New index calculates equine fat based on body measurements

Researchers have developed a body condition index for horses and ponies that can objectively assess the amount of body fat on the animal.

Samantha Potter, Madison Erdody and their fellow researchers, reporting in the Equine Veterinary Journal, said weight issues are a major problem among ponies and some breeds of horses in many developed countries.

Studies in Australia, the United States and the United Kingdom have found the prevalence of obesity in horses and ponies in these countries to be 23% to 33%.

Excess weight is commonly associated with altered metabolic states, most commonly equine metabolic syndrome, carrying with it an increased risk of laminitis.

Furthermore, many horse and pony owners perceive their animals as being an ideal or healthy weight when they may be obese or overweight.

The researchers said the most commonly used standardised practical field method of assessing fat levels in horses and ponies is by body condition scoring (BCS).

“However, it has become clear that these BCS systems have a number of limitations that may cause inaccuracy. From a practical view, further inaccuracies may arise through the need to differentiate between condition scores by visual appraisal or manual palpation — both subjective.”

The authors noted that objective scoring methods have been developed to estimate bodyweight in various equine breeds. However, bodyweight does not necessarily correlate with the amount of fat.

It would, they said, be useful to have a body condition index based on measurements of animals which can be used to accurately estimate body-fat percentage.

The study team, with the University of Melbourne, the Royal Veterinary College in England and several other institutions, set out to develop such an index.

Their study centered on seven Standardbred horses, seven mixed-breed ponies and seven Andalusian horses in a university research herd.

Initially, all began in moderate body condition. The animals were placed on a high-calorie ration for 20 weeks until they had reached between BCS 7 and 8 out of 9 on the Henneke body scoring system. This was followed by a weight loss phase, which concluded once animals reached moderate body condition (a BCS of 5) or the 12-week period ended.

Weights were measured weekly using calibrated weigh scales.

The percentage of body fat for each animal was determined using the deuterium dilution method, which is considered a gold standard method of assessing the percentage of body fat. It involves administering a dose of a deuterium isotope to a subject and calculating its dilution volume.

Measurements taken of each horse and pony including the heart girth circumference, belly girth circumference, mid-neck circumference, body length and height to the withers were obtained using measuring tapes.

Measurements were taken at the beginning and end of the weight gain and weight loss periods, and were always taken by the same investigator.

The new body condition index was derived to give the optimal correlation with body fat, applying appropriate weightings.

The formula expresses the overall outer circumference of the animal (the weighted average of neck circumference, heart girth and belly girth) for a given size of animal (that is, divided by its height and length). The length measurement was included to give a more accurate estimation of the size of the frame/skeleton of the animal.

To assess the appropriate weighting for each of these measurements, regression coefficients were derived. More than 1000 iterations were performed to derive the regression coefficients to get the best possible results.

The index was then validated by assessing inter-observer variation and correlation with body fat percentage in a separate population of Welsh ponies; and finally, the correlation between the new index and body condition score was evaluated in larger populations from studies undertaken in Australia, the UK and the US.

The authors found that the new index correlated well with the percentage of fat in the ponies and horses. However, it was found to slightly overestimate the percentage of body fat in leaner animals and underestimate it in more obese animals.

In field studies, the correlation between the body condition index and body condition score varied, particularly in Shetlands and miniature ponies, presumably due to differences in body shape.

The researchers said their index compared favourably with body condition scores in terms of accuracy in estimating how fat the animals were; however, being based on objective measurements, it was more consistent and repeatable than body condition scoring when used by inexperienced assessors.

Therefore, the index may be a useful measure for horse owners to assess body condition or fatness in their animals. It may be more useful and sensitive than body condition scoring for tracking weight gain or weight loss in individual horses or ponies.

However, it should be acknowledged that the index appeared to overestimate the percentage of body fat in Shetlands and miniature ponies, perhaps due to their different body shape, and further work may be required to adapt it to being more applicable for these breeds, they said.

The study team comprised Samantha Potter, Madison Erdody, Nicholas Bamford and Simon Bailey, all with the University of Melbourne; Edward Knowles and Nicola Menzies-Gow, with the Royal Veterinary College; Philippa Morrison and Caroline Argo, with Scotland’s Rural College; Bridgett McIntosh and Katelyn Kaufman, with Virginia Polytechnic and State University; and Patricia Harris, with the Equine Studies Group, part of the Waltham Petcare Science Institute.

Potter, SJ, Erdody, ML, Bamford, NJ, Knowles, EJ, Menzies-Gow, N, Morrison, PK, et al. Development of a body condition index to estimate adiposity in ponies and horses from morphometric measurements. Equine Vet J. 2023. https://doi.org/10.1111/evj.13975

The study, published under a Creative Commons License, can be read here.