Whether you're raising a pedigreed foal with promise of showring glory or you're bringing up a homebred baby destined for recreational trail riding, developmental orthopedic disease (DOD) can put your dreams in jeopardy. A collection of bone and joint abnormalities that arise in young, rapidly growing horses, DOD can lead to lifelong soundness issues, ranging from mild to crippling.
Therefore, it's no wonder that DOD has been a topic of ongoing research for decades, with breeders closely following any resulting recommendations for preventing the condition. Trouble was, early studies were far from conclusive or even consistent, and the subsequent advice given to horse owners based on these studies has been contradictory or even outright wrong. To further complicate matters, recent advances in our understanding of factors contributing to the problem have not yet reached many veterinarians, who continue to give the outdated advice they were taught years ago.
A case in point is the protein mantra that has dominated discussions of DOD since the 1970s. In the 15 years I've spent investigating dietary causes of DOD, I, along with other researchers, have found that the evidence points to an interplay of the young horse's intake of minerals and metabolic responses to carbohydrates in the ration.
We don't have all the answers yet--exactly how these metabolic processes affect bone growth and mineralization is still not clear--but I can say with confidence that if you want to prevent DOD in a young horse, stop obsessing about restricting protein (which can actually be detrimental!) and pay attention to the carbohydrates and minerals.
In utero, equine bone begins as cartilage. In the three or four months prior to birth, the rubbery cartilage starts to be converted into hard bone through a process called ossification, in which the cartilage cells are mineralized with calcium, phosphorus and magnesium compounds. The trace minerals copper and zinc also play a key role in this process. By the time a foal is born, 80 percent of his bone is ossified.
Thereafter, cartilage ossification remains critical to normal bone growth. Bones become longer by increasing the number of cartilage cells at either end. These new cartilage cells are laid down in an organized, ladderlike formation, surrounded by supporting material, referred to as the matrix, at the edge of the growth plates located near the ends of the long bones of the legs. At the same time, the oldest cartilage cells, farthest from the bone shaft, become ossified as the capillaries within the matrix carry in the minerals and proteins needed to form new bone. Through this process, a newborn youngster's legs can grow as much as an inch a week.
DOD occurs when the formation of cartilage is overstimulated or the conversion of cartilage to bone is somehow disrupted or inhibited, whether by overly rapid growth or inadequate nutrient intake. When a youngster cannot produce uniform new, strong bone, the result can be orthopedic disease.
In some instances, the cartilage layer becomes too thick for nutrients to diffuse from the blood vessels to the cells at the outer surface. The cartilage then dies, leaving an empty, weak "lesion" in the bone, a condition called osteochondrosis or osteochondritis dissecans (OCD). In other cases, the areas of actively growing cartilage at either end of the bone shaft become too wide to support the weight borne by the bone; mechanical distortion can result as the area is literally crushed under the growing horse's weight, resulting in visible "swellings" at the growth plates, a situation referred to as epiphysitis or physitis.
Epiphysitis is often accompanied by pain and flexure deformities, formerly known as "contracted tendons." Any sort of deviation in the distribution of weight on the growing surfaces can result in uneven growth, which causes angular deformities. All of these problems fall under the collective name of developmental orthopedic disease.
Whatever the specific form of DOD, the first clinical signs are typically mild lameness associated with a visible deviation in the angles of the lower limb and/or epiphysitis. Usually both front legs or all four legs are affected and problems may be seen in one or more areas, though the fetlocks, knees and hocks are the most common sites.
When DOD is recognized and prompt action is taken to address the underlying issues, the condition can be halted and normal bone development can resume--but the areas of incomplete ossification can remain, along with any resulting OCD lesions. A foal with DOD may not be obviously lame, or--as frequently happens with OCD--lameness may emerge only when training begins. Some DODs may never cause lameness: How debilitating a problem will be seems to depend on the severity, location and, to some degree, the individual.
In the search for the causes of DOD, researchers have looked at nearly every facet of the management of young horses. It has become clear that many variables are involved in the development of the disorder, ranging from genetics to joint trauma from forced exercise. Mineral imbalances are a well-documented cause of DOD, which is not surprising given that minerals provide the building materials for new bone. Specifically, deficiencies or excesses of copper, zinc, calcium and phosphorus have all been shown to result in abnormal bone maturation. When considering other dietary factors, however, old beliefs are being replaced with findings from new research.
Back in the 1970s, the idea was brought forward that feeding excess protein--more than 16 percent of the total diet--led to the development of DOD. And although the studies were very limited, the notion caught on that protein was the culprit. It seemed like everyone became obsessed with how much protein to feed--or not feed--growing horses. Several well-controlled studies published in the 1980s disproved the protein link, but it remained a key consideration in the minds of many veterinarians and horse owners.
Then, in the early 1990s, I was conducting a series of studies comparing the effect of high and low grain diets on growth and DOD in Standardbred weanlings. In addition to taking radiographs to detect the presence or absence of OCD lesions, we were looking at a number of blood parameters, including glucose/insulin responses to meals of the sweet feed being used.
I'd been looking at glucose/insulin metabolism in horses since my PhD thesis work in the '80s on the control of appetite in horses and have been fascinated by the factors that influence the insulin response to glucose challenges. Insulin is released by the pancreas in response to glucose in the blood and facilitates the movement of glucose into the cells. However, it also affects other hormonal systems, including the release of growth hormone and thyroid hormones. As a result, I usually run glucose/insulin trials in most of the studies I conduct, whether or not it is the primary focus of the research.
When I looked at the insulin results of the weanling Standardbred study, I saw some numbers that seemed to be hugely out of the normal range. When fed grain, certain horses produced dramatically higher levels of insulin than did others receiving the same ration under the same conditions. On any predominantly hay diet, however, their responses were normal. Initially, I thought my laboratory assistant, Anthony Calo, (who is now himself a veterinarian) was making an error in his assays.
Tony, however, knew his techniques were good and began looking for another potential cause of the abnormally high values--searching for any similarities in the horses with the high insulin responses. It was Tony who made what would turn out to be a critical observation: Every horse that had the abnormal insulin response to the grain diet also had had radiographic evidence of OCD lesions in the hocks.
A Eureka Moment
A lightbulb went off in my head. In the 1980s Michael Glade, PhD, advocated the idea that the high glucose/insulin responses associated with high grain intake were a predisposing factor to OCD, and many others also had noted the association with high grain intake. In addition, a genetic predisposition to OCD had been documented in Standardbreds. Both I and a colleague in Europe, Luka Krusic, PhD, had documented that weanlings between 3 and 12 months of age tended to produce more insulin in response to a standardized glucose challenge than they did when younger or older.
Significantly, other researchers had conclusively documented that OCD lesions are formed almost exclusively in the first year of life. No new lesions were found after horses reached the age of 12 to 16 months, though many were not diagnosed until much later when the clinical signs of lameness during work began to occur. Could there be a link between diets high in glucose and soluble carbohydrates, the resulting high insulin levels and the disruption of the normal mineralization of cartilage in rapidly growing young horses?
In subsequent studies we again documented that weanling Standardbreds with radiographic evidence of OCD had higher insulin responses to standardized glucose or carbohydrate (grain) challenges than did unaffected weanlings in the same studies. Meanwhile, laboratory studies done in England confirmed that high insulin did indeed affect growth hormone and mineralization of cartilage cells in culture. Later, an extensive survey of Kentucky Thoroughbred farms conducted by equine nutritionist Joe Pagan, PhD, showed that facilities that fed high-glycemic-index grain concentrates (the most common being sweet feed) in amounts that resulted in high body weights had a higher incidence of severe OCD lesions than did farms feeding lower amounts of carbohydrates to their young stock.
All of this evidence strongly links at least OCD to abnormally elevated insulin levels in response to diets high in glucose and soluble carbohydrates or metabolic disorders in individuals that make them less sensitive to the normal actions of insulin on glucose ("insulin resistance"), resulting in abnormally high insulin responses to even "normal" meals of concentrates.
In retrospect, I can understand how protein was implicated years ago. In forages and many of the early mixed grain feeds, the percent protein was highly correlated with soluble carbohydrate and starch content. The researchers were focusing on protein, but the effect they were seeing was probably linked instead to the soluble carbohydrates and starch in the ration.
That said, many healthy horses with good legs have been raised on sweet feed or other high-carbohydrate rations. The problem apparently lies in feeding excessive amounts of such feeds. In study after study, exceeding recommended energy intakes with rations that were more than 50 percent grain-based, low-fiber, high-starch feeds consistently caused an increase in the incidence of DOD, especially if the mineral intakes were not increased at the same rate as the energy. Many pelleted and extruded feeds cause lower glucose and insulin responses than do sweet feeds, but when fed in large enough quantities, and if not supplemented with the minerals critical for growth, can also cause problems.
What does all this mean to owners looking to prevent DOD in their growing horses? It means you may have to feed carefully and even conservatively. Offer only as much concentrate, formulated specifically for the young horse, as necessary to maintain good body condition: not too fat or too thin, with ribs that are easily felt or barely visible, and a coat that is shiny and soft.
Start by introducing nursing foals to small amounts of concentrates when they are 1 to 2 months of age. The best choice is pelleted and extruded feeds formulated specifically for growing horses with 14 to 16 percent protein and added calcium, phosphorus, copper and zinc. Most feed manufacturers make a growth diet, and some even make two or three with slightly different formulations. The reputable companies, large or small, have formulations that are based on the most recent research. No additional supplementation should be necessary if these products are fed as recommended.
If you have a foal from bloodlines known to have a high incidence of OCD, I'd avoid sweet feeds entirely, focusing on finding a feed formulated for growth that has little to no molasses added and a fiber content of 10 percent or higher. High-fat (7 to 10 percent) feeds may be suitable for hot-blooded breeds like Thoroughbreds, but warmblood or draft breeds definitely do not need the extra calories to maintain optimal growth. We are still working on a test to identify individuals who are genetically at increased risk of DOD and are developing feeding programs especially for them.
Start offering whatever feed you choose at a rate of 0.25 percent of the foal's body weight, cautiously advancing up to 0.75 to 1 percent of his weight and dividing the ration into two or three meals a day. Also, be sure to make good-quality hay, grazing or forage available free choice. Although the soluble carbohydrate content of some hays and pasture grasses can be 22 percent or higher under certain conditions (after a drought or frost, during rapid growth after a dry spell), under most circumstances it is advisable to allow a young horse unlimited access to forage.
Do not allow your foal to become obese. If he develops an obvious crease down his back or you can't easily feel his ribs, he is getting more energy from his diet than he needs. Monitor the condition of young horses daily and separate the thin youngsters from the fatter ones, customizing their feeding programs, if necessary. And, if it's working well, maintain the same feeding program after weaning as before, again keeping a close eye on the body condition of the youngster--the idea is to promote steady growth while avoiding sudden increases or pauses.
Usually, however, as the weanling gets older, the growth rate slows somewhat: I usually try to avoid feeding more than eight pounds of concentrate to a young horse per day, regardless of body weight. You can maintain a young horse on the same diet until he is at least 18 months old. At that point, you may reduce the amounts of the growth formula fed or switch to a feed that provides less protein and fewer minerals.
If signs of DOD appear, temporarily reduce the amount of concentrates fed while reassessing the total ration's nutritional content. Many people will starve a foal that develops any form of DOD because it is associated with rapid growth. However, that's like turning off your car when the engine makes a funny noise. The noise stops but you haven't fixed the problem and it will still be there when you turn the car back on. Instead, cut the rations in half or more while a trained nutritionist helps you identify any problems with your youngster's nutritional intake; then introduce the resultant balanced ration gradually over the course of four to six days.
Helping a horse grow from a wobbly legged newborn to a strong, athletic adult is a most challenging and rewarding experience. No small part of the challenge is a continuing series of decisions regarding nutrition, each of which needs to be made with the aim of supporting optimal growth without providing so much of a good thing that biological processes run amuck. And when it comes to feeding to prevent DOD in young horses, the best decisions are made in light of the most current findings regarding soluble carbohydrates, glucose and insulin.
This article originally appeared in the February 2005 issue of EQUUS magazine.