5 is generally considered to be in good condition. However, in some cases a leaner or fatter condition may be desired, as described below.

Determining an ideal weight for a horse is difficult, in part due to vast breed differences affecting bone and musculature. Muscle accounts for more than 50% of body weight in most athletic horses (and is usually still around 45% in non-athletic horses), compared to 30% to 40% in other species. As in humans, muscle weighs more than fat; therefore, a muscled horse will weigh more than a fat one for a given height and body type. Thus, it is difficult to make claims such as a 16-hand horse should weigh 500 kg. In reality, based on breed differences a 16-hand horse may weigh anywhere between 450 and 550 kg. This is another reason why body condition scores are useful.

Horses with body condition scores equal to or greater than 7 are considered overweight, while those with scores equal to or greater than 8 are considered obese. In terms of the girth-to-height ratios, a horse would be considered overweight if the G:H was 1.26 and obese if the G:H was greater than 1.29. A pony would be considered overweight if the G:H was 1.33 and obese if the G:H was 1.38 (Carter et al., 2009a).

In domestic species such as dogs and cats, it is well established that leaner animals live longer and healthier lives. One study in dogs found those that consumed 25% less food than their counterparts lived significantly longer (1.8 years) and had lower incidences of chronic disease (Kealy et al., 2002). It is unknown if horses fare the same way, though it isn’t unreasonable to speculate that a horse with a body condition score of 4 may be very healthy. In fact, a leaner body condition score may be ideal for a horse with any kind of chronic lameness, as the amount of body weight on the limbs would be less.

In some cases it might be wise for a horse to have a slightly higher body condition score. The original work by Henneke studied reproductive efficiency in mares and found that mares with more condition (higher BCS) had higher conception rates than their leaner counterparts. So, it may be wise to keep a broodmare at a slightly higher condition (for example, around a BCS of 6), though keeping any horse at a body condition score greater than 7 may increase the risk of metabolic issues. It may also be recommended that older horses be kept in higher condition (BCS 6). As the ability to maintain weight during disease or times of stress becomes increasingly difficult with increasing age, having a bit of a “buffer” in body weight could be beneficial. However, the overall health of the animal should be taken into consideration, as an older horse with arthritis or a history of laminitis may do better without the excess weight.

Only a few studies have reported body conditions of performing horses. It has been reported that horses competing in endurance events are more successful with more condition. Specifically, horses entered in the 100-mile Tevis Cup successfully completed 20 miles more with each additional body condition score; for example, a horse with a score of 4 averaged 20 miles more than a horse with a score of 3. Horses with body condition scores less than 3 did not finish the race (Garlinghouse and Burrill, 1999). From this it would be taken that endurance horses should be kept in moderate condition (BCS 4–5). Most racehorses are reported to be in the 3.5–5 range, though it is unknown how this relates to overall performance. Pagan and coworkers (2009) reported body condition scores in sporthorses and found that pony hunters had average scores of 7; dressage, hunters, and jumpers had average scores ranging from 5.5–6.5; and polo horses were close to 5.

Horse owners should work with their veterinarians and trainers to determine the ideal body condition for their horse and discipline, as serious consequences can result when a horse is too thin or too fat. However, prior to discussing health concerns regarding weight management, the concept of energy balance must be introduced.