Appendix Horse Breed Guide: Characteristics, Health & Nutrition

The Appendix horse is a mixed breed produced by crossing an American Quarter Horse with a Thoroughbred. The cross is also known as an Appendix Quarter Horse.

Appendix horses can become eligible for permanent registration with the American Quarter Horse Association (AQHA) if they meet specific requirements. Some famous Quarter Horse sires were originally Appendix horses with Thoroughbred ancestry.

The Appendix combines the athleticism and stamina of the Thoroughbred with the agility and power of the Quarter Horse. This versatility makes them suitable for various equestrian disciplines including racing, jumping, and western events.

This profile will discuss the history, breed characteristics, health problems, and nutritional needs of the Appendix Quarter Horse. Keep reading to learn more about feeding and caring for Appendix horses.

Appendix Horse History

Thoroughbreds and American Quarter Horses are two of the most influential horse breeds, each with distinct characteristics that have been honed through centuries of selective breeding.

Introducing Thoroughbred bloodlines to the Quarter Horse gene pool was a strategic move aimed at enhancing the breed’s capabilities and broadening its appeal across various equestrian disciplines.

The AQHA initially established the Appendix registry to identify Quarter Horse crosses that did not have full AQHA registration. Today, the AQHA only recognizes crosses with particular breeds as Appendix Quarter Horses.

Origin

The American Quarter Horse is the most popular horse breed in North America. These horses trace their origins to early horses brought to the Colonies by English settlers, which were crossed with ponies bred by the indigenous Chickasaw and Cherokee people. ^[1]

Imported Thoroughbreds significantly influenced the development of the American Quarter Horse. One of the founding stallions of the breed, Janus, was an English Thoroughbred descended from the Godolphin Arabian. ^[1]

The Appendix registry was formed in 1949 after two Quarter Horse associations merged to form the AQHA. Initially designed to identify horses previously registered with the other groups until they passed inspection, the registry also included Quarter Horse-Thoroughbred crosses.

When concerns arose over the Appendix registry’s impact on the breed’s integrity, the AQHA refined requirements for registered Appendix horses. However, the term Appendix is often used to refer to any horse with mixed Thoroughbred and Quarter Horse blood.

Historic Use

Colonial settlers used the ancestors of Quarter Horses for short sprint races of a quarter-mile distance. Thoroughbred blood helped improve the stamina of these powerful sprinters.

When longer-distance Thoroughbred racing came into fashion on the Atlantic coast, the hardier Quarter Horses moved west. On the other side of the Mississippi, these horses crossed with Mustangs to produce a versatile mount ideal for working cattle on the range.

Crosses with Thoroughbreds continued even after the first official Quarter Horse breed organizations formed. A Thoroughbred stallion named Three Bars was one of the most influential sires of the Quarter Horse breed in the 20th century. ^[2]

DNA studies in Quarter Horses confirm significant genetic contributions from the Thoroughbred. One study found racing Quarter Horses had the closest relationships to Thoroughbreds. ^[1]

While Thoroughbred crosses are relegated to the Appendix registry, Appendix Horses that pass inspection are eligible for permanent registration with the AQHA. Some Appendix stallions, such as Impressive, become influential sires after gaining their registration.

Breed Registry

The American Quarter Horse Association still operates an Appendix registry in addition to the main registry. AQHA combined tentative and permanent registries in 1959 by automatically registering all foals born after 1962 to two fully registered parents.

Foals of registered Quarter Horses crossed with approved Thoroughbreds received numbers with the new Appendix registry. Crosses between registered Appendix Quarter Horses and fully registered Quarter Horses are also considered Appendix Quarter Horses. These horses could gain full registration as adults if they passed inspection or attained a register of merit.

The Thoroughbred parent must have breeding approval from the AQHA. Appendix horses are only eligible for breeding with fully registered Quarter horses. Foals with two Appendix parents or one Appendix Parent and one Thoroughbred parent are ineligible for the Appendix registry.

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Breed Characteristics

The AQHA approves Thoroughbreds for crossbreeding based on their potential to improve the Quarter Horse’s breed standard. Appendix Quarter Horses are judged against this standard to earn permanent registration, but many Appendix horses inherit traits from both breeds.

Conformation

Most Appendix horses stand between 15 and 17 hands tall at the withers. These horses are often taller than purebred Quarter Horses due to their Thoroughbred ancestry.

Their overall appearance can vary between stock and racing horse types depending on the percentage of Thoroughbred and Quarter Horse bloodlines.

Many Appendix horses have a significant amount of Thoroughbred blood, resulting in a leaner, taller, and more long-legged build. More influence from stockier Quarter horse bloodlines produces a more compact horse with greater muscle mass.

The AQHA evaluates Appendix horses for registration based on the same characteristics preferred in purebred Quarter Horses. These breed characteristics include:

• Attractive head
• Refined throat latch
• Well-proportioned neck
• Sloping shoulder
• Short back
• Deep heart Girth
• Strong Loin
• Long hip
• Straight, correct legs
• Uniform muscling

Colours

Appendix Quarter Horses have the same coat colours as purebred Quarter Horses. Standard coat colours recognized by the AQHA include:

• Chestnut
• Sorrel
• Black
• Brown
• Gray
• Bay

Roan patterns and diluted colours are also found in the breed, including palomino, buckskin, cremello, grullo, bay roan, blue roan, and red roan.

The AQHA traditionally excluded horses with excessive white from registration but rescinded this rule in 2004. Overo is the only pinto pattern found in American Quarter Horses.

Temperament

There generally aren’t significant personality differences between Appendix horses and Quarter Horses. However, Appendix horses with more Thoroughbred blood tend to have hotter and more sensitive temperaments.

The ideal Appendix horse combines the steady mind and work ethic of the Quarter Horse with the intelligence and athleticism of the Thoroughbred.

Disciplines

Appendix horses are versatile pleasure mounts that can excel in many different disciplines. The steady Quarter Horse influence also makes these horses suitable for riders with a wide range of abilities.

Most share similar talents to Quarter Horses and commonly compete alongside them in AQHA events. Popular discipline classes for Appendix horses in AQHA events include:

• Reining
• Western Pleasure
• Cutting
• Trail
• Western riding

Appendix horses also inherit a talent for English disciplines from their Thoroughbred ancestry. These horses often excel in hunter under saddle and working hunter classes.

However, Thoroughbred blood has the most significant influence on racing Quarter Horses. Many of the horses competing in quarter-mile races today are Appendix horses.

Health Profile

Appendix Quarter Horses can inherit mutations associated with several genetic diseases commonly found in Quarter Horses. However, these horses are also susceptible to health problems that affect Thoroughbreds more frequently than Quarter Horses.

Genetic Diseases

Some veterinarians recommended screening Quarter Horses and related breeds with a 5-panel plus test for GBED, HERDA, HYPP, MYHM, MH, and PSSM1.

Glycogen Branching Enzyme Deficiency

GBED is an inherited fatal disease characterized by an inability to store glycogen correctly. The brain, heart, and skeletal muscles are unable to function normally. Most affected foals are stillborn or aborted. Foals that survive until birth typically die by 18 weeks. ^[3]

Studies estimate approximately 10% of Quarter Horses in the United States carry the mutation associated with the disease. It is most commonly found in Western pleasure and cutting horse bloodlines. ^[3]

Hereditary Equine Regional Dermal Asthenia

HERDA is an inherited skin condition associated with hyperextensible skin, severe lesions, and scarring. The skin along the neck and back stretches and tears easily, and lesions occur primarily under the saddle area. This condition makes affected horses unsuitable for riding. ^[4]

Researchers linked the disease to a genetic mutation that causes collagen defects. Some studies suggest these defects can contribute to additional health problems in the horse’s eyes and heart in affected Appendix Quarter Horses. ^[4]

Hyperkalemic Periodic Paralysis

HYPP is a muscle disease characterized by sporadic episodes of tremors and paralysis. The condition is caused by a missense mutation that affects muscle sodium channels. Affected horses need low-potassium diets to manage clinical signs. ^[5]

Studies identified this genetic defect in descendants of the stallion Impressive, who was born an Appendix Quarter Horse. ^[5]

Myosin-Heavy Chain Myopathy

MYHM is another inherited muscle condition found in Quarter Horses. This condition has two different disease presentations, but both are linked to the same gene variant. ^[6]

Some Appendix horses with the disease have immune-mediated myositis, which causes severe muscle atrophy after an autoimmune event. Others experience non-exertional rhabdomyolysis, also known as tying up. ^[6]

Malignant Hyperthermia

MH is an inherited disease that causes a life-threatening hyper-metabolic state when triggered by halogenated anesthetics or succinylcholine, a muscle relaxer. Stress and excitement can also occasionally trigger this state in Quarter Horses. ^[7]

This condition has an autosomal dominant method of inheritance. Affected horses only need one copy of the MH allele, so Appendix Quarter Horses with one Quarter Horse parent and one Thoroughbred parent are still susceptible to the disease. ^[7]

Polysaccharide Storage Myopathy

PSSM is characterized by abnormal glycogen accumulation in skeletal muscles. Researchers have identified two types of PSSM in horses. PSSM Type 1 is associated with a GYS1 mutation. The cause of PSSM Type 2 is still under investigation. ^[8]

One study identified the GYS1 mutation in 72% of 99 Quarter Horses with clinical signs of tying up and PSSM. PSSM1 is an autosomal dominant condition, so Appendix horses only need to inherit one copy of the mutation. ^[8]

Research suggests subclinical PSSM may also affect 6-12% of overtly healthy American Quarter Horses. ^[9]

Health Problems

Thoroughbreds can also suffer from intermittent tying-up episodes. However, one study comparing exertional rhabdomyolysis in Thoroughbreds and Quarter horses found no PSSM in affected Thoroughbreds. Appendix horses with PSSM likely inherited it from a Quarter Horse parent. ^[10]

Appendix horses may inherit an abnormally large heart size from Thoroughbreds, which increases cardiac output during races. However, human research has linked large heart size to an increased risk of arrhythmias, heart valve disease, and heart muscle ischemia. ^[11]

These horses can also inherit a predisposition to hoof problems. Poor hoof quality is a common problem in Thoroughbreds. Many Quarter Horses have narrow feet for their body size, which research suggests can predispose the breed to navicular syndrome. ^[12]

Care and Management

Thoroughbreds, Quarter Horses, and Appendix horses all need quality basic care that meets their physical, mental, and emotional health needs. Work with your veterinarian to develop a preventative wellness program with routine vaccinations, deworming, and dental exams.

Genetics, nutrition, environment, and care can all impact hoof quality. Good farrier care is especially important for Appendix horses with a history of hoof problems. Maintaining hoof balance is also essential for managing horses with navicular syndrome.

Free exercise supports a healthy musculoskeletal system in all horses. Appendix horses with PSSM and other muscle disorders should get regular exercise and daily turnout.

Thorough grooming routines support muscle health by stimulating circulation. Daily grooming also helps keep your Appendix horse’s skin healthy and hair in good condition.

Nutrition Program

Quarter Horses and Thoroughbred horses generally have different nutritional needs. The best diet for Appendix horses can vary depending on workload and their metabolism.

Weight Maintenance

Quarter Horses are typically easy keepers. But some Thoroughbreds are notoriously hard keepers.

Most Appendix horses have efficient metabolisms. But horses with more Thoroughbred blood may need more dietary energy to maintain weight. Monitor your horse’s body condition score to determine if you need to adjust his diet.

Sample Diet

The following sample diet is intended for a mature 450 kg (1000 lb) Appendix horse with normal body condition at maintenance (not exercising).

Most Appendix horses do best on a forage-only diet. Commercial concentrates are commonly fed to Thoroughbreds, but they aren’t necessary to meet the energy needs of most Appendix horses.

However, forage-only diets need fortification to fill the nutrient gaps in hay. A vitamin and mineral supplement can help balance your Appendix horse’s diet without adding unnecessary calories or starch.

Mad Barn’s Omneity® is a vitamin and mineral premix designed to fortify forage-based diets. This formula contains balanced minerals, vitamins, and amino acids commonly deficient in hay. The biotin, yeast, and enzymes in Omneity® also support hoof health and digestive function.

All Appendix horses need free access to fresh water and salt. A salt lick encourages thirst, but our nutritionists also recommend feeding loose salt to meet sodium requirements.

Forage

All horses need a constant supply of forage to maintain optimal digestive health. The best type of forage for your Appendix horse depends on their metabolism and health needs.

A 450 kg (1000 lb) Appendix horse should eat approximately 9 kg (20 lb) of average-quality, low-starch grass hay daily. Appendix horses with metabolisms similar to Thoroughbreds need good-quality grass hay to meet their energy requirements.

Low-starch forages are the best option for most horses, as high dietary starch can contribute to digestive and metabolic problems. Starch and sugar (ESC) are collectively known as hydrolyzable carbohydrates (HC) which differ from NSC by their ability to stimulate insulin spikes in the blood.

Some forages, such as Alfalfa hay, are inappropriate for Appendix horses with PSSM due to their high potassium content.  A forage analysis is the best way to ensure your hay is safe for your Appendix horse.

While free-choice forage supports digestive function, some Appendix horses may gain excess weight with unrestricted forage. Consider using a slow feeder or grazing muzzle to slow intake.

Feeding Recommendations

Large meals and high-starch grains can increase the risk of digestive disturbances. If you feed your Appendix horse a concentrate, split the daily ration into multiple small meals spread throughout the day to reduce these risks. ^[13]

Consult an equine nutritionist before adding a new feed to your PSSM Appendix horse’s diet. Appendix horses with PSSM need low-starch diets. Research suggests these horses can safely get up to 20% of their dietary energy from fat. ^[14] However, to support exercise at that high intake of fat the horses must also get considerable amount of grain to provide the needed substrates, particularly oxaloacetate, for energy generation via the Krebs cycle to operate. ^[15]

The combination of regular exercise and a reduced hydrolyzable carbohydrate intake works well for Quarter horses with PSSM. ^[16] Supplementation with acetyl-l-carnitine can also help.

Commercial concentrates often contain too much starch for Appendix horses with metabolic or muscle health concerns. Consider swapping grains for soaked hay pellets or beet pulp if you need an alternative supplement carrier.

Nutritional Supplements

Good overall health starts with a balanced diet. Focus on filling any nutritional deficiencies in your Appendix horse’s feeding program before adding additional supplements for support.

• W-3 Oil is an essential fatty acid supplement with high levels of DHA, an omega-3 fatty acid. This fat supplement is a good choice for Appendix horses who need extra sources of dietary fat for energy. The natural vitamin E in W-3 also supports muscle health in these horses.
• Appendix horses with hoof health or joint problems can benefit from MSM. MSM is a natural supplement commonly used to support the body’s own homeostatic mechanisms to moderate inflammation.
• Performance Appendix horses with intense training schedules may need extra gut support to maintain digestive health during stressful periods. Mad Barn’s Visceral+® is a comprehensive gut formula that supports stomach and hindgut health in these horses.
• Acetyl-l-carnitine can support efficient energy generation and muscle relaxation in horses with PSSM by directing glucose into energy pathways and away from glycogen, acting as an antioxidant and promoting biogenesis of mitochondria.

Submit your Appendix horse’s diet online for a free evaluation and consult with our experienced equine nutritionists for help formulating a balanced diet.