How we feed, train, and manage horses can significantly impact their body weights. But one factor we have less control over also plays an important role: genetics.

While this isn’t a revelation in itself, what isn’t clear—or, rather, hasn’t been known until now—is which genes code for body weight. In a recent study Japanese scientists identified four genes that appear to have direct consequences on body weight in the Thoroughbred racehorses they studied.

The genes—LCORL, ZFAT, TRIB2, and MSTN—are found, respectively, on Chromosomes 3, 9, 15, and 18. Combined, these genes accounted for 17% of the body weight differences in more than 800 3-year-olds. When factored in with sex differences, they accounted for a full 30%.

“When considering body weight in Thoroughbreds, we’ve found up to four genes that appear to have a major impact,” said Teruaki Tozaki, PhD, of the Laboratory of Racing Chemistry Genetic Analysis Department, in Tochigi.

“This is a smaller number of genes than that affecting body weight in humans, according to previous studies,” he said. “But since Thoroughbred horses are a domesticated breed subject to selective breeding within a single restrictive breed, it isn’t surprising that the number would be so low.”

It’s possible that other genes within the genome might contribute to body weight, but to a lesser extent, he added.

Researchers already knew that LCORL affects wither height, Tozaki said. So it seems logical it would also affect body weight. However, Tozaki’s team found no contribution from two other genes known to impact wither height (HGMA2, on Chromosome 3, and LASP1, on Chromosome 11) on body weight in their study.

To identify the “body weight genes,” Tozaki and his fellow researchers weighed and collected blood samples from 851 Thoroughbreds that came to the track to race. The horses’ weights ranged from 390 to 568 kilograms (roughly 860 to 1,250 pounds), with an average of 474 kilograms (about 1,045 pounds).

They ran a genome test on the horses’ blood to detect associations between individual genes and measurable data (in this case, body weight). The test, called a genome-wide association study (GWAS), takes advantage of basic equine genome data from the Horse Genome Project.

Their analyses pointed them to the four aforementioned genes as strong “candidates” for influencing body weight.

This new information is useful from a research perspective and could eventually lead to more precise breeding for better performance, health, and welfare, Tozaki said. It could even help scientists tailor medical care, feeding, and training to individual horses in the scope of “personalized medicine.”

However, it should not be used for unfair sport, Tozaki said.

“Our genetic information should not have any application in the development of genetically modified animals, which hinders fair horse racing,” he said.

The study, “A genome-wide association study for body weight in Japanese Thoroughbred racehorses clarifies candidate regions on chromosomes 3, 9, 15, and 18,” was published in the Journal of Equine Science.