Salt for horses: Researchers delve into this important balancing act

Owners will often add salt to the daily rations of horses in work, but how much is too much?
Owners will often add salt to the daily rations of horses in work, but how much is too much?

Common salt is a cheap and important supplement for working horses, but how much is too much?

A German study has delved into supplementation of salt – that’s sodium chloride – in horses in moderate work and compared their findings to widely accepted recommendations.

The level of sodium and chloride in typical horse forages and feeds, especially those non-commercially manufactured, was generally low, the study team noted.

Recommended intakes of sodium and chloride for exercise performance were therefore unlikely to be met by non-supplemented diets, meaning the addition of salt to the diet was commonly recommended.

Annette Zeyner and her colleagues, writing in the open-access peer-reviewed journal PLOS ONE, said that since equine sweat is rich in sodium, potassium and chloride, diets marginal in these electrolytes may disturb fluid and mineral balance, potentially sparking health problems in exercised horses.

“However, recently the value of supplementing, especially in the non-endurance horse, at the currently recommended levels has been questioned and this topic has become highly controversial.” they said.

The study team noted that giving salt to horses in moderate work had been shown to cause acidosis after meals. Acidosis is the name given to increased acidity in the blood and other body tissues.

The researchers wanted to find out if this salt-induced acidosis persisted beyond the post-meal period, and whether it still occurred after a two-week adaptation period.

Six stabled adult warmblood mares in moderate work were used in the study. They received a daily diet of 1 kilogram of meadow hay per 100kg of bodyweight, together with 0.64kg of unprocessed oats and barley per 100kg of bodyweight. They also received a salt-free mineral mix.

Salt supplementation occurred at rates of either 0, 50 grams or 100 grams a day, fed in two meals per day together with the concentrates for 3 weeks.

The study was performed in a Latin Square pattern, in which the horses were organized across the three diets in groups of 3 x 2 horses over three study periods.

Urine and blood samples were collected at appropriate times to be analysed for pH, acid-base status, creatinine and electrolyte concentrations. Feces samples were analysed for mineral digestibility.

Average apparent sodium digestibility ranged between 60 and 62% whereas chloride digestibility was consistently above 94%, the researchers found.

Supplementing 100 grams of salt, but not 50 grams, resulted in a significant reduction of blood pH (acidification) and base excess, as well as urinary pH and urine acid excretion.

Both 50-gram and 100-gram salt supplementation caused a significant reduction in base and net acid-base excretion, urine density and potassium concentration, but increased urine sodium concentration. This suggested that a high proportion of the recommended salt doses were excreted via the kidneys, they said.

The described effects of salt supplementation persisted over the two-week measurement period.

The results, they said, suggested that feeding 100 grams of salt to moderately exercising horses resulted in mild metabolic acidosis, whereas feeding 50 grams according to current recommendations resulted in compensated acidosis. Compensated acidosis is where blood pH is maintained within normal limits, although blood bicarbonate levels generally fell below normal.

The calcium intake was equal in all diets and met commonly accepted requirements. Despite this, the acidotic metabolic changes − that’s changes related to acidification −  following salt intake increased the renal calcium output with both 50 grams and 100 grams daily salt intake. This effect was probably caused by an activation of the parathyroid hormone, seen in dairy cows due to ingestion of so called “acid salts”. In horses, this may prove critical for bone health when occurring over a longer period.

All horses remained clinically healthy throughout the study period, with no obvious change in behaviors. Body temperature and respiratory rates were within the normal range, with the salt changes having no apparent effect.

There were no changes in bodyweight attributable to the salt intake. All meals were eaten completely, although the horses seemed to take longer to eat the meals enriched with salt.

The addition of salt appeared to have no significant effect on daily water intake, with only minor increases when 50 grams or 100 grams of salt were fed.

Discussing their findings, the researchers said: “Although this study did show that sodium chloride supplementation can result in a mild metabolic acidosis, further work is required in order to evaluate the clinical relevance of this effect, in particular any long term effects on calcium balance, especially in the young growing, heavily exercising, animal.”

They said that the amount of sodium found to have been excreted by the horses in the study suggested that the sodium requirements of horses in moderate work may be even lower than that stipulated in some internationally recognized guides.

The authors noted that the sodium requirement of a 600kg horse in moderate work was estimated by both the Washington DC-based National Research Council (NRC) Nutrient requirements for horses and Germany’s Energie und Nährstoffbedarf landwirtschaftlicher Nutztiere. Nr. 2. Empfehlungen zur Energie- und Nährstoffversorgung der Pferde (Energy and nutrient requirements of farm animals. No. 2. Recommendations for the energy and nutrient supply of the horses) to amount to about 20 grams a day.

The addition of 20 grams of sodium – that would require 50 grams of sodium chloride – to a daily ration that already provided 13 grams of sodium would result in an intake of 33 grams. That, the researchers noted, would be around 150% of daily sodium requirements.

It was therefore not surprising that clearance of sodium by the kidneys increased even at this level of supplementation, they said.

Even on the basal diet with a sodium intake of only 13 grams, compared to a recommended intake of 20 grams, there was considerable sodium content in urine, they noted.

“Since horses can down-regulate renal sodium excretion to very low values if they do not ingest sufficient sodium, the excretion of considerable amounts of sodium by urine suggests that sodium requirements of horses in moderate work may be even lower than the NRC and GfE recommendations – at least under similar conditions as in the present study (i.e. cool environmental temperature).”

In principle the same was true for chloride, they said. Their results, they said, suggested that a reduction of the recommendations for chloride intake for working horses under the GfE was justified.

The study team comprised Zeyner, Kristin Romanowski, Andreas Vernunft, Patricia Harris, Ann-Marie Müller, Carola Wolf and Ellen Kienzle. The researchers are from a range of German institutions except for Harris, who is with the Equine Studies Group within the Waltham Centre for Pet Nutrition in Britain.

Zeyner A, Romanowski K, Vernunft A, Harris P, Müller A-M, Wolf C, et al. (2017) Effects of Different Oral Doses of Sodium Chloride on the Basal Acid-Base and Mineral Status of Exercising Horses Fed Low Amounts of Hay. PLoS ONE 12(1): e0168325. doi:10.1371/journal.pone.0168325

The study, published under a Creative Commons License, can be read here

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