12/23/2024
Salt and electrolytes
I have recommended for many years at least 2 tablespoons of salt (about 50 g or so) and more in hot weather for horses on untested forage on the basis of the NRC Nutrient Requirements of Horses and Dr Eleanor Kellon VMD. Horses use sodium, chloride and potassium for chemical processes in the body (maintenance needs) AND for cooling down when sweating (sweat losses). Fortunately potassium is relatively high in all feeds including grass and hay so from a daily, maintenance aspect, the two electrolytes we mainly have to supplement is sodium and chloride. Fortunately the easy way to do this is with NaCl, salt. This amount of salt will hopefully cover maintenance requirements AND contribute to electrolytes lost in sweat.
I've been concerned for some time by some people saying to base horses salt supplementation on 5-10 g/100 kg BW (bodyweight). This is not going to be enough in many cases. It's not a BW calculation when you take into account the horse can excrete excess electrolytes easily, and the potential for high losses in sweat in hot conditions. If the horse is a miniature, I have halved the general recommendation but understand that excess salt (excreted easily as long as plenty of drinking water) is vastly better than an electrolyte deficiency.
Some years ago, a visiting vet told me she gives her horses 6 (six!) tablespoons of salt over a day on extremely hot days.
It makes sense. If we only supplement at one time in a day and knowing that electrolyte excess is excreted easily and efficiently within a 4 hour window, even within one hour, then it makes sense that if a horse is sweating in the heat in the paddock, it's not going to be enough. I've now stepped it up to 4 tablespoons, split between two small feeds (we have plenty of grass thank goodness).
If your horse does not have access to water (huh?) never force salt into your horse as it can cause hypernatraemia. Always have drinking water available.
Sodium is what the brain ‘reads’ in determining when to trigger thirst and when to regulate the amount of sodium and water the body excretes in the urine.
At normal body levels, the horse has 1.58 g of sodium per kg of body weight. That's 632 g of sodium for a 400 kg horse. Slightly over 50% of that is stored in the bone and only 10% of the body's sodium is in the blood. If blood levels of sodium have been low for a long time, when you start to provide salt, sodium will need to be replenished in the skeleton and other tissues, not just the blood.
When a horse is sodium deficient, they drink less, they urinate less to conserve sodium. A sodium deficiency = fatigue. Drinking less may make you think you shouldn't give salt but salt is what they need. By supplementing salt, it can trigger drinking. Many endurance riders know how well this works.
You may ask, why do horses not drink when they are obviously dehydrated (pinch test)? The two triggers that can cause the thirst mechanism to be activated are loss of body fluids and loss of concentration of sodium.
When body fluids around cells drop due to sweating (horse looks dehydrated, tucked up, skin stands up in tent, slow to lower), fluids are forced out of the interstitial spaces (around tissues and organs) to compensate for this. With continued exercise and sweating, fluid is absorbed from the stores in the gastrointestinal tract. (Jenkinson et al., 2006) This helps maintain the fluid volume of the plasma. Due to the large reservoir of fluids that the horse maintains, plasma volume is able to be maintained, even with a fairly large loss of total body volume.
The plasma concentration of sodium is also fairly stable. When the sodium levels drop, the kidney concentrates the urine and less is secreted. When the fluids move from interstitial spaces or from the colon, sodium is moved with it. This allows for a more stable plasma concentration of sodium, so the cells do not recognise that there is sodium depletion.
The notion that all grass and hay provides enough salt, or that horses will always get what they need from a salt block is not supported by pasture and hay test results or research. Horses are amazing at conserving sodium if the need arises, and will even excrete potassium as a substitute for sodium in urine. I don't want to confuse people, salt is so far the only documented craving in horses, they will travel long distances to a salt lick but the above hopefully will help with understanding why some horses simply won't access a salt block. The best way to supplement salt is in a feed, in addition a bucket of loose salt in the paddock is sensible too but NOT to be relied on. And please don't be fooled into thinking your horse knows always when he needs to grab salt from your salt block. Some don't even like the flavour.
Effects of feeding frequency and voluntary salt intake on fluid and electrolyte regulation in athletic horses
https://journals.physiology.org/doi/full/10.1152/jappl.1999.86.5.1610
Sweating. Fluid and ion losses and replacement
https://pubmed.ncbi.nlm.nih.gov/9561689/
Voluntary salt (NaCl) intake in Standardbred horses
https://www.researchgate.net/publication/279566836_Voluntary_salt_NaCl_intake_in_Standardbred_horses
