Direct measurement

Bodyweight is most accurately measured in horses by use of a calibrated weigh scale. Various scale designs are available, with some more portable models available. It is important that the scale is properly calibrated and leveled in order to obtain an accurate weight measurement. Fluctuations in bodyweight of up to approximately 5 kg may occur with feed and water intake, defecation, urination, and hydration status (Webb & Weaver 1979). Therefore, it is important to weigh at the same time of day with respect to feeding and exercise, in order to minimize the influence of normal daily variability on measurements.

Indirect measurement

Given that scales are relatively expensive to purchase, they are often only used at larger institutions and are usually not available in a small farm setting. Therefore, calculations have been developed for estimating bodyweight based on measurements of body size. The simplest and most practical application of estimating bodyweight is the use of commercially available weight tapes, which estimates bodyweight from girth circumference. Weight tapes are easily used by placing the tape at the base of the withers and running it around the girth circumference just behind the elbow and forelimb (Fig. 22.1). The measurement is taken while the horse is standing square and at the time of exhalation, so that additional lung volume is not included in the calculation. The accuracy of this technique is decreased for horses at the extreme ends of the scale, and may not be applicable for certain classes of horses, including pregnant mares, growing foals, and thin, fit horses (Gee & Harris 2005). Although accuracy of weight tapes is increased when height-specific tapes are used (Ellis & Hollands 2002), more accurate methods of estimating bodyweight include calculations that contain variables of height or length (Ellis & Hollands 1998).

Figure 22.1 Location of common morphometric measurements used in body weight estimation and body condition assessment, with the horse in an appropriate square stance and neck position for measurments.

NC, neck circumference; H, wither height; G, girth circumference; L, body length.

Many formulas have been described for estimating bodyweight in horses (Table 22-2). In order to improve accuracy, equations have been developed for specific types of horses, including Warmbloods (Kienzle & Schramme 2004) and Thoroughbred foals (Staniar et al 2004). When using weight formulas, it is important to make certain that measurements are taken according to the method from which the original equation was calculated. For example, girth circumference may be measured over the highest point of the withers (Carroll & Huntington 1988, Ensminger 1977), behind the slope of the withers (Fig. 22.1; Leighton-Hardman et al 1980, Milner & Hewitt 1969), one inch behind the highest point of the withers (Staniar et al 2004), or around the abdomen at the point of the umbilicus (Jones et al 1989). Additionally, some formulas use the body length from the point of buttock (tuber ischium) to the point of shoulder (head of humerus) (Fig. 22.1; Carroll & Huntington 1988, Leighton-Hardman et al 1980, Staniar et al 2004), while others use the length from the point of buttock to elbow (olecranon) (Ensminger 1977, Jones et al 1989), or from the point of shoulder to the point of hip (anterior part of trochanter major) (Milner & Hewitt 1969).

Table 22-2 Formulas for Estimating Bodyweight (BW) from Morphometric Body Measures in Horses

B = body circumference; CB = cannon bone circumference; N = neck circumference; C = carpus circumference; F = left forelimb length.

a As referenced in Milner & Hewitt (1969)

b y-value dependent on body condition and breed

c Body condition score, scale 0–5 according to Leighton-Hardman (1980)

d Body condition score, scale 1–9 according to Kienzle & Schramme (2004)

A widely used equation that has been evaluated in ponies and light breed horses (Reavell 1999) was developed by Carroll and Huntington (1988), and is applied as bodyweight = girth² × length/y, where y = 11 877 for measurements in metric units (cm and kg) or y = 330 for measurements in imperial units (in and lb). For larger horses, the equation of Jones et al (1989) was determined to be most accurate (Reavell 1999), although is awkward to use because of nonunity exponents: bodyweight (kg) = [umbilical girth (cm)^1.78 × length (cm)^1.05]/3011.

Assessment of body condition score (BCS)

The most commonly used method for assessment of adiposity in horses is the rating of body condition based on a numerical scale with specific criteria for each category. This method is based on visual and physical (palpation) assessment of “superficial flesh” deposition in specified body regions. The term superficial flesh has been largely substituted by “subcutaneous fat” in description of BCS systems for horses.

Several BCS scales have been developed specifically for use in horses. The 0–5-point scale was first described by Leighton-Hardman (1980) and later adapted by Carroll and Huntington (1988). This BCS system assesses subcutaneous fat deposition in 3 body areas (neck, back and ribs, and pelvis) on a scale of 0 (very poor) to 5 (very fat). Each area is scored separately, then the pelvis score is adjusted by 0.5 point if it differs by 1 or more points from the back or neck scores to obtain the overall BCS.

A widely used BCS system, developed by Henneke et al (1983), has a broader range of numerical increments and encompasses more bodily areas for evaluation. This system involves scoring six areas of the horse’s body (neck, withers, back, tailhead, ribs, and behind the shoulder) (Fig. 22.2) to categorize body condition on a scale from 1 (poor) to 9 (extremely fat) (Table 22-3, Fig. 22.3). Each body area is scored separately, and then scores are averaged to obtain the overall body condition score. The Henneke BCS system, originally developed for use in Quarter Horse broodmares, may be most appropriate for use in mature light breed horses (e.g., Quarter horses, Thoroughbreds and Arabians) that share similar body type and patterns of fat deposition. Application of the BCS system to Thoroughbred geldings has been described (Suagee et al 2008), and alterations in the BCS system have been developed to increase its suitability for Warmblood breeds (Kienzle & Schramme 2004). However, the application of the Henneke BCS system to classes of equidae with unique conformation or fat deposition patterns, such as ponies, draft breeds, or young horses needs to be performed and interpreted with caution.

Figure 22.2 Location of subcutaneous adipose depots over the six bodily areas assessed by body condition scoring, according to Henneke et al (1983).

A, neck; B, withers; C, behind the shoulder; D, ribs; E, topline; F, tailhead.

Table 22-3 Description of Individual Body Condition Scores

Adapted from Henneke et al (1983).

Figure 22.3 Illustrations of individual body condition scores as described by Henneke et al (1983).

Regardless, Kohnke’s modification (1992) of the Henneke BCS system has been applied to pony breeds in research studies and clinical situations (Freestone et al 1992, Treiber et al 2006, Carter et al 2009c, Dugdale et al 2010). However, it is unknown whether the relationship between BCS and actual adiposity is similar between horses and ponies. Recent work on pony cadavers has shown an exponential association between BCS, as assessed using the Henneke or Carroll and Huntingdon systems, and body fat content (Dugdale et al 2011a). There was, however, a linear association between BCS (both systems) and overall “flesh” (total somatic soft tissues) content (Dugdale et al 2011a). This work supported the exponential relationship described between BCS (INRA system of 0 to 5) and body fat content in 20 French Sport horses (Martin-Rosset et al 2008). Once bony landmarks become obscured by superficial adipose tissue, adequate descriptors to differentiate “obese” from “super-obese” animals are lacking.

Many factors may influence the accuracy of condition scoring within an individual animal, including conformation, coat length, pregnancy, evaluator bias, gut fill, and muscle development. Ideally, the BCS systems would assess subcutaneous fat tissue independent of muscle mass. In reality, even with an experienced evaluator it is often difficult to differentiate adipose tissue from muscle, and in animals with relatively low muscle mass there can be underestimation of fat mass (e.g., aged horses).

Morphometric measurements

Bodyweight alone is uninformative for assessing relative adiposity or body condition, unless it is put into context by relating it to the size of the animal. For example, a 400 kg, 13 hh pony would have a drastically different body condition than a 400 kg, 16 hh horse. Body mass index (BMI) aims to adjust bodyweight for body size by dividing the weight of an individual by a measurement of body size. In humans, BMI is calculated as bodyweight/height². This index has been applied to horses with moderate correlations (r_s = 0.35 to 0.60) between BMI and BCS (Carter et al 2009a, Donaldson et al 2004). In other animals including cats, a BMI has been applied where body length is integrated into the adjustment for body size, calculated as bodyweight/(height × body length) (Hoenig & Ferguson 2002). When this BMI was applied to horses, it similarly correlates with BCS (r_s = 0.37 to 0.50). Additionally, a simple bodyweight to height ratio has been demonstrated to be correlated to BCS (r² = 0.58) and body fat percentage (calculated from rump fat thickness, see below) (r² = 0.43) (Henneke et al 1983). However, all of the above correlations are relatively weak, suggesting that one-off measures will not necessarily be accurate and precise predictors of actual body fat content.

Additional morphometric measurements that are useful for assessing relative body condition include the use of a body circumference in relation to body size. This assumes that an increase in circumference is mainly caused by an increase in fat mass. To support this assumption, a ratio of girth circumference to withers height was correlated to BCS (r² = 0.51) and body fat percentage (r² = 0.44) in Quarter horses (Henneke et al 1983). When girth and abdominal circumferences were adjusted by withers height, body length, or bodyweight, it was found that the girth to withers height ratio was the strongest predictor of BCS in horses and ponies (r_s = 0.64 and 0.83, respectively), with a greater predictability than both calculations of BMI (Carter et al 2009a

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