Several factors determine how quickly and thoroughly starch is digested by horses. These include properties of the starch granule; the effect of processing; associated food structures such as plant cell walls; transit time through the small intestine; and the availability and concentration of enzymes. These factors will affect the horse’s glycemic response to feeding and the subsequent production of insulin. Resistant starch, together with undigested starch, can pass into the large intestine, where it may be fermented to produce short-chain fatty acids.
Resistant starch may escape digestion in the small intestine of the horse because of physical entrapment within a food, such as in partly milled grains and seeds (RS1 starch), or because starch granules have a B or C crystalline structure, which is highly resistant to digestion (RS2 starch). RS1 and RS2 are quantitatively the most important forms of resistant starch found in horse feeds.
Regardless of starch source, total tract apparent digestibility is usually above 95%.
Differences are apparent between starch sources when measuring small intestinal or prececal apparent digestibility of starch, with rates of 78.2%, 91.1%, and 94.3%, respectively, for corn, oats and sorghum. These differences are mostly due to the differences between the starch granules contained in different plant materials, with oat starch granules being small and easily digested.
Crude physical treatments such as rolling, crushing or grinding do not significantly improve digestibility of corn or oat starch, but fine grinding (<2 mm) can improve small intestinal starch digestibility of corn. Cooking, either by micronizing or popping, can lead to significant improvements in prececal starch digestibility, particularly with respect to corn.
Small intestinal starch digestibility generally declines as starch intake increases, which is one reason that horses should not get grain meals larger than about 4 to 5 pounds (2 to 2.5 kg) at a time. Depending on the starch source, larger meals increase the chance that the capacity of the small intestine to degrade starch could be exceeded.
Normal variability between horses means that some horses are better able to digest starch than others. For example, horses that eat rapidly and/or do not chew feed thoroughly will have reduced small intestinal starch digestion because the pieces of swallowed grain will be larger.
Other differences may exist between horses in terms of enzyme production. Small intestinal passage rate will vary between individual horses, even when fed in the same way. Thus, the notion that there is a “safe” upper level for meal-feeding horses or ponies with starch is questionable, and owners should err on the side of caution when deciding on meal size for their horses when the meal involves grain.
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