Signs of a salt deficiencyWhen 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 needsMany 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 ofthe 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 andmanagement situation.Salt and Coping with StressModern 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 fromstress 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.Cattle: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 saltsupplements 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.Pigs: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).Source: University of Illinois