Standardbred

The Standardbred is an American horse breed best known for its ability in harness racing where they compete at either a trot or pace. Developed in North America, the Standardbred is recognized worldwide, and the breed can trace its bloodlines to 18th-century England. They are solid, well-built horses with good dispositions.

Standardbreds are generally well-muscled and similar to, but a bit heavier than, a Thoroughbred, with a large head often with a Roman nose profile, straight neck, sloping shoulder, defined withers, a deep girth, strong bone (legs) and hard feet. Standardbreds average 15 to 16 hands (60 to 64 inches, 152 to 163 cm) and typically weigh between 800 and 1,000 pounds (360 and 450 kg). They are most often bay, and less frequently brown, black, chestnut, gray or roan. Standardbreds are considered easy-to-train horses with a willing submissive nature.

Standardbreds race either at a trot or pace. In the trot, the horse's legs move in diagonal pairs; when the right foreleg moves forward, so does the left hind leg, and vice versa. In the pace each foreleg moves in unison with the hind leg on the same side. However, the breed is able to perform other horse gaits, including the canter, though this gait is penalized in harness racing.

Pacers still retain the ability to trot and do so regularly; however, they are labeled by the gait they race at. Additionally, some trotters are able to pace, but prefer to race at the trot. Today's Standardbreds race in the same gait for their whole career, with rare exception. And although the gaits are similar, the pace is faster than the trot.

Today's Standardbreds are commonly bred trotters to trotters, and pacers to pacers. Although it is possible for trotters to produce a pacer, these gait-specific breeding practices are causing Standardbreds to differentiate into two subpopulations based on gait, more distinct than some breeds.

The ability to pace is linked to a single-point mutation in gene DMRT3, which is expressed in the I6 subdivision of spinal cord neurons; this area is responsible for coordinating the locomotor network controlling limb movements. The point mutation causes early termination of the gene by coding for a stop codon, thus altering the function of this transcription factor. Uniquely, both pacing and trotting American Standardbreds are fixed for the DMRT3 variant. This suggest that there is at least one additional genetic variant that controls trotting and pacing in Standardbreds. However, European Standardbreds are not yet fixed for this DMRT3 variant, although homozygous individuals have better performance records.

Osteochondrosis (OC), osteochondrosis dissecans (OCD), and other related developmental orthopedic diseases (DOD) appear at a high frequency in Standardbreds. OC/OCD is a moderate to highly heritable condition in Standardbreds and similar breeds, and genetic risk factors have been identified. These findings suggest that a change in breeding practices could lower the prevalence within the breed; however, lesions may resolve on their own or can be surgically corrected.

In the 17th century, the first trotting races were held in the Americas, usually in fields on horses under saddle. However, by the mid-18th century, trotting races were held on official courses, with the horses in harness. Breeds that have contributed foundation stock to the Standardbred breed included the Narragansett Pacer, Canadian Pacer, Thoroughbred, Norfolk Trotter, Hackney, and Morgan.