Salt Smart: Preventing Deficiency and Toxicity in Livestock

What is Salt?

Salt is an essential mineral for many cellular functions in all mammals. Chemically, salt is composed of sodium and chloride (NaCl), both of which are regulated to keep a steady level in the blood. Both minerals are efficiently absorbed from the intestines, and the kidneys regulate the amount excreted. When water is lost from the body, sodium levels in the bloodstream increase, triggering the cow's thirst center in the brain to stimulate drinking. Additionally, the kidneys conserve water. This becomes visually apparent with darker urine. Cattle are believed to have a true salt appetite, meaning they can control how much salt they consume when given free access to both salt and water.

Salt Deficiency:

 Two unpublished cases below illustrate the importance of salt. The first case involved a 60-cow Holstein dairy, where the farmer noticed that the cattle began licking dirt and urine. At the same time, milk production decreased, but this was blamed on a change in forage. The local veterinarian was called when three cows stopped eating.

The cows did not exhibit an elevated temperature, ketosis, or a displaced abomasum ("twisted stomach") on physical exam. However, the cows did show a decrease in rumen activity, and they had relatively stiff manure. Blood samples from two sick cows and one healthy cow, eating normally, were evaluated. The analysis showed that these cows were all below normal in sodium and chloride. (See Table 1).

It was discovered that salt had been missing in the lactating supplement, and these cows had developed a deficiency. The cows exhibited behavioral changes in response to salt cravings. The diagnosis was confirmed by allowing access to salt blocks. The cows aggressively licked the salt block, physically competing with herdmates. Milk production steadily increased to normal levels in the weeks that followed. It was never determined when the problem began, but it likely developed over a few weeks. 

Salt Toxicity:

Elevated levels of sodium and chloride in the blood become toxic. Typically, this occurs when animals have normal access to salt but are restricted from free access to water. A different farm had an issue with salt toxicity, but water restriction was not the issue. On this farm, the cows had been housed in a tie-stall barn without free choice access to salt. Early in spring, the farmer decided to supplement free choice salt with the cows now grazing. He distributed an entire bag of salt for the 30 milk cows. The herd devoured the entire 50 lbs. of salt in one afternoon. That evening, three cows could not rise, and many others had loose manure. There was also evidence that there was excessive urine production. It was confirmed that the cows were never without access to water.

Blood tests showed that the sickest cow had severely elevated levels of sodium and chloride. (See Table 2). The aggressive desire for the loose salt suggests that the herd was marginally deficient in salt. At normal intake rates, blood sodium levels increase, and the desire for salt decreases. Unfortunately, in this case, the cows ingested salt so rapidly that by the time blood levels increased, toxic amounts were already in the rumen. After the desire for salt stopped, the blood levels continued to rise.

At toxic levels, the sodium content in the brain tissue is abnormally high. This triggers thirst in an attempt to lower sodium levels. Unfortunately, as water moves into the brain tissue to balance levels, the brain swells. Because the brain is fully encased in bone, swelling results in an increase in pressure, leading to clinical signs such as inability to rise and tremors. Sadly, several animals died from the toxicity. These cows were likely the most aggressive animals and had eaten the most. The excessive liquid levels in the barn gutters the next day provided evidence of the high water intake.

These two cases show the extremes of deficiency and excess, but both were entirely preventable. Always having free choice salt available allows cows to regulate their intake. As the second case illustrates, introducing salt to cows without prior access must be done cautiously. Salt blocks slow intake compared to loose salt, but intakes must be monitored following reintroduction.

Medical Application:

Dehydration is a medical condition where an animal loses more fluid than it ingests. Intravenous fluids are highly beneficial in treating dehydration. For example, young calves with diarrhea can significantly improve their condition with just a liter of intravenous fluids and electrolytes. While it is possible to perform the same treatment in an adult cow, it is typically impractical in a farm setting.

The principles of sodium regulation and the thirst center can be used to treat cows medically. Large volumes of 0.9% sodium chloride (the normal concentration in the blood) can be used to correct dehydration. Hypertonic (hyper meaning over or above) saline is 7.2% sodium chloride. Administering 2 liters of hypertonic saline intravenously to an adult cow raises the sodium level in the bloodstream. This stimulates thirst, which often prompts a cow to drink 5 to 10 gallons of water and replenishes the fluid in the bloodstream. This treatment is a practical way to correct dehydration on the farm. Animals that don't drink following treatment should have fluid pumped into their rumen.

Key Points:

1. Always discuss changes on your farm with your veterinarian and nutritionist before making them.
2. Cows can regulate their salt intake. Have free choice white salt available for your cows.
3. Discuss the uses of hypertonic saline with your veterinarian.

References

Angelos, Stephen M., and David C. Van Metre. 1999. "Treatment of Sodium Balance Disorders." Veterinary Clinics of North America: Food Animal Practice 15 (3): 587–607. doi.org/10.1016/s0749-0720(15)30165-1.

Constable, Peter D. 1999. "Hypertonic Saline." Veterinary Clinics of North America: Food Animal Practice 15 (3): 559–85. doi.org/10.1016/s0749-0720(15)30164-x.

McGuirk, Sheila M., and Susan D. Semrad. 2005. "Toxicologic Emergencies in Cattle." Veterinary Clinics of North America: Food Animal Practice 21 (3): 729–49. doi.org/10.1016/j.cvfa.2005.08.001.

National Academies of Sciences, Engineering, and Medicine. 2021. Nutrient Requirements of Dairy Cattle: Eighth Revised Edition. Washington, DC: The National Academies Press. doi.org/10.17226/25806.