THE ROLE OF ROCK SALT IN ANIMAL NUTRITION
Historical need for salt
For thousands of years it has been known that domestic and wild animals need salt just as
man does. The virtues of salt for animals were extolled by the ancient Greeks. Early explorers in Africa, Asia and North America recorded observations of grazing animals traveling to salt springs or deposits to satisfy ravenous appetites for salt. Animals deprived of salt will risk grave danger or resort to unusual behavior to obtain it. Considerable evidence exists that early nomads and hunters took advantage of this fact to lure and capture animals by locating areas with salt and waiting for animals to come there periodically.
That livestock and poultry need salt was recognized long before scientific knowledge of foods or nutrition became available. In the early 1800s the value of salt for experimental animals was demonstrated. Since then, many studies have been conducted and a summary of these results are reported herein.
The role of salt in animal nutrition
Common salt contains both sodium and chloride and is also called sodium chloride. Salt is unique in that animals have a much greater appetite for the sodium and chloride in salt than for other minerals. Because most plants provide insufficient sodium for animal feeding and may lack adequate chloride content, salt supplementation is a critical part of a nutritionally balanced diet for animals. In addition, because animals have a definite appetite for salt, it can be used as a delivery mechanism to ensure adequate intake of less palatable nutrients and as a feed intake limiter.
Even though the body only contains about 0.2% sodium, it is essential for life and is highly regulated. About half of the sodium in the body is in the soft tissues of the body; the other half in bones (18). Sodium makes up about 93% of the basic mineral elements in the blood serum and is the chief cation regulating blood pH. The ability of muscles to contract is dependent on proper sodium concentrations. Sodium plays major roles in nerve impulse transmission and the rhythmic maintenance of heart action (18). Efficient absorption of amino acids and monosaccharide from the small intestine requires adequate sodium (42).
The other nutrient in salt, chloride, is also essential for life. Chloride (cl- ) is the primary anion in blood, and represents about two thirds of its acidic ions. The chloride shift, movement of chloride in and out of the red blood cells, is essential in maintaining the acid-base balance of the blood. Chloride is also a necessary part of the hydrochloric acid produced by the stomach which is required to digest most foods.
Unfortunately, it is often assumed that if the sodium requirement is met, the chloride requirement will automatically be met also. However, recent evidence indicates this may not always be the case.
For example, Belgian studies showed a close co-relation between potassium and chloride in the urine of cows (2). They concluded that the necessity for the ruminant to eliminate high amounts of dietary potassium (as potassium chloride) can dramatically increase the chloride requirement.
Therefore, since many ruminant feedstuffs are quite high in potassium, the potassium-to-chloride ratio in the diet is important.
In monogastrics, a chloride deficiency can also develop when low levels of salt are fed. Leach and Nesheim, (3) reported that a chloride deficiency in chicks results in extremely poor growth rate, high mortality, nervous symptoms, dehydration and reduced blood chloride.