Effect of Various Diets on Glycemic Response in Horses

When horses eat grass, hay, or grain meals, digestion of these feedstuffs sends glucose into the blood. This rise in blood glucose is known as a glycemic response. Insulin is secreted into the blood in relation to the glycemic response, and if a large rise in glucose is met with a great amount of insulin, the horse can suffer an energy slump when the glucose level falls abruptly. Therefore, feed management strategies should seek a dietary treatment that will produce moderate rather than extreme rises in plasma glucose concentrations.

At Kentucky Equine Research, studies were designed to measure the differences in glycemic response when horses were fed different diets at three different levels of intake. Six Thoroughbred geldings used in the study were fed mixed grass hay. Each horse was also fed  one of six diets: cracked corn, whole oats, sweet feed (45% cracked corn, 45% whole oats, and 10% molasses), sweet feed plus 10% corn oil, alfalfa forage, or a low-starch, high-fermentable fiber mix (25% rice bran, 25% soy hulls, 25% wheat bran, and 25% soaked beet pulp).

The horses were randomly assigned to low (0.75 kg, about 1.65 lb), medium (1.5 kg, about 3.3 lb), or high (2.5 kg, about 5.5 lb) intake levels at each feeding during six test periods of three days each. Diets were split into two feedings each day. Only hay was fed during transitions between test periods. Horses were given access to free exercise on pasture during the day, although they were not allowed to graze.

On test days, morning feeding levels equaled 750, 1,500, or 2,500 g of the treatment diets in every case except sweet feed plus corn oil. Horses on this diet received 750, 1,500, or 2,500 g of sweet feed with an additional 75, 150, or 250 g corn oil, respectively. Blood samples were taken prior to the morning feeding on test days to determine baseline glucose values and at 30-minute intervals following feeding until glucose levels returned to or dropped below baseline for 120 minutes. The morning allotment of hay was fed following completion of sample collection. Area under the curve, mean glucose, peak glucose, and time to peak glucose were determined, and plasma glucose concentrations were analyzed. Using area under the curve for whole oats as a standard of reference, a glycemic index was generated for all diets and intake levels.

Differences in glycemic response were found between low and high intake levels of all diets. Sweet feed and whole oats demonstrated the greatest glycemic response, while alfalfa and sweet feed plus corn oil provided the lowest response. Plotting the glycemic index by feed and level of intake revealed an appreciable drop in the index for whole oats fed at 2.5 kg, compared to that at 1.5 kg and relative to glycemic indexes generated for other feeds and intake levels.

Mean glucose was highest for sweet feed, whole oats, and the low-starch, high-fiber mix. Mean glucose was lowest for the alfalfa diet. Peak glucose was similar for all diets except alfalfa forage. Time to peak glucose was greatly increased in the sweet feed plus corn oil diet, while the remaining diets demonstrated similar responses. Increasing the level of intake from 0.75 kg to 1.5 kg per feeding increased time to peak glucose by an average of 45 minutes.

Results of this study indicate that different grain diets demonstrate different glycemic responses. Adding fat reduces both area under the curve and peak glucose values as measured within this experimental design. More research is required to determine the relevance of glycemic response in predicting the effects of different feed ingredients on a horse’s performance or behavior.