Heritable Heart Traits Can Help Endurance Horses Succeed
When it comes to breeding good endurance horses, many traits aren’t heritable or have little to do with performance. But French researchers said one feature does matter: the heart.

“An ‘athletic’ heart is a heritable characteristic that’s favorable for performance in endurance,” said Céline Robert, PhD, DVM, professor and researcher at the National Veterinary School of Maisons-Alfort and researcher at the French National Agricultural Research Institute in Jouy-en-Josas, France.

Unlike Thoroughbred racehorses, Arabian and Arabian-cross endurance horses start training at around age 4 or 5 and reach high-level (160-kilometer) races starting at about 8 years old. “If you have to invest eight years of training and maintenance into a horse before you know if you’ve got a champion, you want to be sure you’re starting off with the right horse,” Robert said.

But success doesn’t always pass down through the genes. That’s why Roberts and her fellow researchers set out to determine what genetic factors affect performance and how heritable they are.

They found that most measurable traits—whether related to morphology, gaits, or cardiology—have little to do with success on endurance rides. And that’s consistent with her 2014 findings, which were limited to morphology alone.

“Few traits are related to performance, and few of those have strong heritability,” she said.

However, several heart parameters did appear to affect performance and were also more or less heritable. The most significant were related to the size of the left ventricle and the volume of blood ejection.

“The larger a horse’s heart is, the more blood it can contain, the more powerfully it can contract, and the better the horse performs,” Robert said during her presentation at the 2017 French Equine Research Day, held earlier this year in Paris.

In their ongoing GenEndurance project, the scientists measured 79 traits in several hundred competing endurance horses. Those included five “direct” measurements (height, weight, body length, girth, and skin thickness), 43 morphological measurements (angles, bone lengths, etc.), 20 gait measurements, and 11 cardiac readings. Their analyses aimed to determine how those traits related to performance (distance, times, and rankings in races) and how heritable they were.

Heritability is a trait’s likelihood of being passed down to offspring. It’s rated between 0 and 1. Anything over 0.4 is considered high heritability; below 0.2 is low; and between 0.2 and 0.4 is moderate.

Of the 79 traits, 18 appeared to be at least weakly correlated with performance, of which seven had a strong correlation, Robert said. Weak correlations included wither height—which is actually strongly heritable at 0.72—and barrel depth. The latter is probably related to better respiration, she added.

Stronger correlations included total body surface area, which would contribute to better cooling. This is particularly useful in endurance, she said, “a sport where a major challenge is eliminating the heat produced by muscular effort.”

But generally, morphology and gait characteristics were weakly or not at all correlated with performance. Stride speed at a walk did appear to have some relationship with performance, however, and is a moderately heritable trait.

Half of the cardiac measurements seemed related to performance. And they’re also fairly heritable. Left ventricular internal area in diastole (0.42), aortic diameter (0.36), left ventricular internal diameter (0.29), and stroke volume (0.34) all showed strong or very close to strong heritability levels. Veterinarians can measure these parameters in live horses if owners would like to evaluate their own horses.

“These cardiac capacities improve with training, but they also seem to have genetic origins, as the heritability of these measurements appears elevated,” Robert said. “This reinforces what has already been determined in previous studies showing that cardiac recovery time is an important and heritable performance criterion.”