Reshaping horse breeding for the digital age

Dr. Annik Gmel outlines research showing how equine joint angle measurements reveal how genetics can affect conformation. To date the work has been carried out using European riding horses, but could be of significant future use in the thoroughbred

“BREEDING THE BEST with the best and hoping for the best” has been the main horse breeding strategy for centuries. But how can we select for performance and longevity without waiting for the end of the horse’s career?

Imaging and genomic tools bring new insights to the selection of horses, focusing on conformation as the traditional base.

In equine breeding, conformation regroups a set of traits describing the shape and proportions of an animal, strongly determined by the underlying musculoskeletal apparatus.

The shape of the head, the length of the limbs in relation to the body, and the angulation of certain joints are important traits involved in the aesthetics and functionality of a breed. Currently, across Europe, judges visually assess riding horses according to a breed-specific scoring sheet. These scores are used to select the best horses for breeding.

Conformation assessments – more art than science

However, scorings in most European horse breeds are an unreliable source of information for breeding.

Certain traits are difficult to evaluate with vaguely described breeding standards, while differences between judges can also occur as every human has a slightly different opinion depending on previous experiences.

The outline of each measured horse is overlapped with the rest to visualize the average shape, and the extremes within a group of horses

Scoring each breed differently also limits the number of horses scored each season, leaving judges poorly trained.

As the scoring is breed-specific, the underlying traits are often the same. For example, the shape of the head is as important in the Spanish Purebred as in Arabian horses, but the former selects for a roman nose while the latter prefers a dished face.

One common measurement system for all horses

To obtain uniform information on conformation traits, researchers from Austria and Switzerland have developed an image-based method to objectively measure variation in shape and joint angles – the “horse shape space”.

On a single photograph taken from the side, one person traces the outline of the horse, and places dots for specific anatomical landmarks such as the point of the shoulder, the patella (stifle) or the hock with a computer program (Figure 1).

The outline of each measured horse is overlapped with the rest to visualize the average shape, and the extremes within a group of horses (Figure 2).

This simple, low-cost method has many applications: comparing outlines of horses from different disciplines, e.g. Flat racing and steeple chasing, can reveal differences in the morphology, and what makes a good performer in each discipline.

Figure 1: Angular measurements from the horse shape space were developed in FranchesMontagnes horses and can be calculated by tracing straight lines between three anatomical markers (https://doi. org/10.1371/ journal. pone.0202931. g001) Figure 2: Overlapping the mean Lipizzaner shape (in black, out of 224 horses) with the mean FranchesMontagnes shape (in red, out of 300 horses) reveals differences in the shape of the head, the croup and the underbelly between the two breeds

Overlapping mean outlines can also show breed-specific variation in conformation.

As these measures are based on anatomical structures present in all horses, researchers can now pool data from different breeds to identify links between conformation measurements and the horses’ genome.

Hereditary predispositions

The genome consists of DNA and contains all the genetic information within an organism. It is the blueprint to the individual’s phenotypes – what you can see and measure.

Unlike coat colour, which is entirely determined by the genotype at certain genes, most phenotypes are influenced by several genes and, to some extent, also by an individual’s environment.

For example, even with genetic variants coding for higher size, malnourished animals may not become as tall as those that have been well fed.

Essentially, the individual genotype will give the horse predispositions for certain traits, which can be positive (performance) or negative (hereditary defects).

Identifying genetic variants

Genome-wide association studies (GWAS) investigate whether animals who differ in their phenotype also show genetic differences, and where these differences occur in the genome.

For example, using this method Irish researcher Dr. Emmeline Hill was the first to discover a variant within the so-called speed gene, MSTN, giving thoroughbred horses the predisposition for either speed or stamina.

For conformation traits, Swiss researchers identified several genetic regions associated with joint angles measurements in FranchesMontagnes and Lipizzaner horses using the horse shape space.

The association with the elbow joint angle in particular could be relevant to the thoroughbred.

The elbow joint angle measurement was associated with a genetic variant near two genes involved in bone density.

... the humerus bone, situated just above the elbow joint, tends to be more susceptible to stress fractures in racehorses coming back to racing after a training break

According to Robinson’s Current Therapy in Equine Medicine, the humerus bone, situated just above the elbow joint, tends to be more susceptible to stress fractures in racehorses coming back to racing after a training break. The incidence of these injuries may be affected by the incline of the humerus, as well as a genetic predisposition for more brittle bones.

Perspectives

To confirm these results, the specific genotypes and phenotypes from thoroughbreds could be cross-checked with the incidence of fractures.

Studying the shape of thoroughbreds could also reveal new variants not found in other breeds.

In addition to Franches-Montagnes and Lipizzaner horses, the Swiss-based research group is currently not only studying the conformation of multiple European breeds, such as Warmblood, Spanish Purebred and Shagya Arabians horses, but also biomechanical traits such as stride length, frequency, or the way the limb is moving forwards and upwards during the stride.

The project, called Shape and Gaits 2.0, is ongoing, with the aim to link conformation, gait performance and genetics to select the best, healthiest horses in each breed for optimal performance.

The team taking direct measurements for their research

Photo: Yvonne Wyss