The Need for Salt

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.

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.

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 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. The basal diet was grass silage and ground barley. The final diets contained 1.0, 6.0 and 11.0 g sodium/ kg dry matter for the control, low and high sodium diets, respectively. The salt supplements were mixed with the barley and no feed refusals observed. Cows were individually housed and milked twice daily in their tied stalls. Each cow was observed for a total of 18 5-minute periods and the amount of time doing various behaviors recorded. Stereotypies behavior recorded included: mouthing the feed trough bars or tethering chain, rubbing against feed trough bars or tethering chains, pawing the ground or self-grooming. None of the individual sterotypies behaviors was significantly affected by sodium level, but collectively there was a reduction in total time spent in stereotypic behavior at the high sodium level. The fact that stress increases the sodium appetite of other herbivores suggests that the reduction of stereotypies measured in this experiment may be a consequence of the physiological relationship between stress and sodium status.

In a second experiment (354), 16 British Friesian female calves were selected at birth and allocated to pairs of similar weight. Within each weight, calves were assigned to no additional salt or 13.5 grams of salt/kg of concentrate fed. Adding the salt to the concentrate increased the sodium concentration from 4 to 9 g sodium/kg concentrate. Calves were housed in individual pens and weighed weekly for 6 weeks. Behavior was recorded for 12 hours after the calves received their concentrates on day 1 of each week. Adding sodium to the concentrates increased feed intakes, water intakes, and live weight. Calves with supplementary sodium spent less time grooming themselves, licking the pen, licking the buckets and ear sucking. The sterotypies behavior was more pronounced in calves than in the cows in the previous experiment. The sodium intake of the control treatments was greater than the requirement given by the British Ministry of Agriculture. In that sense they were not sodium deficient diets. However, the stress experienced may have increased the desire for sodium that resulted in behavior patterns associated with stress. Increasing the sodium level was helpful in controlling abnormal behaviors.

Abnormal behaviors may also be influenced by sodium levels in the diets of other farm animals. For example, tail biting in finishing pigs can be a real problem in high-density confinement buildings. Tail biting begins with the occasional chewing of another pig’s tail. Once a wound has been established, the biting becomes more frequent and intense. Docking the tail at birth has become standard practice to try to avoid this problem later.

Diets containing less than 0.3% salt are associated with high levels of tail-biting (352). Most swine nutritionists recommend 0.5% salt in the diet. However, salt concentrations are often raised to 1% of the diet following an outbreak of tail biting. Other factors that may contribute to tail biting in pigs includes protein deficiency, amino acid imbalance, thermal stress, high ammonia levels, overcrowding, large group sizes, and poor ventilation.

Because blood is relatively high in sodium, some researchers have proposed that tail biting was an effort to find more sodium. Canadian researchers (356) have tried to determine if this was the case by allowing pigs access to ropes (similar to pig tails) soaked in blood, salt water, and pure water. The pigs were given ACTH injections to simulate stress conditions. In this study the blood-soaked ropes were the most popular, but there was no difference in the number of pigs that preferred the salt water and pure water ropes. This suggests that salt taste may not be the only factor that makes blood attractive to stressed pigs (356).

A similar problem associated with a salt deficiency in poultry is cannibalism. Cannibalism occurs when birds peck at the feathers, toes, heads, and vents of other birds. If there is bleeding and further pecking, it may result in the death of the bird. Poultry nutritionists often recommend that the diet contain 0.15 to 0.20% sodium to minimize cannibalism. If cannibalism does become a problem, sometime it can be controlled by adding 5-10 grams of salt per gallon of drinking water. Other factors that can contribute to cannibalism include vitamin and amino acid deficiency, feed deprivation, over-crowding, over-heating, inadequate ventilation and bright lighting.

The drive to consume adequate sodium can have a powerful influence on behavior. Dr. Derek Denton in his book The Hunger for Salt builds a strong argument that the incidence of cannibalism in primitive people was highly correlated with a lack of sodium in the their diet. Cannibalism was most common in the tropical areas of the world that lacked access to salt. The equatorial jungles and mountains are noted for their very low sodium status. Requirements for sodium were also increased in these hot environments due to its loss in sweat.

Production environments that increase the stress in farm animals will also increase the appetite for sodium. The endocrine changes in the brain as a result of stress will stimulate the appetite for salt. Stereotypies behaviors that seem to have no meaningful purpose may be aimed at increasing the contact with and consumption of sodium sources. Salt requirements have been determined in production settings where efforts have been made to minimize stress. Sodium levels required to minimize undesirable animal behaviors in stressful environments may be greater than that required to meet nutrient needs in a low stress production settings. In that sense animals may have two sodium requirements, one to maximize animal production and another to modify behavior.

Previous researchers (1,2) have noted that the sodium concentrations reported in NRC feed composition tables were inflated for many common feedstuffs relative to values being reported. Minson (217) reported that the distribution of sodium concentrations in pasture samples was skewed to the low values. In this study 50% of the samples contained less than 1.5 grams of sodium per kg dry matter. Other researchers had shown that the sodium concentration decreased rapidly in some forages as they mature. Morris (218) found that California rangeland pastures in September only contained 0.1 to 0.2 grams of sodium per kg dry matter while the same pastures in the spring contained 0.5 grams of sodium per kg dry matter.

With this in mind, a survey was conducted to compare the mineral concentrations of common feedstuffs as determined in commercial laboratories with NRC values as reported in the third revision of the United States-Canadian Tables of Feed Composition. Data from laboratories in New York, Indiana, Idaho and Arizona were pooled and summarized. Feedstuffs from all 50 states were analyzed by these laboratories, but no attempt was made to summarize the data by region. Of all the minerals analyzed, sodium was found to be consistently below the NRC values. For example, the sodium concentration in corn was 23%, barley 47%, oats 12%, wheat 18%, corn silage 31%, alfalfa hay 10%, distiller’s grains 57%, brewer’s grains 16%, and whole cottonseed only 3% of the NRC value. With most feedstuffs, over 100 samples were analyzed and with corn silage and alfalfa over 3500 samples were involved. Soybean meal was the only common feedstuff compared where sodium was equal to or greater than NRC values (185).

More recent data suggests that sodium concentrations in grazed forages are also lower than many book values. Between 2001 and 2004, 1021 forage samples harvested by county agents across the state of Tennessee were analyzed for sodium (293). The average sodium concentration was 0.01%. Because most of these samples were not from pure stands of forage, it is difficult to compare with book values. However, 0.01% is below book values for most grasses.

The reason for this discrepancy is difficult to explain. Regional difference could be a contributing factor since most of the feeds were grown in the eastern half of the U.S. However, this logic does not hold for feedstuffs like whole cottonseed or wheat that were from the South and High Plains regions, respectively. One explanation that has been offered, but not confirmed, is that with some of the early feed analysis procedures, chlorides were measured and then multiplied by 0.649 to get the sodium concentration (188). This assumed that sodium was present in equal molar concentrations to chloride. The practical ramification of this information is that many diets formulated for specific sodium concentrations are actually below the desired amount because NRC values were used in the formulation.

Water quality is an issue receiving national attention. High quality water is essential for successful animal production systems. Rarely is salt the only mineral in high concentrations in saline water. Besides sodium and chloride, calcium, magnesium, bicarbonate, sulfate and nitrate ions are commonly present. Each ion may exert its own specific toxic effects separate from osmotic effects normally associated with the total dissolved solids. The nitrate ion is a good example.

The amount of sodium and chloride tolerated in drinking water is largely determined by the total soluble salt content of the water. Consequently, the NRC (189) guidelines for the use of saline waters by livestock and poultry are based on the total soluble salts (Table 1). The NRC committee (189) suggests the following points should be considered when making recommendation based on Table 1.

Table 1. A Guide to the Use of Saline Waters for Livestock and Poultry (189)

Total Soluble Salts Content of Waters, ppm	Comment
Less than 1,000	These waters have a relatively low level of salinity and should present no serious burden to any class of livestock or poultry.
1,000 – 2,999	These waters should be satisfactory for all classes of livestock and poultry. They may cause temporary and mild diarrhea in livestock not accustomed to them or watery droppings in poultry (especially at the higher levels) but should not affect their health or performance.
3,000 – 4,999	These waters should be satisfactory for livestock, although they might cause temporary diarrhea or be refused at first by animals not accustomed to them. They are poor waters for poultry, often causing watery feces and (and the higher levels of salinity) causing increased mortality and decreased growth, especially in turkeys.
5,000 – 6,999	These waters can be used with reasonable safety for dairy and beef cattle, sheep, swine, and horses. It may be well to avoid the use of these waters approaching the higher levels for pregnant or lactating animals. They are not acceptable waters for poultry, almost always causing some type of problem, especially near the upper limit, where reduced growth and production or increased mortality will probably occur.
7,000 – 10,000	These waters are unfit for poultry and probably for swine. Considerable risk may exist in using them for pregnant or lactating cows, horses, sheep, the young of these species, or for any animals subjected to heavy heat stress or water loss. In general, their use should be avoided, although older ruminants, horses and even poultry and swine may subsist on them for long periods of time under conditions of low stress.
More than 10,000	The risks with these highly saline waters are so great that they cannot be recommended for use under any conditions.

(1) Alkalinities and nitrates should be considered when using water containing more than 3,000 ppm total salts. Hydroxides are more detrimental than carbonates that are, in turn, more detrimental than bicarbonates. (2) Animals can consume high salinity water for a few days without risk if given access to good quality water thereafter. (3) If given a choice between highly saline and good quality water, animals will not drink the saline water. (4) Water consumption will usually increase as salinity increases up to the point where they refuse to drink. (5) Depressed water consumption will decrease feed intake and reduce performance. (6) Abrupt changes from low salinity to high salinity water will have greater detrimental effects on animal performance than gradual changes.

Information on the tolerance of livestock and poultry for sodium chloride in the water will be given in each species section. This information should be considered as one evaluates the tolerance of farm animals to sodium chloride in drinking water.

Salt concentration in manure is a matter of concern only with the confinement rearing of livestock and poultry. Usually, the reference is to total mineral salts in the manure, not just sodium chloride (salt). The percent "salt" in manure is estimated by adding the percents potassium, calcium, sodium and magnesium and then multiplying by 2 (190). In most cases, the order of contribution to the salt concentration is potassium, calcium, magnesium, sodium. Often the contribution of sodium chloride to the total salts concentration in manure is overestimated. In fact, sodium accounts for only about 7% of the four major salts in manure from livestock and poultry (Table 2). Consequently, if the total salt concentrations contribute to a waste disposal problem, then diets should be evaluated as to the optimal concentration of each mineral before sodium chloride is removed.

Table 2. Estimated Quantities and Constituents of Livestock and Poultry Manures Produced Yearly¹ (190)

Animal		Quantity	Per	Animal-	Year
Type	N²	P	K	Ca	Na	Mg	COD
Dairy	123	21.0	98	72.0	15.0	22.0	3,340
Beef	61	18.0	39	11.5	4.2	5.7	1,510
Swine	32	7.4	11	11.0	1.9	2.9	416
Layers³	94	40.0	40	170.0	18.0	13.0	1,741
Broilers³	78	22.0	25	91.0	9.2	9.2	1,183
Turkeys³	304	84.0	99	355.0	36.0	36.0	4,599

¹ Manure production was derived from ASAE standards and Midwest Plan Service and Gilbertson et al. The values are commonly used for calculating storage volume and equipment requirements and do not indicate quantities available for land application. Based on average animal weight as follows: Dairy and Beef, 1,000 lb; swine, 200 lb; sheep, 100 lb; layers 41 lb; broilers, 2 lb; and turkeys, 10 lb. These values do not include bedding or materials such as spilled feed, soil or water from precipitation. Neither do they reflect the decomposition processes that start as soon as the manure is voided by the animal.

² Nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), sodium (Na), magnesium (Mg) and chemical oxygen demand (COD)

Several research studies have been conducted to evaluate the effect of sodium chloride in manure on crop yields when manure is applied at various rates. Soil type, rainfall and crop grown are the major agronomic factors determining the optimal manure application rates. Under the conditions of west Texas, Texas Tech studies have shown that 10 to 15 tons per acre every 3 to 4 years will not affect crop yields or runoff problems (49, 51). In contrast, Michigan studies with sandy loam soil showed 10 tons per acre yearly to be the optimal application rate (50). This loam soil could handle 10 to 15 tons of manure annually, without nutrient accumulation, especially if the corn was harvested as silage.

A new concept that is being researched in areas where ground water contamination is not a problem, is for the livestock producer to rent the land for 1 to 3 years and apply very high levels of manure. The farmer can grow crops on the land during this period but recognizes that some reduction in yield may occur. Crops that can tolerate saline soils such as bermudagrass, tall wheatgrass, barley, cotton or sugar beets are best suited for this system (190). For example, Texas A&M has experimented with adding up to 900 tons of manure per acre in the El Paso and Pecos, Texas area. Crop growth was achieved at this application rate, but special management considerations are required for this approach to be used successfully.

In addition to the solid manure, feedlot runoff needs to be monitored. Often the runoff is collected in a settling basin and the water is used for irrigation purposes. Using runoff from the Pratt Feedlot, Kansas State researchers showed that approximately 20 inches of water could be applied before corn yields were depressed due to soil salinity (191). USDA scientists at Nebraska reported that application of up to 24 inches of feedlot runoff per irrigation season did not cause a build up of nitrate, nitrogen or salts in the soil after two years of application.

These and other studies show that dietary sodium chloride concentrations required for optimal performance does not create major manure disposal problems. If the total salt level in manure does restrict land application rates, the concentration of potassium, calcium and magnesium in the diet may be adjusted to bring it into an acceptable range.

Animals have a more well defined appetite for sodium chloride than any other compound in nature except water. Ruminants have such a strong appetite for sodium that the exact location of salt source is permanently imprinted into their memory which they can then return to when they become deficient. Bell (133) showed that when steers were trained to receive their sodium in response to pressing a panel, maximum effort to receive the sodium occurred at eight days and after, on a sodium deficient diet. Cattle also have a keen sense of smell for sodium (192). Sodium deficient steers were offered a cafeteria of 12 buckets of water with only one containing moderate levels of sodium salts. Steers would quickly choose the water containing sodium salts without having to taste the water sources.

Horses have been shown to have a specific appetite for salt if the diet is deficient in sodium (132). This is not true for the other nutrients. For example, horses do not develop a preference for calcium supplements when fed a calcium deficient diet (193). This natural appetite for salt is what makes salt such an excellent delivery mechanism for other nutrients that need to be consumed regularly, but where a natural appetite is lacking. The 1984 NRC Beef Cattle committee (157) recognized this fact in stating that minerals lacking in the diet can be provided by "self-feeding" common salt-mineral mixtures when the mixture is consumed in amounts to satisfy the animals’ appetite for salt.

Cattle are also able to detect when they are chloride deficient. In a Cornell study, lactating cows fed a chloride deficient diet (.18% chloride) consumed 337 grams of salt a week while those fed the chloride adequate diet (0.40% chloride) ate 149 grams per week (134). Sodium levels were adequate so that cows were consuming the salt to satisfy their chloride requirement.

The study described above shows the risk of assuming that if the sodium requirement is met then the chloride level is also automatically met. Fettman et al. (194) reported that lactating dairy cows fed a diet containing 0.10% chloride developed clinical deficiency signs characterized by reduced feed intake, body weight, and milk production, lethargy, cardiovascular depression and mild dehydration. Many vegetable protein supplements are low in chloride (0.3-0.7 grams/kg dry matter). It is quite possible that mixtures of the protein supplements and cereal grains can be deficient in chloride for pigs and poultry (219). More attention needs to be paid to diet chloride levels, especially where some of the sodium needs are being met with sources other than NaCl.