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Fats
Fats are triglycerides which are composed of one molecule of glycerol and three long-chain fatty-acid molecules, which may be the same or different. In contrast to the short-chain or volatile fatty acids produced by bacterial fermentation of ingested carbohydrates, which contain only two to five carbon atoms, most long-chain fatty acids in fat contain 16 to 20 carbon atoms. The more unsaturated the long-chain fatty acids (i.e., the greater the number of double bonds between its carbon atoms) and the shorter their chain length (i.e., the fewer carbon atoms they consist of), the lower the melting point of the triglyceride. When the melting point is less than room temperature, the triglyceride is referred to as an oil; when it is greater than room temperature, it is referred to as a fat. Triglycerides, like other lipids (phospholipids, sphingolipids, glycolipids, steroids, and waxes), are soluble in organic solvents such as ether. Other nonmineral constituents of plant and animal tissues are water soluble and are relatively insoluble in organic solvents. This difference is the basis for extracting and determining the amount of lipid in a sample, and the reason lipids are referred to as ether extracts, which in horse feeds is almost entirely fats and oils.
Fats or oils are needed in the horse’s diet for the absorption of the fat-soluble vitamins A, D, E, and K, and as a source of the unsaturated fatty acid linoleic acid. Linoleic acid is essential in the diet of all animals. The amount needed by horses hasn’t been determined, although at least 0.5% linoleic acid in the diet dry matter is recommended for all horses, the requirement is probably much lower than this. The horse can synthesize in sufficient quantities all other fatty acids needed, including linolenic and arachidonic acids, which are also often referred to as essential fatty acids. Therefore, fatty acids other than linoleic acid are not needed in the horse’s diet.
Unsaturated fatty acids such as linoleic acid become oxidized (rancid) with increasing time, temperature, and humidity if sufficient antioxidants aren’t present. Rancid fats don’t provide the animal’s linoleic acid needs. Fat rancidity also impairs the utilization of vitamins A, D, and E, as well as several B vitamins, and decreases the fat’s palatability so that the horse may refuse to eat feed containing it. Rancidity may occur: (1) if unsaturated fatty acids that do not contain sufficient antioxidants, such as Vitamin E, are added to the diet, and (2) with prolonged storage of cereal grains, with or without added fat or oil. The time required for rancidity to occur decreases with increasing temperature and humidity, both of which may increase sufficiently for rancidity, as well as mold growth, to occur if cereal grains contain more than 12 to 15% moisture when stored.
An essential fatty acid deficiency in most species causes a dry, lusterless hair coat and scaly skin, and may predispose to skin infection. If the deficiency persists, hair loss, edema, and exudation from localized areas of the skin, resulting in moist inflammation, may ensue. Reproductive efficiency may be impaired. If pregnancy occurs, there may be neonatal abnormalities and death. However, in the horse, an essential fatty acid or linoleic acid deficiency is not known to occur. No clinical symptoms, visual indications of skin or hair coat alteration, or decreases in plasma concentrations of triglycerides or free fatty acids, including linoleic acid, occurred in ponies fed extremely low-fat diets for 7 months. However, at the lowest fat intake (0.05% in the diet), tissue and plasma vitamin E concentrations fell significantly, perhaps indicating inadequate absorption of this fat-soluble vitamin. Common horse feeds contain 2 to 6% fat, i.e., many times more than that present in the diets used in this study. The fats present in common horse feeds are also high in linoleic acid; e.g., corn oil is 55% and safflower oil 73% linoleic acid. In contrast, coconut, palm, and olive oils are 1 to 2%, butter fat 2.5%, beef fat 4%, linseed or flax seed oil 14%, pork fat 18%, and poultry fat 22% linoleic acid. Thus, a dry, lusterless hair coat in the horse is unlikely to be due to a fatty acid deficiency. Instead, it is more likely to be due to a dietary protein, energy, or Vitamin A deficiency.
Ingested fats are used primarily as a source of dietary energy, although if a diet contains adequate available carbohydrate and protein, fats are not needed as a source of energy. Digested fats provide over 2.25 times more utilizable energy than an equal weight of digested carbohydrate or protein. Because of their high energy density, these feeds may be added to a horse’s diet to increase its dietary energy intake in an effort to increase its growth rate, milk production, reproductive efficiency, or physical performance.
This article is from "Feeding and Care of the Horse", second edition, by Lon D. Lewis, Lippincott Williams & Wilkins, 1995. Reprinted with permission from the publisher.
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