This natural feeding regime is very sympathetic to the structure of a horse’s digestive tract but the domestication of the horse as a work animal, including the subsequent and necessary changes made to their dietary and feeding habits, has helped contribute to the many feed-related problems that can now occur.
The digestive tract of equines can be envisaged as divided into two sections: the first of which (stomach and small intestine) is similar to that of a monogastric such as a pig, man or dog: and the second section comprises a well-developed hindgut, which is similar, in a way, to the rumen of a cow. Horses are called ‘hind-gut fermentors’ and this is where the serious digestion really occurs. The caecum and colon can hold over 100 litres of feedstuff in various stages of digestion.
As with all animals the digestive process starts in the mouth with thorough chewing of the food materials to reduce particle size as an aid to digestion and to allow easier swallowing. During chewing saliva is produced; this acts not only as a lubricant to ease the passage of the chewed food but also saliva contains bicarbonate, which helps to protect the vulnerable areas of the stomach from the strong acids that are produced during digestion.
Ideally, all equines should be fed a high fibre diet with at least half their energy needs coming from roughage/ fibre: those that are fed a ration that is high in long fibre (hay, haylage, grass…) chew more and so produce more saliva.
After chewing and the start of its digestion (breakdown) in the mouth, the food material moves along to the stomach. In the acidic conditions of the stomach food is mixed thoroughly; the stomach has two major functions during the digestion process in the horse: firstly to continue the breakdown of the feed by digestive enzymes and secondly to regulate the rate of passage of feedstuffs from the stomach; the food matter only remains in the stomach for a short period of time before it passes into the small intestine: horses have a relatively small stomach, 5 – 15 litres - that is designed for trickle/ little-and-often feeding.
The feed also travels quite quickly through the regions of the small intestine; this is about 21-25 metres in length and is where much of the enzymatic digestion of fats, starch, sugars, and protein takes place, and also the absorption of many minerals such as calcium, zinc, copper and fat-soluble vitamins. Starch is digested (broken down) by glycanase enzymes, oil is digested by lipase enzymes and protein is digested by protease enzymes that are produced in the pancreas or small intestine.
Diets fed as large meals only once or twice a day result in the rapid passage of food through the stomach and small intestine reducing the efficiency of the digestion, increasing the possibility of undigested starch reaching the hindgut. The hindgut is adapted to digest fibre in the horse’s diet through a process of fermentation and here its rate of passage slows considerably.
Horses are not able to digest fibre using their own digestive enzymes and so are dependent on the microbial populations found throughout the length of the digestive tract from the stomach to the terminal colon; the caecum contains billions of coliform forming units (CFU’s) microflora per millilitre of caecal fluid consisting of bacteria, yeasts, fungi and protozoa.
Basically the hindgut is a highly developed large intestine, consisting of the caecum, large colon, small colon and rectum; in the hindgut the feed is turned into usable nutrients that will be absorbed through the gut linings. Digestive enzymes and yeasts break down the feed; it is the billions of bacteria that further process the simple sugars, amino acids and fatty acids for absorption through the gut wall. Beneficial bacteria assist in the fermentation of fibre plus the synthesis of protein and a host of B vitamins; pathogenic microflora contribute to disease, which may cause digestive disturbances, scouring and colic.
Probiotics (yeasts, non-harmful bacteria, fermentation products and enzymes) can usually curb digestive upset symptoms within a 24 hour period, but it takes longer, probably 3 to 7 days to change the microflora balance and re-establish good hind-gut fermentation.
During the process of fermentation fibrous material is broken down and converted into substances called volatile fatty acids (VFAs). The horse absorbs these VFAs from the hindgut into the body, where they are used as a source of energy; when enzymes provide better digestion of fibre the animal gets more calories and more varied nutrients from the feed.
The starches and simple sugars found in cereals that are commonly fed as part of animal diets would normally be digested in the small intestine; a horse’s G.I. tract does not have the capability for large amounts of starch digestion in the small intestine and any starch that remains undigested on reaching the hindgut will be fermented rapidly, leading to a build up in acidity.
Despite the knowledge that large amounts of cereals can be detrimental to the horse, affecting the hindgut environment, many of the diets fed to competitive horses place great stress on their digestive systems: they are often fed large high cereal-content rations and have limited access to forage.
Diets containing high levels of cereal starch and limited quantities of forage have a negative effect on the hindgut environment and alter the balance of microflora; the population and balance of different microflora in the equine gut is changing constantly; this balance of microflora can be affected by numerous factors including the composition of the diet, particle size of the ration, time (of day) of feeding, environmental stress, drug treatments (wormers, antibiotics…) etc.
The Value of Yeast
When hind gut efficiency is impaired, for example on low-fibre diets, it is important to supplement the diet with the B vitamins that are usually ‘manufactured’ in the gut. Probiotics will help to restore hindgut efficiency i.e. yeasts will help the beneficial (cellulolytic) bacteria to multiply because they are a rich dietary source of B vitamins and amino acids, whilst oligosaccharides (prebiotics) will bond with undesirable (pathogenic) bacteria in the gut and effectively remove them, excreted in the faeces.
As previously mentioned, increased acidity results from a change in the relative proportions of the different volatile fatty acids produced during the fermentation of starch, and is largely responsible for imbalances in the microbial flora.
These imbalances can be common because millions of beneficial bacteria die naturally and are lost from the digestive tract every day; maintaining the healthy balance is essential to support normal digestion and immune function thereby helping the horse maintain health, vitality, weight and condition.
The term yeast designates a group of fungi which have certain outstanding characteristics in common; live yeast is scientifically proven to help maintain a healthy balance of beneficial bacteria.
Yeasts (fungi) are microscopic living organisms that are responsible for the fermentation process; there are many different strains of yeast with varying characteristics present throughout the natural environment that we live in; some strains with desirable characteristics for food production/ digestion are now commonly utilised in human and animal nutrition.
The prominent genus in nutrition is Saccharomyces; selected strains of Saccharomyces cerevisiae are used in the food and feed industries and for other fermentation purposes (baker's yeast, brewer's yeast, wine yeast, in the manufacture of alcohol…)
It is important to reiterate that yeasts are now recognised as rich dietary sources of B vitamins and amino acids for both humans and animals; more specifically for equines, yeast enhances feed digestion improving growth, bone development and stamina.
Available nutritional yeast products fall into three distinct categories; namely live yeasts, yeast cultures and dead yeasts:
Live Yeast - Live yeast is the preferred nutritional product: live yeasts, separated from the media at the end of the yeast culture process, yield a nutritional product of greater quality and consistency; this, when fed as a nutritional aid, benefits digestion in the aforementioned ways.
Yeast Cultures - All commercial yeasts are produced by growing them in media containing the necessary nutrients for optimal growth. At the end of this process the entire culture can be harvested and supplied as a yeast product (a yeast culture). However, this will not only contain pure live yeast cells, but also dead yeast cells, growth media and metabolic by-products.
Dead Yeast - Dead yeast products are also marketed (Brewer’s yeasts etc) that have been separated at the end of the brewing process and thus retain no fermenting capacity (beneficial action) although they are still a source of nutrients.