Water Losses in Horses

Written By Dr. Kris Hiney

This month we will discuss the most important nutrient in your horse’s diet, but maybe the most overlooked. Because providing our horse with water may seem obvious, many believe water requirements may not warrant discussion. But how much do you really know about how much your horse should be drinking per day?

The amount of water a horse needs to consume per day is directly related to how much water the horse loses per day. Horses lose water through four ways: their manure, urine, sweat, respiration, and if a broodmare, lactation. All of these variables must be taken into account when determining how much water our horses should be drinking. When we increase these losses due to variations in diet, work or environment, we must allow the horse greater access to water. Sometime that may mean we need to be creative in encouraging the horse to consume more water.

One of the greatest water losses to a horse is often overlooked, horse manure. While we tend to think of it as a rather solid form that we must continually scoop, pick up or shovel, horse manure is mainly water. This is especially true if the horse is eating primarily roughages. On an all forage diet, horse manure contains as much as 72-85% moisture. In fact, the water lost through their manure may represent almost 60% of a horse’s daily water intake. If we switch the horse to a grain based diet, the manure actually becomes much drier. Now, that does not mean that this may be a great strategy to minimize water losses. Overly dry feces can lead to impactions and colic, which is certainly to be avoided! When a horse consumes forage, it must be digested through fermentation which requires a fairly liquid environment in the hindgut and therefore normal gut health. In part, this is why it is recommended to always provide at least 1% of a horse’s body weight in forage per day. Consumption of forage therefore encourages water intake.

Variations in diet beyond just forage versus grain, can influence water losses in horses. The total amount of feed the horse eats will alter its water requirements. As consumption of feed increases, the horse must consume more water in order to allow normal digestive processes to occur. While we mentioned already that forage does increase water losses and thus water intake, the type of forage the horse consumes alters its water needs. Obviously fresh pasture grass contains a much higher moisture content compared to dry feed which is typically only 10-15% moisture. Growing grass may contain as much as 80% moisture. When taking into account the total amount of grass a horse can consume, simple grazing may approach a horse’s basic water requirements. Don’t be surprised then if your horse visits the water trough less frequently while he is grazing compared to when you feed hay.

Urine obviously contributes to water losses in horses, but remember that the volume of urine may reflect the water balance in the horse. Urine actually represents the most variable water loss in the horse, as other losses are more directly tied to diet, metabolic demands and environment. Some horses simply consume more water than others, and as a result will excrete more dilute urine in order to rid the body of excess water. Alternatively, if we fail to adequately meet our horse’s water needs, the kidneys will act to limit water losses and concentrate the urine. Additionally, if the feed contains components that need to be excreted by the kidney, water losses will increase. For example, when horses are fed protein beyond their requirements, the extra amino acids are broken down into components that can be used for fuel. This process involves the removal of the nitrogen found in amino acids. The kidney incorporates the excess nitrogen into urea, which is then excreted through the urine. Excess electrolytes, in particular sodium and potassium, must also be excreted by the diet. If you have a horse that likes to consume his salt at a greater rate, you may notice that his stall may be wetter than horses which do not perform such a practice. If you own an enthusiastic salt eater, you may want to limit his intake to strictly his salt requirements.

Sweat represents a tremendous variable in water losses for the horse, dependent on temperature and exercise. Remember that horses are most similar to humans in that we both dissipate heat through sweating, compared to other species that may rely primarily on respiratory cooling or panting. As horses must breathe through their nostrils, panting is simply not an option for them. Increasing the environmental temperature can increase evaporative losses between 45 to almost 400% of the horse’s normal water losses. The addition of exercise on top of environmental losses can quickly lead a horse to dehydration and heat stress if water losses are not replenished. For example, cross country horses have been reported to lose as much as 50-75 lbs of water during a competition due to the sustained duration of activity. Respiratory water losses are also directly tied to temperature and work load as these have the greatest influence on respiration rate. Horses increase respiration rate, either to aid in cooling, or due to the increased demand for oxygen delivery to the muscle tissue. However, relative to sweating, respiratory losses are relatively minimal.

The good news is that horses, through training or adaptation to their environment, do become more efficient at heat dissipation and begin to minimize their water losses. However, full acclimation to increased environmental temperatures may take up to 3 weeks. While it would be nice if weather patterns would gradually increase over time allowing our horses to adapt, we all know that this is simply not reality. Therefore, when the temperature gage rises dramatically, or even sporadically, we must acknowledge that our horses may not easily be able to dissipate heat. This will require more caution on our part when working our horses during this abrupt changes in temperature.

Lactating mares also have a significant loss of water through the milk. The amount of milk produced can be very variable between mares, with an average of 2 to 3% of their body weight per day. This will increase their water requirements somewhere between 50 and 75% over their normal requirements. If we also remember that lactating mares have a very high energy demand on their bodies, their feed intake increases as well. Remember that as feed intake increases, the horse must increase their water consumption to maintain digesta flow, and to counter the losses of water through the manure.

So what does all of this mean relative to what we need to offer our horses? My basic recommendation is to always allow the horse access to water beyond what they are willing to drink. In general a horse will consume around 10 gallons of water per day. This is easily accomplished by offering two full buckets twice a day. However, if you find that the bucket is empty when it is time to refill it, consider hanging an additional bucket. Your horse will thank you!

Next month we will delve more deeply into the current research on strategies to maximize your horses water intake. We all are familiar with the adage that you can lead a horse to water but you can’t make them drink. However, sometimes we really need them to drink! We will also discuss some feeding strategies that may help your horse stay hydrated through the various activities he may encounter such as traveling, endurance exercise, or exercise during hot temperatures. Remember, there is more to know about water than just filling a bucket!

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