Food is moved along the digestive tract by muscular
contractions or waves known as peristalsis. The
gut is designed to receive a small but regular and
frequent supply of food (trickle feeding) to encourage
these 'waves' to continue. A period of as little
as eight hours without food may cause a slowing
of these muscular contractions and impair the digestive
The Mouth or Buccal Cavity
Digestion starts in the mouth. The horse has very
sensitive and mobile lips and together with the
tongue and front teeth, are used to locate and cut
the herbage and bring it into the mouth. The horse
has 'open rooted' teeth which grow continuously
reflecting the effects of the constant grinding
of foodstuffs containing high levels of silicon
compounds. The modern horse is fed a higher proportion
of 'softer' feeds such as cereals (despite the fact
that cereals or concentrate feed is often referred
to as 'hard' feed!). Hours spent chewing are restricted
and the use of haynets changes the way the horse
'grinds' its feed and thus his teeth do not wear
evenly. The modern horse's teeth need regular rasping
- at least once a year - in order to ensure that
sharp edges are removed, which would otherwise impair
the chewing function, and cause bitting problems.
The horse must chew all feed in order to:
1. Break the food into smaller
particles which can be swallowed.
2. Disrupt the hard outer covering - cellulose -
of plant cells, releasing the starches, proteins
and other nutrients contained within.
3. Add saliva for lubrication and to reduce it to
a semi-liquid pulp. There is a very small amount
of enzymic digestion in the mouth. (Unlike humans
and dogs, horses do not salivate in anticipation
of food, but produce saliva in response to the presence
of food in the mouth).
The presence of food in the stomach stimulates the
secretion of digestive enzymes and strong acids.
Protein and fat digestion begin in the stomach.
Under normal circumstances food will start to exit
the stomach after approximately 45 minutes.
The Small Intestine
In a typical 16.hh horse the small intestine will
be approximately 21.5m (70 feet long). Digestion
continues and absorption of nutrients into the bloodstream
1. Starch and sugar digestion
is mainly completed (as long as the quantity fed
does not exceed the capacity of the small intestine)
and the end products - mainly glucose - are absorbed.
2. The end products of protein digestion - amino
acids - are absorbed.
3. Fats and oils are 'emulsified' and broken down
into small droplets, aided by the addition of bile
from the liver (the horse has no gall bladder to
store bile - it is produced as required).
4. Mineral and vitamin absorption begins.
The Large Intestine
This consists of the caecum, the large colon, the
small colon and rectum. The large intestine houses
millions of beneficial micro-organisms (including
bacteria, yeasts and protozoa) which are the only
organisms capable of producing the enzymes necessary
to digest cellulose and other components of fibre.
The types and proportions of these micro-organisms
are dependent upon the diet composition. It is therefore
extremely important that all changes to the diet
are made very slowly in order to allow the microbial
population time to adapt (7-10 days is a sensible
practical time over which to introduce new feeds)
Rapid dietary changes - such as an influx of water
soluble carbohydrates from, for example Spring grass,
which passes too quickly through the small intestine
- cause major disruptions to the gut microflora,
and can lead to metabolic disturbances ranging from
laminitis to filled legs.
The normal end products of microbial digestion (from
digestible fibre or starches and sugar) are volatile
fatty acids, which enter the bloodstream and are
ultimately broken down at a cellular level to produce
energy. If the horse is fit and healthy and provided
with good quality, mould-free forage, the microflora
in the hindgut manufacture some of the B Complex
vitamins and Vitamin K. Performance horses under
stress or horses fed poor quality or mouldy forage,
or foals with an undeveloped microbial population,
will not perform this function adequately and will
need an additional dietary supply of these vitamins.
Water resorption occurs in the large intestine and
mineral and vitamin absorption continues. Metabolic
wastes from all other parts of the body are transported
via the bloodstream to the rectum. The waste products
together with undigested material are formed into
faeces and voided via the anus.
The stomach capacity of the horse is between eight
and 15 litres, depending on the size of the horse.
Optimum digestion occurs when the stomach is not
more than two thirds full. For the average 16hh
horse weighing approximately 500kg it has been calculated
that a feed of 4.5 lbs (2kg) will fill approximately
two thirds of the stomach. The stomach phase of
digestion is less important in the horse who receives
a high fibre diet because fibre is mainly digested
in the hind gut. However, in the horse fed a high
proportion of concentrates, the stomach plays a
more significant role in digestion.
B. If food continues to enter the stomach once it
is full, the pyloric sphincter will open to allow
feed to enter the small intestine. This is the normal
function for the roughage fed horse. In the horse
fed a high level of concentrates this trickle mechanism
does not allow the food to stay sufficiently long
in the stomach for efficient digestion to occur.
C. If intake is very great and rapid, the trickle
mechanism is not activated and the stomach swells
to beyond capacity. This distension switches off
stomach function by inhibiting normal blood flow
and neural activity. Fermentation of the stomach
contents then occurs and gases are produced which
further distends the stomach. This malfunctioning
can lead to disorders ranging from mild colics to
stomach rupture and death.