Mineral Deficiencies

A deficiency of minerals, including calcium, phosphorus, copper, and zinc, may lead to developmental orthopedic disease. Most commonly fed cereal grains and forages contain insufficient quantities of several minerals. A ration of grass hay and oats supplies only 40% and 70% of a weanling’s calcium and phosphorus requirements, respectively, and less than 40% of its requirements for copper and zinc (Table 55-1). The best method of diagnosing mineral deficiencies is through ration evaluation. Blood, hair, and hoof analysis is of limited usefulness.

TABLE 55-1 Mineral Requirements for Weanlings

Mineral Excesses

Horses can tolerate fairly high levels of mineral intake, but excesses of calcium, phosphorus, zinc, iodine, fluoride, and heavy metals, such as lead and cadmium, may lead to developmental orthopedic disease (Table 55-2^1,2).

TABLE 55-2 Toxic Mineral Levels*

* Adapted from Cunha TJ: Horse feeding and nutrition, ed 2, Orlando, 2007, Academic Press¹; and National Research Council: Nutrient requirements of horses, ed 6, Washington, DC, 2007, National Academies Press.²

Mineral excesses occur because of overfortification or environmental contamination. Massive oversupplementation of calcium (>300% of required amount) may lead to a secondary mineral deficiency by interfering with the absorption of other minerals such as phosphorus, zinc, and iodine. Excessive calcium intake may be compounded by using legume hays as the primary forage source. Iodine and selenium oversupplementation occurs if supplements are fed at inappropriate levels. A ration evaluation is the best way to identify this type of mineral imbalance.

Environmental contamination is a more likely cause of developmental orthopedic disease, because contamination may result in extremely high intakes of potentially toxic minerals. If a farm is experiencing an unusually high incidence of developmental orthopedic disease or if the location and severity of skeletal lesions are abnormal, environmental contamination should be investigated. Blood, feed, and water analysis should be performed. Chemical analysis of hoof and hair samples may reveal valuable information. Farms that are located near factories or smelters are most at risk, although osteochondrosis caused by zinc-induced copper deficiency has been reported on farms using fence paint containing zinc or galvanized water pipes.

Mineral Imbalances

The ratio of minerals may be as important as the actual amount of individual minerals in the ration. High levels of phosphorus inhibit the absorption of calcium and lead to a deficiency, even if the amount of calcium present is normally adequate. The calcium/phosphorus ratio in the rations of young horses should never be below 1 : 1 and ideally should be 1.5 : 1. Too much calcium may affect phosphorus status, particularly if the level of phosphorus is marginal. Calcium/phosphorus ratios greater than 2.5 : 1 should be avoided if possible. Forage diets with high calcium levels should be supplemented with phosphorus. The zinc/copper ratio should be 3 : 1 to 4 : 1.

Dietary Energy Excesses

In the Thoroughbred (TB) industry, large, well-grown yearlings are desirable when offered for sale at public auction because selling price is influenced by body size. Yearlings that sold higher than the median of the session in which they were sold were heavier and taller than yearlings that sold below the session median.³ In addition, TBs that were heavy and tall as yearlings had the most earnings, graded stakes wins, and grade-1 stakes wins.⁴ Because of the premium price paid for mass, young TBs are often grown rapidly to achieve maximal size. Excessive energy intake can lead to rapid growth and increased body fat, which may predispose young horses to developmental orthopedic disease. A Kentucky study showed that growth rate and body size might increase the incidence of certain types of developmental orthopedic diseases in TB foals.⁵ Yearlings that showed osteochondrosis of the hock and stifle were large at birth, grew rapidly from 3 to 8 months of age, and were heavier than the average population as weanlings.

The source of energy for young horses also may be important, because hyperglycemia and hyperinsulinemia have been implicated in the pathogenesis of osteochondrosis.^6,7 Foals that experience an exaggerated and sustained increase in circulating glucose or insulin in response to a carbohydrate (grain) meal may be predisposed to develop osteochondrosis.⁸

In a large field trial, 218 TB weanlings (average age 300 ± 40 days, average body weight 300 ± 43 kg) were studied.⁸ A glycemic response test was conducted by feeding a meal that consisted of the weanling’s normal concentrate at a level of intake equal to 1.4 g of nonstructural carbohydrate per kilogram of body mass. A single blood sample was collected 120 minutes after feeding to determine glucose and insulin levels. A high glucose and insulin response to a concentrate meal was associated with an increased incidence of osteochondrosis. More research is needed to determine whether the incidence of osteochondrosis can be reduced through feeding foals concentrates that produce low glycemic responses.

Types of Evaluations

Ration evaluations can be approached in two ways. One way is to add up what is being fed and compare it with the horse’s requirements. This may not be easy because most horse owners do not know exactly what their horses are eating. Alternatively, a new ration may be developed.

Protocol

Every nutrition evaluation should include a description of the horse, definition of nutrient requirements, determination of nutrients in feedstuffs, determination of intake of feedstuffs, calculation of nutrient intake, comparison of intake with requirements, and adjustment of the ration to correct deficiencies or excesses.

Overfeeding

One of the most common problems of feeding young horses is excessive intake that results in accelerated growth rate or fattening. Both conditions may contribute to developmental orthopedic disease. Unfortunately, there are no simple rules about how much grain is too much, because total intake of forage and grain determines energy consumption. Large intakes of grain are appropriate if the forage is sparse or of poor quality, as often is the case in tropical environments. For example, grain intakes as high as 2% to 2.5% of body weight may be necessary to sustain reasonable growth in weanlings that have access to no forage other than tropical pasture. Conversely, grain intakes higher than 1% of body weight may be considered excessive when weanlings are raised on lush temperate pasture or have access to high-quality alfalfa hay.

The surest way to document excessive intake is by weighing and using condition scoring in the growing horse. Based on a system developed by Henneke and colleagues,⁹ condition scoring measures fat deposition. Horses are scored from 1 to 9 (1 denoting extreme thinness and 9 indicating obesity). In a Kentucky study, fillies tended to have higher condition scores than colts, and the difference was greatest at 4 months of age (fillies 6.48; colts 6.0). These condition scores are considered moderate to fleshy according to the Henneke scoring system. By 12 months of age the condition scores of the colts and fillies had dropped to 5.3 and 5.4, respectively. Both sexes increased condition score slightly from 14 to 18 months.

Managing the growth in horses becomes a balance between producing a desirable individual for a particular purpose without creating skeletal problems that will reduce a horse’s subsequent athletic ability. Growing a foal too slowly results in the risk of it being too small at a particular age or never obtaining optimal mature body size. Therefore it is widely recommended to maintain a steady growth rate by regularly weighing and measuring horses during the growth period.^1,10

If growth rate cannot be measured, excessive intake can often be assessed by ration evaluation. For example, a 6-month-old TB weanling (250 kg body mass; 500 kg mature body mass) was being fed 4 kg of a 16% protein sweet feed and 2 kg of alfalfa hay per day, with access to high-quality fall Kentucky pasture. To support a reasonable rate of growth (0.80 kg/day), this weanling required about 17 Mcal of digestible energy per day. The hay and grain intake of this foal alone would supply about 17.5 Mcal of digestible energy, which is slightly above the weanling’s requirement. If a reasonable level of pasture intake were included (1% body mass or 2.5 kg dry matter), this weanling would be consuming 135% of its digestible energy requirement, a level likely to cause problems.

To reduce intake, the alfalfa hay should be eliminated, if the pasture is indeed adequate. If hay were needed when the weanling was stalled, grass hay would be more appropriate. Grain intake should be reduced to a level of about 3 kg/day. At this level of grain intake the weanling would need to consume about 3.3 kg of pasture dry matter to support a growth rate of 0.80 kg/day, and the ration would be nicely balanced.

Inappropriate Grain for Forage Provided

Occasionally the concentrate offered to a growing horse is incorrectly fortified to complement the forage that is being fed. The problem occurs particularly when the forage is mostly alfalfa or clover. Most concentrates for young horses are formulated with levels of minerals and protein needed to balance grass forage.

For example, a 12-month-old yearling (315 kg body mass; 500 kg mature body mass; 0.50 kg/day average daily growth) is raised without access to pasture, and the only forage available is alfalfa hay, which is fed at a level of intake equal to 1.5% of the yearling’s body mass (4.72 kg/day). At this level of forage intake, the yearling would require only about 2.5 kg of grain per day. If a typical 14% protein sweet feed that was formulated to balance grass forage were used, the ration would be inappropriate for a number of reasons. Calcium would be 183% of the yearling’s requirement, with a calcium/phosphorus ratio of 2.9 : 1. This would not be a problem except that phosphorus and zinc are marginal in the ration. Because calcium may interfere with the absorption of these minerals, the yearling may be at risk for developmental orthopedic disease from a zinc or phosphorus deficiency. The solution is to feed a concentrate that is more appropriately balanced for legume hay. For example, a 12% protein feed with 0.4% calcium, 0.9% phosphorus, and 180 ppm zinc would be more suitable.

Inadequate Fortification in Grain

The most common reasons for inadequate fortification are using unfortified or underfortified grain mixes, using correctly fortified feeds at levels of intake that are below the manufacturer’s recommendation, or using fortified feeds diluted with straight cereal grains. These errors in feeding can be corrected by incorporating a highly fortified grain balancer supplement.

For example, a 6-month-old weanling (200 kg body mass; 400 kg mature body mass; 0.60 kg/day average daily growth) is fed 3 kg/day of a 10% protein sweet feed that is intended for adult horses. To compound matters, the weanling is also fed grass hay, with an estimated intake of 2.3 kg/day. This ration is deficient in protein, calcium, phosphorus, zinc, and copper. The foal would be prone to a rough hair coat and physitis. There are two ways to correct this problem. A properly formulated 14% to 16% protein grain mix with adequate mineral fortification could be used, or 1 kg of a grain balancer pellet could be substituted for 1 kg of the 10% sweet feed. This type of supplement is typically fortified with 25% to 30% protein, 2.5% to 3.0% calcium, 1.75% to 2.0% phosphorus, 125 to 175 ppm copper, and 375 to 475 ppm zinc. This is an extremely useful type of supplement to correct underfortified rations.

Early Pregnancy

Proper feeding of a mare during pregnancy requires an understanding of how the fetus develops during gestation. Contrary to popular belief, the fetus does not grow at a constant rate throughout the entire 11 months of pregnancy. The fetus is small during the first 5 months of pregnancy. At 7 months of pregnancy the fetus equals only about 20% of its weight at birth. At this stage in pregnancy the fetus equals less than 2% of the mare’s weight, and its nutrient requirements are minuscule compared with the mare’s own maintenance requirements (Table 55-3). Therefore the mare can be fed essentially the same as if she were not pregnant. Mare owners often greatly increase feed intake after a mare is pronounced in foal, reasoning that she is now eating for two. Increased feeding is unnecessary and may lead to obesity and foaling difficulties, especially if the mare has access to high-quality pasture.

TABLE 55-3 Expected Feed Consumption by Horses

Late Pregnancy

The fetus begins to develop rapidly after 7 months of pregnancy, and its nutrient requirements become substantially greater than the mare’s maintenance requirements; therefore adjustments should be made to the mare’s diet. Digestible energy requirements increase about 15% over early pregnancy. Protein and mineral requirements increase to a greater extent, because the fetal tissue being synthesized during this time is high in protein, calcium, and phosphorus. During the last 4 months of pregnancy the fetus and placenta retain about 77 g of protein, 7.5 g of calcium, and 4 g of phosphorus per day. Trace mineral supplementation is also important because the fetus stores iron, zinc, copper, and manganese in its liver for use during the first few months after birth, mare’s milk being low in these elements.

New Zealand researchers studied the effect of copper supplementation on the incidence of developmental orthopedic disease in TB foals.¹¹ Pregnant TB mares were divided into copper-supplemented and control groups. Live foals born to each group of mares were also divided into copper-supplemented and control groups. Copper supplementation of mares was associated with a significant reduction in the physitis scores of the foals at 150 days of age. Foals from mares that received no supplementation had a mean physitis score of 6, whereas foals from supplemented mares had a mean score of 3.7. A lower score means less physitis. Copper supplementation of the foals had no significant effect on physitis scores. A significantly lower incidence of articular cartilage lesions occurred in foals from supplemented mares. However, copper supplementation of the foals had no significant effect on physeal cartilage lesions.¹¹ Mares in late pregnancy often are overfed energy in an attempt to supply adequate protein and minerals to the developing foal. If a pregnant mare becomes fat during late pregnancy, she should be switched to a feed that is more concentrated in protein and minerals so that less can be fed per day. This will restrict her energy intake while ensuring that she receives adequate quantities of other key nutrients.

Lactation

A mare’s nutrient requirements increase substantially after foaling. During the first 3 months of lactation, mares produce milk at a rate equal to about 3% of body weight per day. This milk is rich in energy, protein, calcium, phosphorus, and vitamins. Therefore the mare should be fed enough grain to meet its greatly increased nutrient requirements. Mares in early lactation usually require 4.5 to 6.5 kg of grain per day, depending on the type and quality of forage they are consuming. This grain mix should be fortified with additional protein, minerals, and vitamins to meet the lactating mare’s needs. Trace mineral fortification is not as important for lactating mares, because milk contains low levels of these nutrients, and adding more to the lactating mare’s diet does not increase the trace mineral content of the milk. Calcium and phosphorus are the minerals that should be of primary concern during lactation. Grain intake should be increased gradually during the last few weeks of pregnancy so that the mare is consuming nearly the amount that it will require for milk production at foaling. A rapid increase in grain should be avoided at foaling because it could lead to colic or laminitis. Milk production begins to decline after about 3 months of lactation, and grain intake can be reduced to maintain the mare in desirable condition.

Sucklings

If a broodmare has been fed properly during late pregnancy, giving mineral supplements to a suckling is unnecessary until it reaches 90 days of age. At 90 days, moderate amounts of a well-fortified foal feed can be introduced and gradually increased until the suckling is consuming around 0.5 kg of feed per month of age.

Birth month has a significant effect on suckling growth. Kentucky TBs born in the winter (January and February) have been found to be smaller at birth and to grow more slowly during the first 2 months compared with spring-born foals. These winter-born foals then exhibited rapid daily weight gain at 3 months of age, coinciding with a spring pasture flush.¹² This compensatory growth resulted in there being no difference in body weights between foals born in any birth month at 5 months of age. This rapid growth spurt exhibited in winter-born foals may be undesirable owing to the risk of developmental orthopedic disease, and thus growth of the suckling must be carefully managed.

Furthermore, it is critical that the suckling be accustomed to eating grain before weaning. If the suckling does not become accustomed to eating grain, there is a good chance that its growth rate will decrease dramatically at weaning. When the weanling finally starts eating grain, a compensatory growth spurt occurs that may result in developmental orthopedic disease.

Weanlings

The most critical stage of growth for preventing developmental orthopedic disease is from weaning to 12 months of age, when the skeleton is most vulnerable to disease and when nutrient intake and balance are most important. Weanlings should grow at a moderate rate with adequate mineral supplementation. In temperate regions the contribution of pasture to the diet often is underestimated, leading to excessive growth rates and developmental orthopedic disease.

Yearlings

Once a horse has reached 12 months of age, it is much less likely to develop many forms of developmental orthopedic disease than a younger horse. Many of the lesions that become clinically relevant after this age probably developed at a younger age. Proper nutrient balance remains important for the yearling, and delaying as long as possible the increased energy intakes that are required for sales preparation is best, because the skeleton is less vulnerable to developmental orthopedic disease as a yearling ages. Normally, increasing energy intake 90 days before a sale is enough time to add the extra body condition that often is expected in a sales yearling. Physitis in the carpus is often a major concern for the sales yearling. The level of trace mineral supplementation should remain high to reduce the incidence of physitis in sales yearlings, and a substantial portion of the energy normally supplied from grain should be replaced with fat and fermentable fiber. Sales preparation grain mixes can contain as much as 10% fat. Good sources of fermentable fiber include beet pulp and soy hulls.

Physitis

Grain intake should be restricted to a level supplying around 75% of the foal’s normal energy requirement. This restriction, however, should not compromise protein and mineral intake, so a different type of feed formulation may be required. For instance, a 6-month-old weanling (250 kg body mass; 500 kg mature body mass; 0.8 kg average daily growth) on a decent fall pasture would normally consume around 3.5 kg of a 16% protein foal feed. If this foal were to develop physitis, it should be confined and fed grass hay (3 kg/day). Reducing the grain intake to 75% of the foal’s normal digestible energy would result in shortages of protein, lysine, calcium, and phosphorus. These shortfalls could be overcome by replacing 1 kg of the 16% sweet feed with a grain balancer pellet. This ration would supply 90% of the foal’s normal protein requirement, along with a good supply of minerals. As the physitis resolves, intake of the 16% grain mix can be slowly increased and the supplement pellet intake slowly decreased, until the foal returns to its normal ration.

Cervical Vertebral Malformation

A feeding program like the one described previously is also appropriate for a horse with cervical vertebral malformation, except that the degree of exercise and energy restriction may be more severe. In this case a feeding program that combines grass hay (2 kg) with a moderate amount of alfalfa hay (2 kg/day) and 1 kg of balancer pellet would result in a reduction in energy intake equal to 65% of normal intake while maintaining adequate levels of protein and mineral intake.

Osteochondrosis

Once a foal has developed osteochondrosis that is severe enough to produce clinical signs, the effect of diet is minimal in solving the existing lesion, but reducing energy intake and body weight while maintaining adequate protein and mineral intake is advised.