Horse Care, Sept/Oct 1989
Reprinted with permission of author

By Mark Secor, D.V.M.

In an effort to produce large, attractive yearlings for showing or sales, some horsemen feed excessive amounts of grain which frequently results in DOD.

Part II: The cause and effect of DOD

DEVELOPMENTAL orthopedic disease (DOD), is a term currently used to encompass skeletal diseases in growing horses such as epiphysitis, contracted tendons, osteo-chondrosis, and wobblers syndrome. The underlying cause of these diseases is defective cartilage-to-bone conversion at the growth plates/centers of bones in the young horse (for more information see Part I, HORSE CARE July/August '89). Research has focused on genetic, hormonal, and nutritional causes for DOD, and most researchers believe that each plays a role in causing the disease.

Genetics appear to be an important contributing factor in DOD, but while various studies have linked the two, just how genetics influence the disease is not completely understood. In one study at Washington State University, wobbler mares were bred to a wobbler stallion. Nearly half of the foals produced by these matings showed evidence of epiphysitis or osteochondrosis while only one wobbler foal was produced (this study initially linked wobblers disease to DOD). Other studies have shown that foals by certain stallions have a significantly higher incidence of osteochondrosis than foals sired by others.

Many veterinarians believe that the fundamental problem may lie in selecting horses which are genetically programmed for rapid growth and maturation. Many of our modern horse breeds such as the Thoroughbred, Quarter Horse, and Warmblood fit this description. It is felt that in horses with rapid growth potentials, the growth plate/center proceeds at such an accelerated rate that cartilage cells cannot mature correctly and their conversion to subchondral bone is impaired or defective. This results in the various defects seen in DOD (for more information, refer to Part I). This belief is backed by the observation that fast-growing breeds experience a higher incidence of the disease than slow-growing breeds such as ponies.

Because of the possible heritability of DOD, some horse registries (the Danish Warmblood registry, for example) will not register stallions with osteochondrosis for breeding purposes. In my opinion, the incidence of DOD could be reduced if horse registries would only permit registration of stallions and mares which are free from DOD. (Dog breeders have reduced the incidence of hip dysplasia, which is similar in many respects to DOD, by only breeding dogs with no signs of this defect.) Even without such regulations, potential horse breeders should avoid breeding stallions or mares with signs of DOD or a history of producing offspring with the disease. This practice should help to reduce the role that genetics may play.

Nutritional factors contribute significantly to the incidence of DOD and, more importantly, it is an area over which horsemen have direct control. Many nutrients are necessary to the proper growth and development of the horse. The most important, as they relate to DOD, are calcium, phosphorus, copper, zinc, and carbohydrates.

There has been much research to determine the effects of various nutritional components on the occurrence of DOD, but a study by the veterinary school at Ohio State University (OSU) in 1985 and '86 appears to be the most conclusive. In 1985, 384 yearlings from 19 farms in Ohio and Kentucky were examined for evidence of DOD and given a score. Yearlings with the most evidence of DOD received the highest scores while yearlings showing little or no evidence of DOD received the lowest scores.

At the same time, feeds and feeding programs were analyzed and scored. Feeding programs which deviated the most from OSU's feeding recommendations received the lowest scores while programs which were closest to the school's recommendations received the highest scores. Results of the study showed that farms with high scores for their feeding programs (closest to optimum) also had much lower scores for evidence of DOD in their yearlings. More importantly, the study showed that the farms which fed high, levels of calcium, phosphorus, zinc, and especially copper, had the lowest scores for DOD.

In 1986, the study was repeated at the same farms. This time, however, each farm had adjusted their feeding programs in accordance with OSU's recommendations. The results from this study showed that the farms which had received the highest scores the previous year for yearling DOD had reduced these scores dramatically This was especially true for those farms which significantly increased the copper levels in their feed.

From my point of view, this study stressed the importance of feeding adequate levels of calcium, phosphorus, and particularly copper. As explained in Part I, calcium is necessary for calcification of cartilage matrix while both calcium and phosphorus are needed for the formation of subchondral bone. Copper appears to be critical in the cross-linkage of collagen fibers used as the core for subchondral bone spicules. Inadequate quantities of these minerals result in retarded cartilage-to-bone conversion and structurally inferior subchondral bone.

There are many feeding programs for growing horses, yet many contain improper amounts of calcium, phosphorus, and copper. In formulating a feeding program, the objectives should be to provide sufficient levels of protein and energy for the growth of the horse, and yet provide the correct amounts of the essential minerals for normal cartilage-to-bone conversion so that joints and bone are formed correctly. A commercial farm may benefit from having their feeds analyzed (Holmes Lab, 3559 U.S. 62, Star Route, Millersburg, Ohio 44654, 216-893-2933, is one I'm familiar with) and a feeding program formulated by a nutritionist or veterinarian. As this is not very practical for most horsemen who raise small numbers of foals and buy feed in relatively small quantities, it is more practical to provide guidelines for feeding.

The first consideration in feeding young horses is to provide adequate amounts of protein and energy for growth. A good source of protein can be found in quality alfalfa hay, which contains from 15 to 20-percent protein. Most nutritionists recommend feeding a diet of 15 to 16-percent crude protein to foals, 13 to 14-percent crude protein to weanlings, and 10 to 12-percent crude protein to yearlings. These requirements are easily met with alfalfa, however, while alfalfa is a good source of protein, it contains an excessive amount of calcium and an insufficient amount of phosphorus and copper. Additionally, the energy content of alfalfa is too low for its bulk (volume) to meet the requirements for growth, and it is not readily available in all parts of the country.

If you are feeding a grass or oat hay, you don't have to worry about excess calcium, but instead will have to supplement with bone meal, dicalcium phosphate, or some other type of calcium supplement. Also, due to the lower protein content of grass hays a protein supplement will be necessary. I advise consulting a nutritionist or veterinarian when making these adjustments to be sure that your feeding program will adequately meet the young horse's needs.

As previously mentioned, calcium is necessary for proper bone and joint development, yet excessive amounts of the mineral have been shown to interfere with the absorption of copper, zinc, and phosphorus. Much of the alfalfa available is also very low in copper. OSU has recommended feeding levels of 25 to 30 parts per million (ppm), while most hay contains only five to 10 ppm. Also, most alfalfa hay only provides 30 to 50 percent of the phosphorus needed for proper growth.

Finally, growing horses have a high energy requirement. You would have to feed approximately 15 pounds of alfalfa to a weanling to meet his energy requirements, and considering the space this quantity of hay would occupy it would be nearly impossible for a young horse to eat enough to fulfill his energy needs. So, while alfalfa hay may have the necessary protein for a growing horse and provides good roughage, it will cause mineral imbalances and an energy deficiency if it is fed alone. To offset these imbalances, fortified grain mixes or pelleted supplements should be fed with alfalfa.

Selection of the grain portion of the ration should focus on supplying additional copper, phosphorus, and energy to compensate for the insufficient amounts in alfalfa hay. The most convenient way to fulfill these requirements is with the use of pelleted supplements specifically formulated for growing horses. These formulas are made with a grain base which has been fortified with high concentrations of minerals, especially copper. As an example, I checked with a local feed mill in my area (O.H. Kruse Grain & Milling, El Monte, California) about such pelleted supplements. Their nutritionist informed me that their Foal & Weanling Pellet is fortified with copper at 50 ppm, and contains 0.75-percent phosphorus. He recommended feeding the pellet at 0.5 to 1.0 pound of pellets per 100 pounds of body weight. This is a reasonable amount and will meet the young horse's mineral and energy requirements when fed with alfalfa hay.

By the same token, there are national feed brands that can be used for the same purpose. OSU's recommended feeding schedule for foals, weanlings, and yearlings is shown in Tables 1 and 2. In Table 1, the supplement used is either Tizwhiz 30-Plus, Manna Pro Spur or Alfa Spur, or Buckeye Gro' N Win. The supplements are combined with either 14 or 16-percent protein grain mixes, such as Omelene 200 or Tizwhiz Sweet. By combining alfalfa hay with these pelleted supplements and fortified grain products, a balanced and nutritious diet for a growing horse can be fed.

Table 1
Foals, Weanlings, Yearlings
Age (Months)	Supplement (Pounds)	Grain (Pounds)
		14% protein	16% protein
0-1	0	0	0
1-2	1/4	0	0
2-3	1/4 - 1/2	1/2	1/2
3-4	1/2 - 1	1/2	1/2
4-5	1	1	1/2 - 1
5-6	1 1/2	2	1-1 1/2
6-9	11/2	2	1 1/2
	1	4	3
	1/2	5-6	4-5
12-18	0	7+	6+

Table 2
Dietary Proportion of Hay and grain
	Percentage Grain	Percentage Hay (Alfalfa)
Foal	75	25
Weanling	65	35
Yearling	50	50
Mature	0-20	80-100

Besides adjusting rations for growing horses to prevent the occurrence of DOD providing nutrition to the foal before birth may also be important. During gestation, and from birth until he can begin eating substantial amounts of feed, the foal is essentially dependent upon the mate to meet all of his nutritional needs. This is also the period of very rapid bone and joint development. During this period it is believed that the mare will provide all of the foal's nutritional needs, except perhaps that of copper. Mare's milk is especially low in this mineral, independent of the amount of copper that is in her feed.

Before the foal begins eating solid food, he receives a small amount of copper from his dam's milk and the rest from stores in his liver. Copper appears to be concentrated in the foal's liver during late gestation, therefore supplementing the pregnant mate with feeds fortified with copper should help boost the concentration of copper in the unborn foal's liver. Various feed formulations are available which will accomplish this task (see "A Balanced Diet:" HORSE CARE May/June '89). OSU's recommendations for broodmares are shown in Table 3.

Finally, the practice of overfeeding young horses also increases the incidence of DOD--particularly epiphysitis and contracted tendons. In an effort to produce large, attractive yearlings for the show ring or sales, some horsemen feed excessive amounts of carbohydrates (grain). While this practice does produce nice hair coats and good body condition, it frequently also results in DOD Traditionally, the first recommendation for treating affected young horses was to cut back (often drastically) the amount of feed (especially the grain portion). Reducing the ration slows the growth rate in the horse and theoretically allows cartilage-to-bone conversion to proceed at a more physiological rate.

A recent study of young horses fed large amounts of grain, but adequate calcium, phosphorus, and copper, showed that cartilage-to-bone conversion in the metaphyseal growth plates was abnormal. Perhaps more importantly, the study showed that levels of the hormones insulin and thyroxine, which are essential in directing normal cartilage growth, maturation, and conversion to bone were desynchronized. That is, feeding horses increased grain rations resulted in thyroxine and insulin levels which did not rise and fall synchronously as they did in the control horses.

Researchers believe that the uncoupling of these hormones may be responsible for abnormal bone and joint development and DOD. Evidence from this study strongly suggests that feeding large amounts of grain to produce a larger, (more attractive?) young horse is detrimental in the long run.

In summary, the cause of DOD appears to be multifaceted. Nutrition, genetics, and hormones each contribute to the disease. Providing proper nutrition, especially in the form of minerals along with the judicious use of grain, is probably the most effective means of reducing DOD. However, the heritability of the disease should not be ignored, and the breeding of affected individuals, or horses that produce affected offspring, should be avoided.