Enhancing horse productivity and well-being has drawn considerable interest in feeding fat-supplemented diets to horses. Much of this interest has been prompted by a significant amount of research on the subject over the last ten years. While several unknown aspects of specific fatty acid nutrition in the horse still exists, the caloric and some extra-caloric effects of feeding fat to horses are well documented. Within this article, the word “fat” will refer to both fats and oils, except where specific reference to one or the other is made.

Palatability or Acceptability of Fat-Supplemented Diets
Most horses readily accept fat-supplemented grain mixes containing up to 10% added fat. Occasionally a timid or “finicky” eating horse may require more time before readily consuming a fat-supplemented diet. Most horses will resume their normal feeding behavior after consuming three to five meals containing a fat supplement. Of course, this assumes the fat is of high quality and is not rancid.

When feeds contain very large amounts of fat (greater than 10% by weight), it takes longer for horses to readily accept the diet. For best acceptability, fat should be introduced into horses’ diets in small amounts (two to three ounces per meal), then gradually increased over at least a week, until the desired concentration of fat in the diet has been reached.

Generally, high-quality vegetable oils are accepted by horses more quickly than lower-quality oils or animal fats, but most horses will even consume diets containing raw, rendered animal fat provided it is good quality. For best results, fat should be incorporated directly into horse feeds. Top-dressing feeds with fat supplements can be done effectively if the fat supplement is mixed into the feed in the feeding trough. Top-dressing a fat supplement is easier with textured feeds than pelleted feeds.

Digestibility of Fat-Supplemented Diets
The digestibility of energy in fats and oils is almost always as high and most of the time is higher than that in the other energy-yielding constituents of feeds. Thus, adding fat to horse feeds always increases the digestible energy density of those feeds. Interestingly, there are very few reports of any negative effects on the digestibility of the ration from adding fat supplements. This is because fat digestion in the horse occurs entirely in the small intestine, and fat that escapes digestion in the small intestine does not seem to have harmful effects on fermentive digestion in the cecum and colon, particularly when fat provides 20% of the digestible calories or less.

The digestibility of energy in vegetable oils can be 90% or more, while animal fat is usually near 75%. However, over 95% of the digestible fat in fat or oil is digested in the small intestine. Thus, diets for horses can contain greater amounts of fat than those for ruminants without creating negative effects on digestion of other diet components. Therefore, feeding fat is a very effective and safe way to increase energy intake in horses with high energy requirements.

Since fat is digested enzymatically in the small intestine and can be substituted into horse feeds in place of starchy ingredients, fermentive digestion of starch can be reduced in horses with high feed requirements. This will reduce the likelihood of colic or laminitis frequently associated with feeding large amounts of starchy feeds.

Metabolic Effects of Feeding Fat-Supplemented Diets
Several metabolic advantages exist from feeding fat-supplemented diets to horses. Several studies have shown the digestible energy in fat-supplemented diets is used more efficiently by the horse than that in conventional low-fat diets. For example, adding fat to the diet reduces the amount of heat energy loss and increases the net energy available for reproduction, lactation, growth or work. This is due primarily to a reduction in feed requirements and a reduction in the heat of fermentation, both of which contribute to lower heat production. Also, lower heat production reduces the thermal load on the horse, and thus, reduces their maintenance requirement during hot weather.

Perhaps the most dramatic and useful result of feeding fat-supplemented diets to horses is the extra-caloric effect on energy metabolism in the equine athlete. When fat is substituted into the diet of horses that perform short-term, high-velocity exercise, and the horses are allowed sufficient time to become adapted to the diet, it has a sparing effect on muscle glycogen stores. This sparing effect results in increased concentration of glycogen in the muscle of adapted horses, which can be mobilized to produce energy anaerobically during short bursts of maximal power output.

The storage and retrieval of glycogen is essential for the elite equine athlete to have a short-term supply of energy when they reach a state of oxygen deficiency during performances/races. It is important to note that this sparing effect on glycogen stores from adapting horses to a fat-supplemented diet occurs only when the horses are fed a diet containing sufficient carbohydrates to facilitate the enhanced storage of glycogen. Further, the horses must be trained appropriately for the glycogen-sparing effects of feeding fat to be realized.

Effects on Growth
The effects of feeding fat-supplemented diets to growing horses appear to be primarily caloric—most, if not all, of the response is a reflection of the number of calories fed. Weanlings and yearling fed fat-supplemented diets will gain weight faster and may be more efficient due to lower feed requirements than when fed conventional diets, but responses can easily be explained by differences in calorie balance.

It is particularly important to rebalance growing horses’ diets when fat is added to maintain necessary nutrient:calorie ratios. When growing horses have been fed liberal amounts of fat-supplemented diets that were balanced to correct nutrient:calorie ratios, no detrimental effects on skeletal or other developments have been reported from controlled feeding trials.

Effects on Reproduction and Lactation
The benefits of feeding fat to brood-mares have also been related primarily to caloric and metabolic effects discussed previously. However, recent work indicates that feeding fat to broodmares may alter the lipoprotein profile in the blood, thus affecting the synthesis of endocrine compounds involved in regulation of reproduction.

There is some indication that feeding fat-supplemented diets to broodmares, even at a comparable energy intake, may shorten the post-partum interval and increase conception rates. When lactating broodmares were fed fat-supplemented diets, the concentration of fat in milk increased, and their foals grew faster compared to their counterparts which were fed conventional, low-fat diets.

Effects on Performance/Race Horses
Undoubtedly, the most dramatic affects of feeding fat-supplemented diets to horses have occurred in the equine athlete. Further, these effects are more pronounced in horses that must work anaerobically during a significant part of their performance. Adding fat to the diet of any equine athlete will result in the metabolic advantages previously described— reduced feed requirements for a comparable amount of work, reduced heat production, reduced thermal load and maintenance requirements for thermal regulation, increased energy available for work, etc. In addition to these caloric effects, it is now known that production of power and stamina can be increased for several seconds in the horse performing short-term, high-velocity exercise by appropriate adaptation to a fat-supplemented diet.

It has been shown that race horses can run faster at a constant heart rate and cutting horses can stop and turn harder for a longer duration after appropriate adaptation to correctly formulated fat-supplemented diets. Some of these effects are due to the caloric benefits described earlier, but the primary effect is due to the improvement in glycogen storage and mobilization that results from feeding a fat-supplemented, high-carbohydrate diet for a sufficient period of adaptation.

It appears that the initial onset of acute fatigue in the race horse, cutting horse, reining horse, etc. is the depletion of glycogen stores in the liver and muscle. Thus, feeding an appropriately balanced, fat-supplemented diet along with a correctly designed training protocol offers great promise as a way to improve performances and perhaps reduce the frequency of injuries to elite, equine athletes.

An interesting side note: It has been recently revealed that horses suffering from recurrent exertional rhabdomyolysis (RER) may have a specific genetic defect that limits glycogen mobilization in the muscle. I have worked with horses that had a high incidence of RER, and preliminary analyses of the feeding and training program indicated the horses were probably suffering from glycogen depletion following workouts. The horses were placed on a fat-supplemented diet, and anecdotal results from consuming the fat-supplemented diet indicated the horses must have been glycogen depleted in the previous routine, because feeding the fat-supplemented diet dramatically reduced the incidence of RER. Thus, RER in a specific horse may be the result of a genetic defect that prohibits glycogen metabolism when needed, or it may be the result of a feeding and training program that does not promote glycogen storage in the muscle.

Workable Amounts of Fat to Feed and the Adaptation Period
Although larger amounts can be fed, most work to date indicates the effective concentration of fat needed to achieve the effects described is an amount that will provide 18% to 20% of the digestible calories in the diet from fat. This can be achieved by formulating a total diet (grain and forage) that provides 8% to 9% total fat. To obtain the beneficial effects described for performance horses, adequate amounts of carbohydrates must be fed in the fat-supplemented diet. It is doubtful the described effects would be observed if horses were fed a fat-supplemented, high-forage diet.

Another way to provide the desired calories from fat is to add approximately 10% fat to a high-carbohydrate diet, balance the diet for proper nutrient:calorie ratios and feed the diet in approximately a 65:35 ratio with hay. While higher amounts of fat can be fed to horses, any benefits of adding fat to the diet of horses in amounts that provide more than 18% to 20% of the digestible calories from fat are not evident at this time and are not practical from a management standpoint. Feeding smaller amounts of fat may be beneficial to the general health of the horse, but to achieve maximal benefit as described herein, feeding the amounts of fat previously described are necessary.

Equally important in realizing the benefits from feeding a fat-supplemented diet to horses is to be aware that the responses described do not occur immediately following introduction of the fat-supplemented diet—a period of adaptation to the diet is required. Digestive adaptation to the fat-supplemented diet occurs comparatively quickly—perhaps in one week. Thus, horses can be brought up gradually over a week to the amounts of dietary fat previously described. However, metabolic adaptation to the fat-supplemented diet in the performance horse requires three to four weeks. Also, the training regimen during the adaptation period must include sufficient anaerobic work weekly to activate the glycolytic muscle fibers.

Research references provided upon request.

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