It isn’t quite correct to say that the value of a horse determines its owner’s reticence to kill it or put it in harm’s way. Horses have certainly had value across their history as domesticated animals, though that doesn’t imply that the market for horses wasn’t variegated. However, there are numerous (one could say) quirks with equine physiology that don’t lend towards good prognoses in the cases of certain types of ailments.

One of the main drivers of the value of horses, generally speaking, is that horse breeding is a bottleneck. Mares can only have a single foal per year. The gestation period of a horse is 11 months, assuming a mare successfully comes into foal at all. Twins are rare, and extremely dangerous for mare and foal. Horses just simply are not designed to carry two foals to term. Only 9% of twin pregnancies are carried to term, and of those 9%, 65% result in the loss of both foals. However, organized breeding initiatives throughout history have produced enough horses to support a market where high quality horses are recognized with higher prices than low quality horses.

While the decline of the Roman Empire corresponded with a decline in the quality of bloodstock across Europe, an organized breeding culture in the Middle Ages was revived in the 8th century, when the Frankish cavalry coalesced after Charles Martel’s victory over Abd-er-Rahman at the Battle of Poiters. Western European elites, not possessing such fine horses as one could find in the hands of Muslim rulers, both imported and captured Arabian, Turkomen, and Barb stallions to cross on the local marebase as improvers. Horses were agricultural products, and people paid for quality. In the course of founding the Saint-Gabriel-Brécy Priory, John of Fécamp purchased seven horses. The most expensive of these horses was worth 14 times more than the least expensive. Similarly, between 1250 and 1350, on average a cheap riding horse would cost up to 24 times as much as a workhorse, a fine riding horse up to 400 times as much as a workhorse, and a warhorse up to 800 times as much as a workhorse.

England in the 16th and 17th centuries is a fantastic case study in how an organized breeding culture and a widespread demand for horses can create a market for both high and low end animals. Henry VIII’s 16th Century campaigns in Scotland and France had exhausted the supply of horses first at home and then abroad, as the English military vacuumed up nearly 10,000 from the Netherlands in 1544 alone. The rising costs of importing horses forced Henry VIII to make the breeding of English horses official policy.

The stock of English horses began to recover by the turn of the 17th Century. However, not every animal bred by the gentry was fit for service. The diversification of stock intersected with the increased demand for horsepower as the population increased and standards of living rose. The gentry continued to pay top dollar for their coach and high school horses. A fine colt in the mid 1600s could cost 12 or 13 pounds. However, the price of an ordinary colt was only one, two, or three pounds. Yeomen could afford horses not just for work, but for simply riding as well. In 1590, one fifth of the households in the dairying village of Yetminster, Dorset, which was populated by smallholdings, had horses. By the 1660s, three fifths of the households in Yetminster owned horses. Since the farmers in this town did not have large acreages to plow, many of these horses must have been used exclusively as personal transportation. One could even determine the class of a traveler on the 17th Century English roads not by whether or not he had a horse, but by whether or not he was riding in a coach or riding the horse itself.

As with early modern England, the rising demand for equine labor in the United States following the Civil War forced the equine market to differentiate. Equine labor was an integral companion to industrialization, and every type and grade of horse had a niche in the American economy, from a four abreast team of chunks on a Wisconsin wheat farm to the horse at the end of its working years pulling a streetcar through New York City. In its 1880 agricultural census, the USDA recorded that the average value of a working horse was 54.75 dollars. By 1890, demand had begun to push the price of the American workhorse higher, but even at the close of the decade the average cost of a horse nationwide was only 71.89 dollars.

In 1900, the nationwide average value of a horse was 49.07 dollars. The market for horses in the United States had slumped. The Panic of 1893 coincided with major cities electrifying their horse-drawn streetcar lines, crushing the market for older, poorer quality stock. By the time the economic crisis ended in 1897, the average cost of a horse in the United States had plummeted to 31.51 dollars. Good horses, of course, still commanded very high prices. The average horse in Fayette County, Kentucky cost 377.78 dollars. Bloodstock farms that raised thoroughbred horses for the racetrack constituted a major sector of the county economy. However, poorer quality horses being produced in the western states could be had for surprisingly low cost:

The very low average for Arizona, $13.61, was due to the large numbers of Indian ponies on reservations. As a result of this fact, and owing to the inferior grade of many horses on ranges, average values were reduced in nearly all of the Western states and territories.

A 13.61 dollar horse in 1900 would only set you back 423.22 dollars today.

While reliable statistics on recent sales data can be difficult to find, one segment of the modern equine market that keeps meticulous records is the Thoroughbred industry. In 2019, the median price for a Thoroughbred yearling was 23,000 dollars. The average price, 83,317 dollars, however, reflects the eye-opening amounts of money spent at the highest echelons of the market. The most expensive yearling sold in 2019? He went for 4,854,387 dollars.

The reason then, despite the high value of many horses, why so often they are just euthanized when faced with severe injury has to do with the intricacies of equine physiology. Since you brought up broken legs, I’ll focus on that system. The horse’s leg is not delicate per se. In certain phases of the gallop, the force on a single hoof can reach up to 2.5 times the bodyweight of the horse. However, when a horse’s legs do fail, they often fail catastrophically.

Let’s take a look at a diagram of the horse’s lower limb. There is, as you can see, a whole lot going on in a horse’s leg. And since there’s a whole lot going on, a whole lot has the potential to go wrong. The length, shape, and exposure of the horse’s lower limb make it comparatively prone to severe injury. Horses support the majority of their weight on their front legs, and equine bones are relatively light in comparison to the total weight of the animal, so fractures are often worse than in other animals. Further, because all of the vital structures of the leg are packed into a fairly tight space, fractures frequently break skin, tear connective tissues, and disrupt blood supply and nerves. Simply put, the likelihood that a fracture presents as a very complex injury is quite high. Regardless of why a horse suffers a catastrophic breakdown, they are all faced with the same hurdles to recovery.

As humans, we tend to want to anthropomorphize horses. A broken foot or leg to a person isn’t often life threatening anymore. However, the biology of the horse is radically different from the biology of a person. Horses are prey animals whose primary form of defense is flight. They spend very little of their life laying down. The only time a horse is really forced to lay down is to enter REM sleep. Thus, their physiology is not optimized for resting off of their hooves, and thus you can’t make a horse lay down for the length of time it takes to heal from a leg injury. Attempting to do so will cause sores, colic, and pneumonia. Since a horse has to stand even while healing, they often suffer from a complication called laminitis, which is inflammation of soft tissue structures within the hoof. Laminitis can have many causes, but in the instance of the horse suffering from a broken leg, it’s caused by the animal having to support its body weight on three legs instead of four. This decreases blood flow to those limbs, particularly the one opposite to the injured limb, which in turn decreases oxygen and nutrient delivery to the area. In cases of trauma, laminitis is only exacerbated by stall rest. Further, horses have evolved to constantly move. One study found that feral horses in Australia traveled on average 10 miles a day. Keeping a horse constantly stalled is quite detrimental to its health, increasing the risk of colic, gastric ulcers, and stress-induced behaviors.

In short, it’s not the value of a horse that would lead someone to euthanize it should it be injured, it’s the question of can the injury be treated -- which is not a given due to the complexity of the horse’s lower leg -- and if the injury can be treated, will the horse recover and still have a quality life? The textbook case of the calculus behind this decision has to be the racehorse Barbaro, who shattered his right hind leg in the 2006 Preakness Stakes. Despite radical surgery to reconstruct his leg by veterinarians at the University of Pennsylvania’s New Bolton Center, and owners willing to spend an unlimited amount of money on their horse’s care, Barbaro developed laminitis in his three other legs, and in 2007 his connections opted to euthanize the colt rather than let him continue to suffer.