Joe Pagan, PhD, founder and president of Kentucky Equine Research, leads us through a study on developmental orthopedic disease (DOD) in foals on a specific Thoroughbred farm in Kentucky. Pagan concluded that while minerals are certainly important for skeletal development, mineral deficiencies or imbalances are not the only causes of DOD.
Over a four-year period, scientists at Kentucky Equine Research studied the incidence of developmental orthopedic disease (DOD) on a single large commercial Thoroughbred farm in Kentucky. For the purpose of this study, developmental orthopedic disease was defined as osteochondrotic lesions occurring in the fetlock, hock, shoulder or stifle.
During this period, the farm produced a total of 271 foals. Foals were weighed monthly on a portable electronic scale. Lesions were initially diagnosed radiographically after a foal displayed either lameness or joint effusion. Often this diagnosis was confirmed by arthroscopy. Other manifestations of DOD such as physitis, acquired contracted flexor tendons, and angular limb deformities were not included in this study since they were more difficult to quantify. No foals were diagnosed as wobblers during this four-year period.
This farm used a single grain mix for broodmares, weanlings and yearlings. This grain mix consisted of oats, molasses and a protein/vitamin/mineral supplement pellet. The grain mix contained 15.5% protein, 1.0% Ca, 0.81% P, 144 ppm Zn, 59 ppm Cu and 0.5 ppm Se. Broodmares were fed 6.6 to 11 lb (3 to 5 kg) of the grain mix during pregnancy and 8.8 to 12 lb (4 to 6 kg) during lactation. Foals were first offered grain starting at 90 to 120 days of age. From 90 days until they were weaned at five months of age, the foals received grain at a level of intake equal to 1 lb (0.5 kg) per month of age. At weaning, each foal’s grain intake was increased to 6.6 lb (3 kg) per day. Grain intake for the weanlings and yearlings was then adjusted based on growth rate and body weight, but averaged between 6.6 and 11 lb (3 and 5 kg) per day until the yearlings were sold or sent away for training. In certain instances, a weanling’s or yearling’s grain intake might fall below 6.6 lb (3 kg) per day. When this happened, the protein/vitamin/mineral pellet was supplemented at a level equal to 0.73 lb (0.33 kg) of pellet for each 2.2 lb (1 kg) of grain intake below 6.6 lb (3 kg).
Mares with foals born before April 15 were kept in box stalls from 3:00 p.m. until 8:00 a.m. the following morning. During the day, the mares and foals were housed in small grass paddocks. After April 15, mares with foals older than four weeks of age stayed out during the night in large grass paddocks and were kept in box stalls between 8:00 a.m. and 3:00 p.m. The mares were fed grain in two meals while in their box stalls. The paddocks housing the mares and foals were well-drained and level, but the ground could become quite hard during dry weather or when frozen. During the winter months, the mares and foals received a grass/legume hay free choice both outside and while in their stalls. During the spring, summer and fall months, the majority of forage intake for all horses came from pasture.
The frequency of DOD was figured as a function of sex, month and year of birth, age, and lesion location. A total of 30% of the foals in this study were born in January or February, and 30% were born in March. The study found 23% of the foals were born in April and 17% of the foals were born in May or June.
DOD was divided into four categories based on the location of the lesion and the age at diagnosis. Lesions in the fetlock were divided into early lesions (102 ± 48 days) and late lesions (379 ± 140 days), while lesions of the stifle and shoulder were grouped together. Hock lesions were grouped separately.
A total of 10% of the foals in this study suffered one of these types of lesions. The incidence of total lesions was evenly distributed between January-February, March, and May-June foals. April foals had a lower incidence of total lesions (3.2%) and all of these lesions were confined to the hock. Fetlock lesions only occurred in January-February and March foals. Early fetlock lesions occurred in more than one joint 83.3% of the time. Stifle, shoulder, and hock lesions tended to occur most often in a single joint. Foals with early fetlock lesions tended to be of average size and their growth rates were similar to the average of a large population of Thoroughbred foals raised in Kentucky. Foals that were diagnosed with fetlock lesions at a later age tended to be of normal size during the first 120 days, but grew heavier than the Kentucky average after weaning. Those foals that developed hock OCDs averaged 11 lb (5 kg) heavier than the Kentucky average at 25 days of age. By 240 days, these foals were 30 lb (14 kg) heavier than the population average. Foals that developed stifle or shoulder lesions averaged 12 lb (5.5 kg) heavier than the Kentucky average at 25 days of age and 37.4 lb (17 kg) heavier at 120 days of age. By 300 days of age, these foals were 26 lb (12 kg) heavier than the Kentucky average.
The nutrition program on this farm was carefully monitored and both pregnant mares and growing foals received adequate quantities of both macro- and micro-minerals throughout the year. In spite of this, this farm still experienced a 10% incidence of DOD. Therefore, other factors besides mineral intake must have played a role in the development of skeletal lesions in these foals.
Overloading of the immature skeleton may have been a factor in the foals that developed early fetlock lesions. All of the foals that developed early fetlock lesions were born in January, February and March. These foals were all housed indoors at night until mid-April. Perhaps these foals did not develop adequate subchondral bone to withstand the forces placed on it once they were allowed greater amounts of exercise as the weather improved.
The etiology of developmental orthopedic disease is almost certainly multifactorial. A primary focus of much of the research concerning DOD has been on mineral intake and balance. While minerals are certainly important for skeletal development, mineral deficiencies or imbalances are not the only causes of DOD. In the present study where mineral intake was carefully monitored, 10% of the foals still developed DOD.
A major factor to consider in the etiology of DOD is whether the lesion develops because of a failure in proper cartilage formation and maturation or whether the lesion develops in normal cartilage. The types of lesions seen in the present study may have been the result of excessive biomechanical forces exerted on otherwise normal cartilage. Early fetlock OCDs may have resulted from inadequate subchondral bone formation due to restricted activity in foals born early in the year and housed indoors at night. Hock and stifle lesions may have occurred in heavy foals that grew rapidly after weaning.
This study raises several intriguing questions about which factors are involved in the development of orthopedic disease under commercial management conditions. Controlled research is necessary to determine if management of growth rate and exercise can reduce the incidence of developmental orthopedic disease in horses.