In the last blog, we discussed the first steps to take when reading a feed tag which included evaluating protein, amino acids, fat, fiber, and the list of major feed ingredients. We also touched on the differences between least-cost formulations, open labels and locked formulas. This month we will turn our focus to vitamins and minerals.
The first minerals to evaluate are calcium and phosphorus. They will be the next items listed on the guaranteed analysis after protein, fat, and fiber. Calcium will be listed with both a minimum and a maximum percentage guarantee while phosphorus is listed with only a minimum percentage guarantee. Most of the calcium within the horse’s body is located in the bones and teeth. Calcium is responsible for the structural components of the bones and teeth but also has other functions such as muscle contraction and blood coagulation. It is critical that calcium levels in the blood remain consistent because if there is not enough calcium in the diet the body will pull calcium from the bones. Phosphorus is also a major component of the skeletal structure of horses and is required for several other functions in the body. Phosphorus competes with calcium for absorption in the small intestine. As phosphorus levels increase in the diet the absorption rate of calcium decreases. It is this reason that the ratio of the two minerals is important. The ideal calcium to phosphorus ratio is 2:1, but ratios in excess of 5:1 have been reported safe as long as the diet contains appropriate levels of phosphorus. Horses that are fed very large volumes of grain-based feeds such as oats or wheat bran are at higher risk of having an inverted calcium to phosphorus ratio which may cause skeletal abnormalities. It is important to remember that the total diet, including forage, must be taken into consideration. Lactating mares and growing horses have the highest requirements for calcium and phosphorus.
Magnesium and potassium are important because they are involved in skeletal structure and muscle. Magnesium and potassium are electrolytes which are responsible for maintaining water balance, appropriate pH levels in blood, and muscle function. Magnesium deficiency may result in nervousness, muscle tremors, and ataxia. Potassium levels are critical for horses with hyperkalemic periodic paralysis (HYPP); management practices include restricting the amount of potassium fed to these horses.
Copper is a micro-mineral measured in parts per million (ppm). Copper is needed in the body for creation and maintenance of elastic connective tissue. Research has shown that just 6 months of feeding low copper levels may result in limb deformities, epiphysitis, and OCD’s. Zinc is another micro-mineral that, when fed in deficient amounts, may result in abnormalities such as poor appetite, reduced growth rate, and hair loss. Horses seem to be tolerant of high levels of both copper and zinc, but extreme over-supplementation is not recommended. Manganese is the third micro-mineral that influences structural integrity, and is essential for synthesis of the chondroitin sulfate necessary in cartilage formation. All three of these minerals may be provided in an inorganic form (i.e. zinc oxide) or an organic form (i.e. zinc amino acid chelate). Chelated forms of these micro-minerals are more easily absorbed and utilized by the body (up to 400% better absorption, according to research). If supplemental copper, zinc, or manganese are added to a feed, the particular form used will be identified in the ingredient listing on the feed tag.
Vitamins A, D, and E are often guaranteed on a feed tag. Vitamin A is responsible for its role in night vision, therefore night blindness is a classical symptom of Vitamin A deficiency. However, since vitamin A is a fat-soluble vitamin it may accumulate in the body when fed in excess. Vitamin A fed at extremely high levels has been reported to cause bone fragility as well as structural and functional abnormalities in the embryo and fetus. Vitamin D plays an important role in calcium levels in the body. It is responsible for calcium absorption in the intestines and mobilizing calcium from the bones when needed. Horses are capable of converting sunlight into vitamin D, therefore there have not been any reports, to date, of vitamin D deficiency in horses that are exposed to sunlight. Vitamin E is most notable as a biological antioxidant. According to the National Research Council, Vitamin E does not appear to be toxic to horses when fed at high levels, however other species have reported negative effects at high levels so extreme over-supplementation is not recommended. Vitamin E is especially important for lactating mares and for growing horses; some research has reported muscular dystrophy in foals whose dams were eating a diet deficient in vitamin E. Vitamin E is also thought to play an important role in exercising horses and may have beneficial effects in horses with PSSM.
Equine nutrition is complex and ever-changing. It is a science. Nutritionists, researchers, and feed manufactures continue to challenge past practices in an effort to improve what we already know. Education is power, we can never know too much, and it is the responsibility of each horse owner to provide the best nutrition possible in the given circumstance. Stay tuned for next month’s column, where we will discuss some less common nutrients that may be guaranteed on a feed tag such as B-vitamins, probiotics, and digestive enzymes.