«©Copyright 1974, 1979, 1983, 1987, 1993, 1999, 2001, 2006 Table of Contents The Role of Salt in Animal Nutrition The Need for Salt Salt for Beef ...»
Salt and Trace Minerals for Livestock, Poultry and Other Animals
by Larry L. Berger, Ph.D.*
*Professor of Animal Nutrition
University of Illinois
1987 and prior editions were written by
Tony J. Cunha (1917-1992)
Published by the Salt Institute
700 North Fairfax Street, Suite 600
Alexandria, Virginia 22314-2040
©Copyright 1974, 1979, 1983, 1987, 1993, 1999, 2001, 2006
Table of Contents
The Role of Salt in Animal Nutrition
The Need for Salt
Salt for Beef Cattle Salt for Sheep Salt for Goats Salt for Dairy Cattle Salt for Swine Salt for Horses Salt for Poultry Salt for Dogs, Cats, Rabbits, Mink, Foxes and Other Small Animals Trace Minerals Zinc for Animals Iron for Animals Copper for Animals Excess Molybdenum Iodine for Animals Cobalt for Animals Manganese for Animals Selenium for Animals Injectable Trace Minerals Bioavailability of Trace Mineral Sources Inorganic and Organic Trace Minerals Trace Mineral Antagonists Trace Mineral Nutrition of Fish The Role of Magnesium and Sodium in Grass Tetany Chromium Helps With Stress New Trace Elements for Animals Nutrition and Disease Interaction Using Salt for Animal Nutrition BSE: Potential Role of Trace Minerals Selected Literature References
THE ROLE OF SALT IN ANIMAL NUTRITION
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 (129). 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 (129). Efficient absorption of amino acids and monosaccharides from the small intestine requires adequate sodium (184).
The other nutrient in salt, chloride, is also essential for life. Chloride 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 correlation between potassium and chloride in the urine of cows (72). 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, (76) reported that a chloride deficiency in chicks results in extremely poor growth rate, high mortality, nervous symptoms, dehydration and reduced blood chloride.
THE NEED FOR SALT
Signs of a salt deficiency
When salt intake is below that required to meet the animal’s need for sodium and chloride, the animal adjusts by conserving (77). Urine output of sodium and chloride nearly stops. A continuous low salt intake affects the health of animals through a loss of appetite and weight. Feed utilization decreases and it takes more feed per unit of gain or product produced (78, 83, 84, 128). Animals soon develop a craving for salt. They may consume considerable amounts of dirt, wood, rocks and other materials. They will also lick manure and urine in an attempt to obtain the needed salt.
Lactating animals are most susceptible to a salt deficiency because milk contains a considerable amount of sodium and chloride. Because the composition of milk is highly regulated, a deficiency of sodium or chloride in the diet will ultimately decrease milk production.
Factors affecting salt needs Many scientists have shown that the salt needs of animals vary. Some of the factors that influence
salt needs are as follows:
1. Diet can have a great impact on the salt needs of animals. Diets containing different amounts of concentrates, pasture, hay, silage or byproduct feeds account for much of the variation in salt requirements due to the wide range of sodium and chloride concentrations.
2. The level of sodium, chloride and other minerals in the water is another important factor.
Animals typically will consume 2-3 times as much water as dry food. Locality can have a major impact on the minerals present in the water and, thus, the need for salt.
3. Level of production can have a great influence on the need for supplemental sodium and chloride.
For example, cow’s milk contains approximately 630 ppm sodium and 1150 ppm chloride. As milk production increases so does the need for salt (130, 131). A Canadian study (123) showed that lactating gilts consumed twice the sodium chloride of open gilts of the same age. Increases in rate of growth, reproduction, egg production, etc. will all increase the need for these minerals.
4. The temperature and/or humidity can be an important factor. The University of Florida (130, 131) showed that heat stress increased the need for potassium in the diet of high-producing dairy cows.
Increased milk production occurred due to 1.5% potassium in the diet. Texas studies verified the Florida finding on a need for up to 1.5% potassium for maximum milk production during hot weather (139). The Florida studies also showed that sodium needs were increased with the higher levels of potassium in the diet (130, 131).
During heat stress, certain animals can lose large amounts of sodium through sweating. For example, working horses have been shown to increase their salt consumption five-fold during heat stress (31). Providing free-choice salt is the best way to meet individual needs in this situation.
5. The sodium concentration of the same feedstuff grown in different areas can be highly variable.
This results in different supplemental sodium needs even though the diets may be similar. A recent survey (185) has shown that sodium concentrations for feedstuffs given in the third revision of the U.S.-Canadian Tables of Feed Composition are often 2-3 times greater than values being obtained by commercial laboratories. Consequently, the animal’s requirement for supplemental sodium may well be greater because the concentration in the basal diet is overestimated.
6. Availability of sodium and chloride in feeds may be over-estimated. Recent work with forages suggest that mineral availability decreases with plant maturity because more and more of it is associated with the indigestible fiber fraction.
7. Potassium concentration in the diet can influence requirements for sodium and chloride. Sodium is required in the kidney for potassium conservation and to balance bicarbonate excretion electrically (186). An excess of potassium can aggravate a marginal sodium deficiency. This can even occur when high forage (pasture, hay or silage) diets are fed. For example, certain pastures may have up to 18 times more potassium than sodium. This helps explain why cattle choose to consume more salt on high forage diets than on high concentrate diets.
Adding supplemental potassium to the diet can have the same effect. Recent research from Florida showed that adding potassium to reduce heat stress markedly increased the sodium requirements of the lactating cow (131).
8. The concentration of chloride and/or sulfate in the diet can impact the sodium requirement.
Cornell studies showed that excessive levels of sulfate or chloride ions depressed growth in the chick unless equimolar amounts of sodium and potassium were also supplied in the diet (59). Their studies provide a possible explanation for why animal performance may be enhanced with salt additions, even when sodium and chloride concentrations are above the NRC requirement.
9. Recent studies with poultry indicate that higher levels of sodium and chloride may be required for normal immunity and maximizing resistance to diseases (187) than is required for maximum growth. Most nutrient requirement studies are conducted under conditions to minimize stress from disease or the environment. It should not be surprising that requirements for sodium and/or chloride may be increased in less than optimal conditions.
10. Genetic differences in animals affect salt requirements. As we select animals for maximum performance while being fed diets with greater caloric density, sodium and chloride concentrations required to achieve maximum performance may be increased.
These factors help explain why salt needs vary among localities and with different feeding and management situation.
Salt and Coping with Stress Modern production agriculture exposes animals to environments that they would not usually be exposed to in the wild. Although efforts are made to minimize the stress these animal experience, some animals do experience increased stress which is reflected in their endocrine profile. Recent research suggests that the changes in hormonal profile may cause an increased appetite for sodium.
This increased appetite for sodium may encourage stereotypies behavior.
In this review, the term “stress” as applied to farm animals is a potential damaging stimulus that evokes a largely adaptive response (349). Stress is a normal part of animal life. Animals raised in the wild are exposed to a lack of food, heat, cold, antagonistic social interactions, predators, etc., all of which cause stress. The point is that animals will experience stress in both “natural” and “production” settings.
Stress and Behavior:
Stress encourages stereotypies behavior in laboratory and farm animals. Stereotypies is defined as behavior of an unvarying, repetitive nature with no direct purpose (353). Rats when they become sodium deficient exhibit stereotyped fixed action patterns that are ingestive in nature (348). Sodium deficient cattle frequently display excessive licking behavior (355). Cattle that are tethered in a restricted area or raised individually as calves in isolated stalls, exhibit similar licking behaviors.
In the past few years scientist have learned a great deal about how hormonal changes resulting from stress can affect brain chemistry and behavioral changes. Animals respond to stress by releasing adrenocortiotropic hormone (ACTH) from the anterior pituitary gland. The ACTH then causes the adrenal cortex to release aldosterone and corticosterone. Aldosterone is the main hormone that controls sodium balance by changing the kidney’s reabsorption of sodium and thus the amount excreted in the urine. Corticosterone increases blood glucose and carbohydrate metabolism to supply energy. These hormones also act directly on the brain through the activation of the neuropeptide angiotensin II. Angiotensin II is a powerful stimulus for thirst and sodium appetite (351). When it is injected directly into sensitive areas of the brain, it causes and immediate increase in water intake followed by a slower increase in sodium intake. However, the appetite for salt is more persistent and may be affected by previous experience. Some researchers believe that the angiotensin II may influence neuronal organization in the brain that can cause long-term changes in sodium appetite (351). Stress has been shown to increase the salt appetite in rats, mice, rabbits and sheep.
Phillips et al., (354) conducted an experiment to determine whether salt intake influenced the behavior of cattle in stressful environments. In this experiment, 36 Estonian Red dairy cows were allocated to three treatments, 0, 200, or 400 grams of salt added to a standard winter ration, daily.