Horse Nutrition: Carbs … good, bad or necessary? Part 2

Last week we told you in part one of this blog that there were five factors that influence starch digestibility in the small intestine. Here they are in more detail.

1) Source of starch
Over 80% of the starch in oats is digested in the small intestine, whereas less than 30% of the starch in whole-corn and barley is digested in the small intestine. Starches are made of sugars, predominantly glucose. So why is glucose from oat starch digested differently than glucose from corn starch? It is due to the architecture of the glucose molecules within the different starches. The following is an analogy to hopefully clarify. Take 45 gallons of wet cement and let it harden in a 45-gallon drum. After the cement sets, remove the drum and with a hammer and chisel start chipping away, it will take hours, if not days, until the cement is completely broken. Now, repeat the exercise but this time set the same 45 gallons of cement in a form to create a ½ inch wall. Using the same hammer and chisel it will probably take only minutes to break this cement wall. In this analogy, wet cement corresponds to unbound glucose, the harden cement to starch and the hammer and chisel is the digestive enzymes in the small intestines. Therefore, the architectural make up of the cement (drum vs. thin wall) influences the efficacy of the hammer and chisel.

2) Processing of starch
Whole or rolled corn and barley are hard on a horse’s digestive system. Research has shown that rolling barley or cracking corn barely improves small intestine digestibility. On the other hand, by cooking barley or corn, either by steam flaking or extrusion can increase small intestine digestibility up to 90%. This is why it is imperative that barley and corn be heat-processed when fed to horses. Referring again to the previous analogy, pelleting, steam flaking and extrusion are all procedures would make that 45 gallons of cement into a very thin wall.

3) Amount of starch intake per feeding
If you over feed starch to your horse a greater amount will pass through the small intestines and enter the hindgut and this could be harmful. A single meal should never supply more than 0.3-0.35% of starch in relation to your horse’s body weight (BW) (reference NSC 2008). Anything higher can lead to starch overload. In some cases, depending on the source and processing of the starch, only 0.2% of BW is recommended. Therefore, the maximum amount of a grain based feed that should be fed to a 500 kg horse at one time varies between 2 to 3 kg.

4) Time of hay feeding in conjunction to grain
Now I will go against the long-standing tradition that hay fed prior to grain slows things down. The foregut (stomach and small intestine) of the horse is anatomically similar to humans. It is common knowledge that fibre increases passage rate in our bodies. Yet, we mistakenly assume your horse’s fibre is actually a blocker, or to slow things down. I know what you’re thinking right now…this makes no sense. Keep in mind that this concept is heavily dependent on particular feeding practices: Amount of feed fed per feeding (a cup or 3 kg of feed), type of feed fed (starch based or a fat/fibre based), hay consumption (horse continuously eats hay or fed 3 flakes twice a day), feed processing (does the feed contain whole, rolled or heat-processed grains), will all determine if the time of feeding grain in conjunction with hay will have a significant effect or not. A research trial demonstrated that when horses were fed 2.3 kg of a starch based feed (sweet feed) with 2.3 kg of hay (either at the same time or 2 hours prior to the feed), the hay increased the passage rate of the digestion through the small intestine. Consequently, decreasing starch digestibility in the small intestine and increasing the amount of starch entering the hindgut. These results demonstrated that hay does not slow passage rate, on the contrary it increases passage rate through the small intestine.

Ultimately, there is no universally right or wrong time to feed grain in conjunction with hay. It depends on all the numerous factors mentioned above and on your individual horse. In an ideal world, the best feeding practice is to provide free choice hay and to provide small frequent feedings. Your horse’s digestive system is designed to eat 16 to 18 hours a day. In the end you will need to balance a practical and acceptable method that will work for you and your horse.

5) Individual differences between horses
Differences in eating behaviour, rate of passage, capacity to produce digestive enzymes, damage to gut lining can all affect a horse’s ability to digest and absorb starch. As you can see this is not something that is always easy to determine so but the good news is that we are here to help. Call your local Purina dealer or send us an email through our website to get one of our professional equine nutrition consultants assisting you with your ration today.

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