The current wisdom is that all horses should be fed forages only and never any grain. While it’s certainly true that most horses do not need grain calories and are better off in many ways on a forage diet, there is still a place for grain in some circumstances.
Glucose is the most important fuel for all cells in the horse’s body. There is a baseline minimum requirement for glucose without which all cells would die. Glucose is so essential the horse’s body is equipped to produce it from other substances if dietary levels are low.
Muscle cells do pull glucose from the blood when working but they cannot get a sufficient amount this way. They are heavily reliant on glucose stored inside muscle cells in the form of glycogen.
With exercise, fat, amino acids and glucose are burned. The relatively percentages depend on the intensity of the exercise but can be influenced to some extent by diet and level of fitness. However, the bottom line for both high speed work and endurance type work (as well as draught work back in the day) is that glycogen stores are the limiting factor.
Racing, eventing and endurance are the sports where horses are most likely to face limited exercise capacity related to insufficient glycogen. Horses doing less strenuous work may be just fine with lower levels of glycogen in their muscles. As you might expect, signs of insufficient glycogen include inability to continue performing beyond a certain distance/time duration with slow work and inability to meet speed targets with faster work.
A study presented in 2014 (ICEEP) showed that even with high calorie intake from fat (23% of the calories) horses receiving only 18% of their calories from sugar and starch were unable to replace glycogen stores after heavy exercise while those receiving 36% and 43% did. Unfortunately, we don’t know if levels lower than 36% may have also been adequate.
What does this look like in a diet? For a 500 kg horse getting 1.25% of body weight in a high quality grass hay you would have to feed about 4 kg of oats or 2.6 kg of a mixed grain sweet feed. If the real requirement is only about 25% sugar/starch this would drop to around 2.7 kg of plain oats or 1.8 kg of mixed grain sweet feed and hay fed should be increased to closer to 1.37% of body weight. (Numbers are approximations.)
This of course is how countless generations of caretakers have fed high performance horses. It probably holds true for the upper level international endurance horses as well, at least during replenishment periods after races or heavy training.
The bottom line is that while grainfree, high forage diets make sense for most horses, grain is not poison and has a place in the diets of our upper echelon performance horses. From ancient Rome to pony express mounts to today’s elite equine athletes, grain is the concentrated fuel they need to keep going.
Eleanor Kellon, VMD