Estimating weight

Whether in a stud farm or stable, it is important to know the live weight (LV) of a horse in order to:

• Establish its ration: a very large proportion of the recommended food intake depends on the live weight of the animal.
• Monitor its growth
• Monitor its general health
• Adjust doses of drug treatments, including antiparasitic drugs

If there is no scale available in the facility, a visual, empirical and often false estimate (except for real experts) is sometimes made.

There are, however, body weight formulae that allow the horse’s weight to be assessed more accurately. From simple-to-take measurements such as:

• Height at withers (WH)
• Thoracic circumference (TC)

To take these measures, make sure you keep the horse straight (neck in the extension of the body) and square (the four limbs flat on the ground). For safety reasons, both handlers should always stand on the same side of the horse.

For the sport horse

 Based on Martin-Rosset - 1990, a working sports horse or one maintaining its weight:

• LW (in kg) = 3 WH + 4.3 TC - 785 (to within +/- 25kg)

For thoroughbred brood mares

According to Blanchard et al - 2005

• Thoroughbred brood mare:                LW (in kg) = 2.3 WH + 3.19 TC - 428
• French trotter brood mare:                LW (in kg) = 4.42 WH + 3.76 TC - 849
• Selle Français brood mare:                 LW (in kg) = 0.64 WH + 5.78 TC - 633

According to Martin-Rosset - Inra 2012:

• Saddle and racing brood mares:        LW (in kg) = 2.6 WH + 5.2 TC – 855 (to within +/- 25kg)

For draft horses

According to Martin-Rosset - 1990, mare and stallion

• LW (in kg) = 7.3 TC - 800 (to within +/- 25kg)

For ponies

According to G. Duchamp and E Barrey - Inra 2012, for any type of pony over 1m tall

• LW (in kg) = 3.56 WH + 3.65 TC - 714.66 (to within +/- 21kg)

For growing foals

During growth, chest circumference and body weight change in the same proportion. The height at withers increases differently, so the formula (INRA) calculates the weight of the growing blood foal without the height at withers.

• LW (in kg) = 4.5 - 370 TC (TC is expressed in cm)

A study carried out in Lower Normandy on Selle Français, Trotteur Français and Thoroughbred horses also makes it possible to estimate the weight to within 20 kg. It also takes into account the age of the animal in the parameters, and formulae differentiate between the sex of the animal.

• For males:    LW (in kg) = 0.20 A+ 1.72 WH + 2.15 TC - 327
• For females:    LW (in kg) = 0.21 A+ 1.85 WH + 2.01 TC - 330.2

A = age in days, TC and WH in cm

Estimating the weight allows the breeder to understand the growth of the animal and to adjust the ration according to their objectives.

The nutritional requirements of the foal depend on the animal's live weight, weight gain, composition (mainly lipid content) and stage of development.

In 2013, a comparison of different height/weight formulae listed in the scientific literature was carried out on 162 adult horses of various breeds. The formulae were compared to the actual weight of the horses as weighed on the scale.

How to take measurements. Focus in photos

Some measures require anatomical elements to be correctly identified and the measurements must be very precise, at the risk of significant deviations, such as different body lengths (LC1, LC2 and LC3 that pass over the belly). Other measures are easier to take, such as height at withers (WH) and thoracic circumference (TC)

The formulae by Hapgood, Milner & Hewitt and Caroll proved to be the most closely correlated to the actual weight of the horses.

Be careful, however, as the Hapgood formula is calculated in inches. These three formulae use measurements of LC1 and LC2 which are more difficult to gauge.

The formulae by Crevat and Martin Rosset are perhaps less precise but appear to be faster and easier to estimate (WH and TC measurements).

Jones' formula proved to be the least accurate for the horses that were studied.