It is important not to underestimate the effect that nutrition and feed management can have on general equine health, ranging from digestive upsets to behaviour abnormalities the list could go on. This article covers the functioning of the horse’s digestive system, natural feeding behaviour and appropriate feeding practices to promote optimal health.
The horse is a grazing animal and will naturally spend 17+ hours a day grazing, continuously consuming small amounts of fibrous material throughout the day, equating to a high intake of feed. Their digestive system reflects this feeding behaviour and hence feeding management should match this as closely as possible. There are several considerations of the anatomy and physiology of both the foregut and hindgut that determine feeding practices. Starting with the foregut:
· Chewing produces saliva
· The stomach constantly secretes gastric acid
· The stomach is small in size
· The rate of passage of digesta through the stomach and small intestine
is relatively fast and is made quicker when larger meals are fed
· The small intestine is the site of digestion of non-structural carbohydrates,
proteins, and lipids.
Each of the above are adaptations to a near continuous flow of feed through the digestive system and disruptions can occur when this is not the case. For example, a horse that is fed a high amount of grain or compound feed, with two meals of hay fed during the day and limited access to pasture may have several negative effects on the horse’s health and wellbeing.
A horse on a low forage, high concentrate diet will have a decrease in chewing. Studies have shown that less chewing is required for concentrates than long stem forage, with 1kg of concentrate taking 800-1200 chews in contrast to 1kg of hay taking 3000-3500 chews. The time taken for the horse to consume 1kg of concentrate is also significantly less than the time taken to consume 1kg of hay. Two main implications of this are that the horse spends less time eating and saliva production decreases. It is estimated that a stabled horse on a low forage, high concentrate diet may spend less than 15% of its time eating. This may contribute to an increase in or the development of abnormal behaviours, also known as stereotypical behaviour as the horse is unable to spend majority of its time grazing as it naturally would so it adopts other behaviours to cope or fill their time. Secondly, saliva is only produced through the action of chewing, as chewing decreases so does saliva production. Saliva contains bicarbonate which aids in the buffering of the constantly secreted gastric acid within the stomach. Decreased buffering leads to more acidic stomach acid, coupled with a lack of feed in the stomach to act as a physical barrier protecting the squamous mucosa of the upper part of the stomach, may further predispose (particularly the ridden horse) to the formation of gastric ulcers.
The horses relatively small stomach and small intestine and already quick rate of passage of digesta mean that it is unable to cope well with large, intermittent meals. When a large meal is fed, the rate of emptying from the stomach into the small intestine increases and the rate of flow of digesta through the small intestine also increases. This decreases the digestion and absorption of carbohydrates, proteins, and lipids and hence the body’s ability to absorb and utilise these nutrients. The method of processing of feed material will also affect the amount of digestion that occurs in the small intestine, with the digestibility of raw grains being far lower than the digestibility of extruded (cooked) grains. Of particular concern here is the decreased digestion of non-structural carbohydrates particularly starch, as undigested material will travel to the hindgut. This brings us to the considerations of the hindgut:
· The hindgut contains a large microbial population responsible for the digestion of fibrous material
· The microbial population of the hindgut thrive in particular environmental conditions
· Material not digested in the small intestine travels to the hindgut
The hindgut (caecum and colon) is home to an extremely numerous microbial population that require certain environmental conditions to survive and function. This population plays a crucial role in the digestion of the fibrous material the horse consumes, which contributes to 60-70% of the horse’s energy intake. This delicate balance can be disrupted by both a lack of fibre and the increased presence of starch in the hindgut. A lack of fibre reaching the hindgut means a lack of feed for the microbial population, generally causing a decrease in numbers and hence the fermentation of fibre which may result in weight loss, vitamin deficiencies, diarrhoea, and poor hoof quality. Large amounts of starch reaching the hindgut because of large meals or unprocessed grains can also cause a decrease in fibre fermenting bacteria. This is mostly due to the rapid fermentation of starch in the hindgut, resulting in the production of lactic acid; this decreases the pH and kills of the fibre fermenting bacteria which prefer an environment around a pH of 6. These starch fermenting bacteria also produce endotoxins that may absorbed and contribute to laminitis. A prolonged decrease of pH to 5.8 or lower may lead to hindgut acidosis, anorexia, epithelial damage, and suboptimal nutrient absorption because of the reduction of gut villi. Hence, keeping the hindgut microbial population and the hindgut pH balanced and stable is key to avoiding disease and digestive disturbances.
In summary, the gut microbial distribution is dependent on the interaction between the following three factors:
DIET - GUT PH - GUT MICROBES
Overall, this highlights the importance of the equine diet and feed management for general health and wellbeing. Fortunately, much of the above can be prevented with appropriate feeding and management:
· Allow constant access (or as close as possible) to pasture or forage
For horses that are stabled or on limited pasture ensure the provision of a minimum of 1.5% of the horse’s body weight per day in forage and allow access to this throughout the day and night. This promotes chewing, saliva production, the buffering of gastric acid, feeds the fibre fermenting bacteria of the hindgut, keeps the gut full and allows the horse to exhibit natural feeding behaviours more closely.
· Feed compound/supplementary feed little and often
A general rule of thumb is if you are feeding more than 0.5% of your horse’s body weight (2.5kg for a 500kg horse) per meal this should be spread out in multiple meals through the day. Studies have shown that horses fed the same quantity of feed over 3 meals a day had less changes to the hindgut population and pH, in comparison to horses fed once or twice a day. This reduces the rate of passage of feed through the small intestine and allows maximal digestion, while also preventing starch from entering the hindgut.
· If feeding grains, feed only extruded grains
Horses have a low capacity for the enzymatic digestion of starch in the small intestine. Hence, current feeding recommendations from studies on the equine hindgut suggest that highly digestible grains are fed in lower amounts to meet energy requirements, over less digestible grains in higher volumes. The extrusion of grains breaks down the barriers of digestion of starch, making the starch readily available for digestion. This allows starch to be effectively digested in the small intestine and prevents starch from entering the hindgut.
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