Body Weight Estimation of Horses

Agdex#:	460/28
Publication Date:	01/99
Order#:	98-093
Last Reviewed:	01/99
History:	Original Factsheet
Written by:	Dr. B. Wright - Veterinary Scientist, Equine and Alternative Livestock/ OMAFRA; G. Reitveld and P. Lawlis - Animal Care Inspectors/OMAFRA

Table of Contents

1. Introduction
2. Body Measurements
3. Nomograms
4. References

Introduction

The body condition of a horse is generally assessed by visual observation. The references to an animal being too thin, in good flesh, or obese all relate to the animal's weight. Therefore, the weight of an animal can be used as a measurement tool to determine its well-being, or the presence of problems which may threaten the health of the horse.

A horse may be underweight for a variety of reasons, including health status, age, pecking order, poor management, or under feeding.

As weight can be used to determine if an animal is thriving or failing, it is paramount that a reliable method of measurement is available.

Calculating dosages for de-wormers or antibiotics requires the accurate determination of the correct weight of the horse or pony. An incorrect weight estimation may result in over-dosing or under-dosing the animal. Both scenarios can be detrimental to the health of the horse. The over-dosing of some medicinal products (e.g., de-wormers) may cause toxicity and colic or other complications which may result in the death of the animal. Under-dosing, in the case of antibiotics, will not produce the desired effects of the drug. The animal will likely experience only limited relief of the disease symptoms, and the disease process will progress, threatening the well-being of the horse.

Several methods can be used to determine a horse's weight. The use of a weigh scale is the most accurate way to determine a horse's weight. Scales are not an item commonly found on most horse farms. However, horses could be taken to locations in the community, where weigh scales are available, such as grain elevators and land fill sites. Standardize when weights will be taken since feeding and watering will significantly affect the differences in weight between readings.

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Body Measurements

Various body measurements are used singly or together to estimate the body weight of horses. The weight tape normally uses the girth measurement. The girth measurement alone has a correlation of 0.87 (p<0.001) while girth and length has a correlation value of 0.90. This is the most accurate estimate as reported by Carroll and Huntington.

Adult horses:

Wt (kg) = [(girth)² x length] ÷ Y

where y=11900
(measurements are in centimetres)

Foal weight 0 - 60 days:

Wt (kg) = [heart girth in inches - 25] ÷ .07

The average thoroughbred or standardbred foal will double its weight in the first 28 days.

Weight Tape
A weight tape is a simple and effective way to estimate a horse's weight at a fraction of the cost of a weigh scale and with minimal effort.

Weight tapes are readily available in most tack and feed shops. The weight is determined by wrapping the tape around the heart girth of the horse, directly behind the elbow (Figure 1), overlapping the ends of the tape, and reading the resultant weight. The reading should be taken with the tape snugly in place, at the time of respiratory expiration.

Tape accuracy is dependent on the user, the breed of horse and age. They are useless on miniature horses and foals. They are also inaccurate on high withered horses. However, on the average horse, they estimate the weight of a horse more accurately than most owners. The following table (taken from Lon Lewis' Feeding and Care of the Horse) will help you to estimate the weight of a horse by its girth measurement when a weight tape is not available.

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Nomograms

The following charts provide alternative ways of estimating a horse's bodyweight. They were developed by C. L. Carroll and P. J. Huntington.

The first method relies on the assessment of body condition score and height at the wither. (For further reading on Body Condition Score, see OMAFRA Factsheet No. 98-101, Body Condition Scoring of Horses or the References by Henneke et al. and Rudman et al. ) It gives good results for all but horses in racing condition. The more accurately height and condition score are established, the better the weight assessment will be. Ensure that the horse is standing squarely on level ground, is relaxed, and with the head in the normal position before measuring its height.

Make allowance for the thickness of the shoes. To calculate, run a straight line on the chart between the horse's known body condition score and height, then read its weight on the centre axis. In the example below, a 14 hand pony, scoring 3 points on the condition scoring chart, shows an estimated body weight of 375 kg.

Method One
Nomogram for estimating bodyweight from condition score and height measurement.

The second method, which relates measurement around the girth to length, is slightly more accurate and is the method to be used for racehorses in full work. The girth measurement is taken immediately behind the elbow after the horse has breathed out. Length is taken from the point of the shoulder to the point of buttocks (tuber ischii). To calculate, run the straight line on the chart between the known girth and length. In the following example, a horse with a girth of 180 cm and length of 175 cm shows an estimated body weight of 475 kg.

Measurements of girth, height, length and body condition scoring are used in the various systems to estimate body weight. None of these systems completely replace the accuracy of a weigh scale when used over an extended period. However, they are often more accurate than a horseman's eye.

Method Two
Nomogram for estimating bodyweight from girth and length measurements.

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References

Carroll, C. L., and P. J. Huntington , (1988) Body Condition Scoring and Weight Estimation of Horses, Equine Veterinary Journal 20 (1), 41-45.

Lewis, Lon D., (1982) Feeding and Care of the Horse. Lea & Febiger, 1982.

Henneke, D. R., G. D. Potter, J. L. Kreider, and B. F. Yeates, (1983), Relationship Between Condition Score, Physical Measurement and Body Fat Percentage in Mares, Equine Veterinary Journal 15, 371-372.

Rudman, R., and R. R. Keiper, (1991), The Body Condition of Feral Ponies as Assateague Island, Equine Veterinary Journal 23 (6), 453-456.

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