Hydration

Christina Boucher Reynolds, PAS

With the oncoming heat and humidity paired with show season and trail riding in full swing, it’s a good time to re-evaluate whether your horse’s hydration is where it needs to be.

To give you a quick snapshot of where the thin line between moderately dehydrated but fit to continue and dehydration that requires IV fluid replacement is, it is just 2-3 gallons of unreplaced fluids. 5% dehydrated (or about 6.25 gallons unreplaced fluid loss) is the point where most endurance horses finish and are judged by a team of vets as “fit to continue”, which means the pair has completed the ride successfully- in endurance you don’t just get to cross the finish line, you must also be judged as “fit to continue” as if you still had another 15 mile loop to complete. At 8% fluid loss (or about 9 gal unreplaced fluid), the horse is in the treatment box being given IV fluids. At 12% dehydration, the horse is likely in organ failure and at risk of dying. The difference between tolerable fluid loss and organ failure is a mere 8 gallons.

There are about 80 gallons of fluid in the horse and about 2/3 of it is not easily accessible because it is inside of cells. 26 gallons are in the extracellular space- ie not inside of cells- in blood, saliva, urine and fluids in the gut. A loss of 2-4 gallons of fluids equals an 8-15% loss in the circulating fluids until the body can share fluid from inside the cells or they are replenished by drinking or administering IV fluids.

At 2% dehydration, which you are likely at before even starting exercise in the summer or after you trailer to an event, the body starts to conserve fluids and the first place it starts is saliva production. You can see this when checking the gums, they will have a “tacky” feeling. Decreased saliva production can increase the chance of esophageal obstruction- choke, especially for horses eating dry hay. Feeding a wet mash, soaked forage or regular grass can help reduce the risk of choke. For every hour of trailer transport under ideal ambient conditions, in a good trailer with a considerate drive, a horse will become 1% dehydrated for every 90 minutes of travel. We rarely have ideal weather conditions in NC! Horses arrive at their show or trail ride already behind the eight ball in terms of hydration. For horses travelling a long way, 8 hours in the trailer will easily have them at the 5% dehydration level where they are feeling the effects of dehydration. They are unlikely to replace their fluid levels by themselves overnight and are likely to begin the competition with a level of dehydration that will negatively affect their performance. Giving the horse extra time to recover after trailering is a good way to increase their ability to perform. In humans, 3% dehydration means a 10% decrease in muscle strength (think driving up a hill or jumping) and 8% decrease in muscle speed (think ability to recover after a stumble or make a quick turn in the ring). We don’t have research numbers on that in horses, but we can assume it will be similar.

The first bodily fluid to be affected by fluid loss during exercise is plasma- the watery portion of the blood. In extreme conditions, horses can lose 4 gallons of fluids per hour in sweat. As the watery portion decreases, the blood becomes thicker and harder to transport around the body, which is where you often see a “hanging heart rate” that doesn’t want to drop below 60 or so. This is a useful indicator that the horse is being adversely affected by the level of dehydration. To get proficient at checking heartrate and using it to understand both cardiovascular fitness and dehydration effects, you can use either your hand or an inexpensive stethoscope. Place it behind the left point of elbow near the girth and listed for the ”lub-dub”, and count each paired “lub-dub” as one heartbeat. You can use your watch or the timer on your phone to count the beats for one minute. Find out your horse’s resting heart rate first and check it a couple times over a couple of weeks to establish what “normal” is for your individual horse. This is helpful knowledge for your vet too, when you call in an emergency. Temperature, heart rate and respiratory rate give the vet a ton of information as they decide how much of an emergency they may have on their hands. Once you know the horse’s resting heart rate, check it after your rides and see what the number is. You can check it every 5-10 minutes post-ride to evaluate your horse’s ability to recover. If you have a fit horse, they will likely come in from moderate exercise around 75 bpm and fairly quickly drop below 60 and then to their resting heart rate. An unfit horse will be slow to drop below that 60 mark but will progress down gradually, and a dehydrated horse will drop a little then remain the same for a while.

After decreasing saliva production to conserve fluids, the next thing a horse will do is decrease sweating. They will do this even while still exercising, which can be a problem as sweating is a major way the body can decrease thermal load and avoid hyperthermia. Clipping horses, keeping heavy manes braided and off their necks, and sponging down regularly with water can help minimize the effects of the decreased sweating.

Unfortunately for us, horses’ thirst mechanism is not well designed for what we ask them to do. In humans, sweat composition is saltier than our blood, so we have a cue to drink (thirst) relatively early in the dehydration process compared to horses whose sweat is less salty than their blood. It takes a while for the blood sodium level to increase enough to trip their thirst cue. Horses also rely more on their blood pressure dropping due to dehydration to signal thirst than the blood-sodium level alert that humans use. This is a slower process than the sodium trigger, and often means horses aren’t interested in drinking until they are much more dehydrated. It’s important to note that horses cannot be encouraged to drink by squirting water in their mouth. In fact, this can prematurely shut off their thirst drive by tricking the brain into thinking the horse has drunk water thanks to a reflex that shuts down thirst when it senses water in the mouth. Good for helping prey animals not get eaten at the water hole, bad for dehydrated horses in the sweaty Southeast who are going to compete after a trailer ride.

So how can we help horses stay hydrated? Establishing good hydration leading up to an event so they start the process with a good hydration status. Make sure clean water is readily available 24/7 leading to the event. Research in endurance horses shows they drink 10% more water if their eyes never go below the rim of their container. University research shows horses drink more from turquoise buckets. Research at New Bolton showed horses prefer to drink water around 45 degrees F regardless of outside temperature, but will drink more water at 65 degrees F if there is no cold water option. Offering “treat water” may entice horses to drink more and may help picky horses. You can add a couple handfuls of rice bran and/or senior feed to the bottom of a bucket before filling and offer it before travel and at stopping points along the way. Dr Garlinghouse’s tip is to make a soupy mash of your horse’s regular feed plus beet pulp or hay cubes and hang it in the trailer, adding a piece of 2x4 floating on the top as a baffle to keep it from sloshing out. Increasing the amount of sodium in the horse’s diet leading up to the event can help them drink more; Dr Vandergrift recommended a couple of tablespoons of baking soda split between their meals in the days leading up to a trailer ride or event. The baking soda provides just sodium which kicks on the thirst drive, whereas salt provides both sodium (thirst drive) but also chloride (signal to pee out extra fluid). Once your horse is exercising and sweating, using a suitable commercially available electrolyte blend will provide the electrolytes your horse is losing through sweat. I personally like the Apple A Day product which provides a high level of the most needed electrolytes, plus a higher level of calcium that heavier muscled horses need, and without added sugar. My preferred paste electrolyte is EnduroMax which is a similar high level of needed electrolytes and no extra sugar but also has a gastric buffer, which I recommend anytime you’re feeding electrolytes by oral syringe.

In summary:

-horses become dehydrated quickly during hot weather

-trailer rides contribute a significant amount of dehydration before the horse even starts work

-re-hydration takes time and some planning

-horse’s thirst reflexes often don’t match their state of dehydration

-dehydration has a marked effect on performance

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