Feeding...the Inside Story

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Your horse is a grass-eating machine! He has evolved over millions of years to survive on a diet of nothing but grass and although he is adaptable and can eat grain and hay when we need him to, his digestive system is designed as, and will always work best as a perfectly oiled grass-processing system.

Ever been told you eat like a horse? It is true horses eat a great deal, usually more than any human could hope to compete with! Even if your horse was the same size and weight as you, he would need to eat a lot more than you to maintain his weight and have the energy to run around and jump fences.

Horses have evolved as grazers and have developed a special way of processing grass and other forage to get the most out of every mouthful. Because grass doesn’t have much energy compared to the foods we eat like meat, potatoes and vegetables, horses have to eat a lot more to supply their everyday requirements.

In the wild, horses eat slowly for most of the day and night, grazing over large areas for up to 18 hours per day, picking out the most nutritious grasses. Food is moving through them almost constantly....as you know all too well if you have the responsibility of cleaning a horse’s stable after he’s been left with a large haynet overnight!

Horse feed is made up of carbohydrates (simple sugars and starches), protein, fat, fibre, vitamins, minerals and electrolytes (salts). These get extracted from the feed at different stages of the digestive system.

Figure 1 Figure 1

If you could look inside your horse, you would see that his digestive system takes up a very large amount of the space in his body (see figure 1). This reflects the importance of the digestive system to the well being of the horse. The system is well adapted to dealing with tough fibrous material such as hay, pasture and chaff. The four major sections of this system are the major topic of this article:

1. The mouth and teeth–where food is chewed and ground into small pieces.

2. The stomach–where acids and enzymes are added to start dissolving food particles.

3. The small intestine–where most parts of the food ard digested and absorbed into the blood.

4. The large intestine–where the horse allows its host of resident bugs to digest the remainder of the food so that the horse can use it, leaving the rest to pass out of the horse as feaces.

The Mouth and Teeth

Looking inside your horse’s mouth you see lots of large teeth. These teeth go right to the back of the horse’s jaw and are wide, flat and perfectly suited to grinding and crushing hay and feed. The tongue helps to ball the food up and move it around the mouth as it is chewed. Horses chew about 60,000 times per day so have very strong jaw muscles. When feeding horses hay and grass needs to be thoroughly chewed before it can pass on to the next part of the system. If your horse cannot chew properly, it makes the job of the digestive system that much harder and the horse will not get the full benefit out of his food.

It’s very important to get your horse’s teeth checked and rasped at least once per year, and more regularly for older horses. Chewing makes the food wet and softer with saliva. Food is ground into small pieces for the next part of digestion: the stomach.

The Stomach

Food passes from the mouth down the oesophagus to the stomach. The horse’s stomach is quite small and can’t hold a lot of food at one time. If you give your horse a large meal and he eats it quickly at one sitting, he may show signs of colic as the stomach gets swollen with too much food at once.

It’s important to obey the ‘feed little and often’ rule with horses to avoid these problems. As your horse chews and eats his hay, the food goes down and passes through the stomach to the next stage fairly quickly (about 20 minutes from one side to the other) so there is a continuous stream of food passing through. In the stomach, the chewed food is mixed with strong acids and enzymes which are made by cells in the stomach wall. These break down and dissolve the food, allowing a small amount to be absorbed through the stomach wall. There is a good blood supply all around the stomach and intestines–when the food is dissolved, some parts can pass through the stomach and into the blood. Once in the blood, the food goes to parts of the body that need it, like the muscles during exercise. If the horse doesn’t need the energy right away, the food is stored for later use either in the muscles, in the liver or as fat. Once food has passed through the stomach it goes to the small intestine.

Figure 2 Figure 2

The Small Intestine

The small intestine is quite long, around 21-25m in length. It’s the first real site of digestion and the part of the system where grain and concentrate horse feeds are digested. Sugar, protein, fat, minerals, vitamins and electrolytes (salts) are taken up from the small intestine into the blood for use in the body. The strong acid of the stomach must be neutralised to prevent it dissolving the walls of the small intestine. Bile and other alkaline secretions produced from cells in the small intestine walls do this job as soon as the food enters the small intestine. Digestive enzymes are added to the partially digested food in the small intestine and these need a fairly neutral environment to work properly.

Food spends between one and eight hours travelling through the small intestine. The protein, fat and about half the available carbohydrate contained in the feed is dissolved and absorbed into the blood, leaving only the tough fibre portion which passes on to the large intestine.

If you feed your horse a large grain meal, or if he gorges himself on rich green pasture, then the small intestine might not be able to digest all the sugar from the food before it reaches the large intestine, so the large intestine gets a sudden rush of sugar that it’s not used to dealing with. This can be bad for your horse as it can cause colic or laminitis.

The quantity of digestive enzymes in the small intestine increases slowly in response to changes in diet so all changes should be made slowly to ensure complete digestion in the small intestine. Some grains have a very tough outer coating which can only be digested in the large intestine. This coating must be broken down before the starch inside can be digested, so these grains should not be fed whole. This is why many grains like barley or corn are crushed, cracked, pelleted or cooked, so the starch can be more thoroughly digested before reaching the large intestine.

The Large Intestine

Although only 7-9m long, the large intestine is very wide, holding a large amount of partially digested food, water and microbes. It’s made up of a large section called a caecum, and two large sections called the colon (see figure 2).

The large intestine is designed as a fibre digestion site. Horses do not have the ability to digest some forms of particularly tough fibre found in hay and grass. To get over this problem, horses have a very special relationship with a number of bacteria and other small bugs (protozoal and fungal microbes) that live in the large intestine. These good bacteria live and work their entire lives inside the horse, digesting the fibrous portion of the diet and turning it into a form the horse can use. The bacteria benefit by having a constant food supply in a protected environment, and the horse benefits from being able to use energy from the portion of the diet that it would not otherwise be able to get to.

It can be beneficial to feed more hay during winter. It can be beneficial to feed more hay during winter.

Fibre digestion takes quite a long time–50 to 60 hours for food to get through this part of the system. The fibre digesting microbes produce some heat as they work which is why it can be very beneficial to feed a horse more hay in the cold winter months. Feeding a lot of fibre means that the hindgut will get very full, the horse drinks lots of water when he eats a large amount of hay and together these make his belly look big and round. Horses that get a lot of grain and not so much hay will have a leaner, more greyhound-shaped belly, like race horses and trotters.

Bacteria and microbes are fairly temperamental and easily upset by changes in their environment which is why you have to be careful about what you feed. If you change the diet too quickly, the microbes don’t have time to adjust to the new food source and may die in large numbers.

If a lot of starch (from grain or green grass) makes it through to the large intestine, the bacteria will process it, but unlike the useful product of fibre digestion, they produce a type of acid, which ultimately kills some of the bacteria. When bacteria die, they release toxins and these are responsible for colic or laminitis after a big meal.

The digestive system of the horse is perfectly suited to their natural diet of grass. By domesticating horses and asking them to do work, we have had to find ways of feeding them a diet with more energy, and one that can be fed in the confinements of a paddock or stable. We have to be very careful to try and mimic the eating patterns of horses in the wild so their digestive system receives a continuous flow of food and is not overwhelmed with large high starch meals that we feed for energy.

It’s best to feed at least two or three small meals of grain per day with plenty of hay, pasture or chaff. If your horse does not have to work too hard he will manage quite well on a diet of hay or pasture alone. Horses are designed to eat grass so the more natural roughage we can feed, the better the digestive health of the horse will be and the less likely you are to see problems like colic or laminitis.

By Sonja Gardner BSc

Kentucky Equine Research
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