With the industrialization of farming, a few breeds of livestock have been standardized for maximum productivity. This expansive growth of the few breeds resulted in the critical endangerment of many other breeds, with several becoming extinct. These pre-industrial breeds, known as heritage breeds, possess a priceless genetic diversity that can help farmers, and society at large, adapt to variable conditions, ranging from new consumer tastes to varied landscapes.

American Mammoth Jackstock are the world's largest breed of ass. The breed was developed in the United States through the cross-breeding of imported large European breeds (predominantly from Spain) and, to a lesser degree, native American and Mexican stock. The breeding of these animals in small numbers precedes the creation of the registry by more than 100 years, well before the Revolutionary War, according to an account written by one of the registry's founders in 1902.

Jackstock were developed out of a need for high-quality jacks to sire mules with size, strength, and vitality for use in the agriculture and transportation industries. The variety of types of mules required in each of these industries in different regions of the country provided the premise for the types of Jackstock bred in the early years. Breeding of various types of Jackstock has been further influenced by the breeds, or types of mares available in different areas, the economy, war, and other factors.

To identify the characteristics of Jackstock, it is helpful to understand the diverse genetic makeup that has given distinction to these animals. There are different types of Jackstock, just as there are different types within the various horse breeds. Some Jackstock may exhibit more qualities inherited from their Majorcan ancestors, while others may resemble the Andalusian, and another may exhibit characteristics of several of the breeds in its ancestral makeup.

This diversity does not reflect the original goals of the registry when it was created in 1888, at a time when the Jackstock industry was flourishing. The registry sought to facilitate the standardization of the breed to include only black animals, ultimately a minimum of 15 hands tall. The records of breeding for many years reflected these goals. Over the years, revisions to the registry rules of entry, economic factors, war, geographical considerations, and other factors altered these breeding practices, and the diverse genetic makeup of Jackstock persevered.

Documentation of breeding prior to the Revolutionary War is lacking in detail; however, one account indicates the first jacks to be imported were from the Cape de Verde Islands, tracing back to the Portuguese. These animals were said to have produced inferior mules that were exported to the West Indies during the time of the Peninsular wars when jacks could not be imported from Spain. Due to their inferior quality, these animals are said to have died out from competition with better animals, and the decline in demand from West India.

The gift of an Andalusian jack and two jennets to George Washington from the King of Spain in 1785 is widely recognized as the event that revolutionized Jackstock breeding in the United States. These foundation stock, and a Maltese jack later bestowed on Washington by his European ally, Lafayette, produced Jackstock and mules of a quality not previously attainable with the inferior domestic stock and those imported from the Cape de Verde Islands.

Between 1785 and the start of the Civil War a number of jacks were imported from Europe. Registry records contain relatively good accounts of the breeds of animals imported and bred during this period. Though this information is not official, it was documented by prominent individuals in the Jackstock industry, and founders of the registry.

One of the most influential jacks in Jackstock history was known as Imported Mammoth, imported to northern Kentucky prior to the Civil War. Accounts of Imported Mammoth indicate he was 16 hands, of extreme length, heavy like a horse, with large feet and bone, the latter being somewhat meaty. Said to be far superior to any other jacks at that time, he was credited with having improved the jack and jennet stock in bone and size more than any other in his nine-year breeding career. Limestone Mammoth, a descendant of Imported Mammoth, is a good example of the Imported Mammoth bloodlines.

In contrast to Imported Mammoth, imported Catalonian jacks contributed style and finish. Prized for their glossy coats ranging in color from black to brown, the Catalonian was said to be "less in size (14-1/2 to 15 hands), bone, and foot but with the finest long, keen, Roman heads, and thin folding, upright ears." Offspring of early imports reportedly exhibited an excess of style and finish with superb heads and ears, and fine silky coats. The bone and foot lacked the size of the Imported Mammoth bloodline, but the bone was flat and clean. Great Eastern was considered by the registry in 1888 to be one of the highest types of the Catalonian breed. The Catalonian thrived in the United States, and the produce of the imported jacks reportedly outgrew their sires in size, bone, and muscle, but retained the desired style and finish. The Starlight line descended from imported Belknap were considered by many to be excellent examples of the fine Catalonian type of jacks produced in the United States in the mid and late 19th century. Crossing descendants of the Starlight line with descendants of the Imported Mammoth line produced excellent Jackstock with size, bone, style, and action.

Maltese jacks were some of the earliest imported to the United States. Considered by many to be too small, their use in breeding Jackstock declined drastically with the increased importations of Catalonian, Andalusian, and Majorcan jacks. The Maltese imported to the United States were rarely over 14 hands. Some had good feet and bone, but were said to be lacking in their limbs. Black or brown in color, they had attractive heads and ears. The Maltese were a more-fiery breed than the other large European breeds, and have been credited with contributing vitality to the Jackstock breed.

The Majorcan was the largest of the European breeds used in the development of American Jackstock. The average height of Majorcan imports was 15-1/2 hands, with exceedingly large bone and body. Their heads were enormous and bulky looking, with the largest, but not so erect, ears of any other breed. The Majorcan's action was sluggish in contrast to the lively action of the Catalonian and Maltese. They were more uniform in size than the Catalonian, and considered to be not only the largest jacks in existence, but also the most pure breed of jacks in Spain. The numbers of Majorcans imported was limited due availability. In 1890 it was reported their native island had been literally stripped of these animals due to their desirability for the production of jennets with size. King Inca, registered as being 15-7/8 hands, was an example of a good imported Majorcan jack.

Andalusian jacks were some of the earliest imported, and considered to be one of the most distinctive breeds of jacks in existence. It is claimed that this breed is of an ancient race used for producing mules before the time of Christ. Andalusians are predominantly gray, though some may be almost white, and in rare instances, black or blue. They range in size from 14-1/2 to 15 hands, with excellent legs, large firm bone, and a good head and ear. The Andalusian was widely used in Jackstock breeding, but lost favor due to the coat color. Black was the preferred color, and the only color that was allowed entry into the stud books during the early years of the registry. An interesting observation of early breeders is that the Andalusian was less prone to developing "jack sores" than any of the other breeds.

Though considered an inferior jack at the time, a fair number of Italian jacks were imported in the late 19th century. It was felt the importation of these jacks was motivated by their cheapness and the strong demand for imported jacks in the United States at the time. They were generally black, though occasionally gray, and believed to be related to the Maltese. The Italian jacks were the smallest jacks imported, ranging from 13 to 14 hands. They were a vigorous jack, considered by some to be of a vicious nature. These jacks were shunned by serious Jackstock breeders, and it is not clear how much they may have been used to breed jennets.

The Poitou of France was widely considered to be the most perfectly formed of all European jacks imported to America, due to its immense weight, bone, and muscle, ranging in height from 14-1/2 to 15 hands. The Poitou is quite distinct in its appearance, being the only breed with a very long coat. Other characteristics include a broad chest, deep body, full hip, and large feet and joints. They were not imported in as great a number as the Spanish breeds, for the most part due to high prices. LaFleur was one of the early Poitou entries in the stud book.

In addition to the above-mentioned breeds, early history indicates some jennets were imported for Jackstock production to the midwest and southern regions of the United States, from Texas and Mexico. These animals were considered to be inferior in quality, lacking size, bone, and style. Their introduction was stated to be a great drawback to the progress of raising the standard of jacks and jennets. L.W. Knight, in a 1902 writing, makes reference to the colors of jennets that would seem to elude to the use of these Texan and Mexican jennets in Jackstock production. He said, "Our jennets in this country are very diverse in color. Grays, blues, and mouse-colored are quite numerous." The "mouse" color was quite common in the Texan and Mexican jennets.

The distinguishing characteristics of good-quality Jackstock are no different today than they were 200 years ago. Registry documentation and other historical accounts consistently affirm that breeders of the 18th and 19th centuries were striving to produce animals with the same qualities as breeders in the 20th and early 21st century.

Jacks with a minimum of 14.2 hands (58") tall and jennets a minimum of 14 hands (56"), plus girth and cannon bone measurements that meet the requirements, are eligible for registration, as they were when the registry was created in 1888. Animals in these size range represent a significant number of the Jackstock population. However, the model jack 200 years ago and today would be no less than 15 hands (60") tall. He should have good width, depth and length of body, a strong loin, and full hip. The neck should be well muscled, but not excessively thick, and of proportionate length. The feet should be large and well cupped. Bone should be of good size, flat, and clean. The legs should not be fine in appearance, resembling the leg of the Thoroughbred horse. The head should be well shaped and not of extreme length or thickness, tapering to a relatively fine, rounded muzzle, and be in good proportion to the body, with large wide set eyes, and well-placed long, thin upright ears. The result of these combined qualities is a jack with style and substance that is well suited for Jackstock production, as well as producing superior quality mules of any type, relative to the breeds of mares he is put to.