Chapter 2

MINERALS FOR HORSES

Although important for a variety of functions, inorganic elements or minerals constitute only a small fraction of the body weight and of the amount of nutrients needed in the diet. On a weight basis, the horse's body consists of approximately 60 to 65% water; 30 to 35% of the energy source nutrients protein, fats and carbohydrates; and 4% minerals. Most of the body's minerals are the major, or macro, minerals: calcium, phosphorus, sodium, chlorine, potassium, magnesium, and sulfur. Macrominerals are minerals for which the amount needed in the diet are best expressed as parts per hundred (percent). Trace, or microminerals, are those for which dietary requirements are best expressed as parts per million (ppm or mg/kg)-units 10,000 times smaller than those used for macrominerals. Trace minerals needed in the diet include selenium, iodine, copper, zinc, manganese, iron, and cobalt.

Macrominerals are needed for body structure, for maintaining the body's acid-base and fluid balance, and for nerve conduction, and muscle contraction. Most trace minerals are needed as components of enzymes that are involved in controlling numerous diverse biologic reactions. Iodine is a necessary constituent of thyroid hormone, iron of blood hemoglobin and muscle myoglobin, and cobalt of vitamin B12.

Trace minerals needed in the diet but for which deficiencies outside the laboratory have not been verified in animals, include: vanadium, nickel, chromium, tin, silicon, and arsenic. Whether the trace mineral fluoride is needed in the diet is unknown. The term heavy metal in science refers not to rock music but to minerals which, even when consumed in relatively small amounts, cause adverse effects. Heavy metals include: lead, nickel, mercury, arsenic, aluminum, and cadmium. However, all minerals, like all other nutrients, can have an adverse effect if sufficiently high levels are consumed, as shown in Figure 2-1.

The width of the curve in Figure 2-1 (i.e., the intakes which are deficient or toxic, marginal and optimal) varies for each nutrient and situation. The amount given as that required in the diet for most nutrients is toward the lower intake side of the range at which animal health and performance are optimal. This amount reduces feeding costs while still maintaining optimal health and performance. However, as Figure 2-1 illustrates, feeding more of a nutrient than that required does not improve health or performance, and impairs them if a sufficient excess is fed. This is an important concept often ignored by those selling or administering nutrient supplements, or trying to maximize an animal's health or performance by nutritional supplementation in the feed or by their direct administration. It is particularly important for all minerals, and for vitamins A and D, for which excesses in the diet can be quite harmful.

Dietarily, minerals should be regarded as a group rather than individually. As the intake of a mineral increases above that needed, the amount absorbed and/or excreted in the urine and/or feces also increases. An excess amount absorbed may be harmful. That not absorbed may bind other minerals, decreasing their absorption and possibly resulting in a deficiency of these minerals.

It is the balanced amount of all minerals in the diet that is important. Indiscriminately adding one or even several minerals to the diet is likely to be more harmful than beneficial. Minerals, therefore, should not be added to the diet unless it is known which ones and how much are needed. Knowledge of which minerals are needed, when, and in what amounts can be obtained from the information given for each feeding situation throughout this book; the amount present in a feed or the total diet can be determined as described in Chapter 6.

Fig. 2-1. Effects of amount of nutrient consumed on animal health and performance. Marginal intake of a nutrient impairs health and performance, cannot be detected clinically, but for some nutrients can be detected by laboratory tests. Clinical signs occur only at an intake below or above that which is deficient or toxic, respectively. The nutrient level required or toxic is near the lower or upper end of the range, respectively, at which health and performance are optimal. The intake range between requirement and toxic levels differs for different nutrients, e.g., from a fewfold for fluoride to 20-fold for selenium, to no toxic level known for some nutrients such as many of the B vitamins.

The mineral contents of feeds and mineral supplements commonly fed to horses are given in Appendix Tables 2, 6, and 7. The recommended minimum and maximum concentrations of each mineral needed by the horses are given in Appendix Tables 1, 2, and 4. These recommended amounts represent the total, not the available, mineral amounts that are either needed or toxic, based on the low or high, respectively, average availability of that mineral from feeds that commonly make up the horse's diet. For example, calcium and phosphorus requirements given assume that 50% of the calcium and 35% of the phosphorus (45% for lactating mares and growing horses because their diets are typically supplemented with phosphorus-containing minerals, which are more available than the phosphorus in feeds) consumed will be absorbed. Since absorption of the calcium and phosphorus from feeds for horses is generally near this amount or higher (50 to 80% for calcium, 30 to 55% for phosphorus, and both higher in mineral supplements), absorption does not usually need to be considered in ensuring that the horse receives adequate calcium, phosphorus, or other minerals. However, as described for each mineral, numerous factors may decrease their efficiency of absorption. Only if one or more of these factors are present does the availability of that mineral need to be considered.

The only minerals generally of concern in feeding horses are calcium, phosphorus, salt (sodium chloride), in some geographical areas selenium, and for growth copper and zinc. Other minerals are unlikely to be present in inadequate or excessive amounts in feeds commonly fed, unless mineral supplements are given, the feed is contaminated by industrial waste, or the horse ingests selenium-accumulating plants (Table 18-9). If this is not the case, these minerals are the only ones whose intake needs to be evaluated. Although other mineral deficiencies and toxicosises are rare as described in this chapter and summarized in Table 2-1, they do occur and, therefore, should be evaluated if a threatening situation is present.

CALCIUM AND PHOSPHORUS

Functions, Utilization, and Needs

Calcium and phosphorus comprise about 70% of the mineral content of the body and from 30 to 50% of the minerals in milk. About 99% of the calcium and over 80% of the phosphorus in the body are in the bones and teeth. However, both calcium and phosphorus play a critical role in numerous other body functions. Phosphorus is necessary as a buffer, for energy metabolism, and for numerous other cellular functions. Calcium is necessary for blood coagulation, cell membrane function, glandular secretion, temperature regulation, the regulation of the activity of many enzymes, and mitochondrial and neuromuscular functions. If the plasma calcium concentration is significantly increased or decreased, it has the opposite effect on muscle membrane excitability; i.e., hypercalcemia decreases muscle tone and hypocalcemia increases muscle tone (see "Hypocalcemic Tetany" in Chapter 11).

Dietary calcium and phosphorus deficiencies or excesses result in excess mobilization or deposition of these minerals in bone, causing skeletal disease. These effects, however, maintain plasma concentrations and, therefore, nonskeletal functions are maintained. Thus, skeletal diseases are the major effect of a dietary deficiency or excess of either calcium or phosphorus. Horses are more likely to suffer from a lack of calcium or phosphorus, and as...