The Digestive System

Introduction

Digestion is the process by which feedstuffs are broken down to their simplest forms.  The resulting nutrients are absorbed into the bloodstream to provide fats, carbohydrates, amino acids, vitamins and minerals for maintenance and growth, or they can be stored for future needs.  Digestion takes place through a complex process that involves peristaltic muscular contractions, enzymatic action, and fermentation. The digestive process is completed when usable nutrients are absorbed and undigested feed residues and waste products excreted (Cassady, 1997).

The digestive tract consists of a muscular tube which begins at the lips and terminates at the anus. The digestive tract consists of the mouth, pharynx, oesophagus, stomach, small intestine, large intestine, rectum and anus (Cassady, 1997).

Animals have different types of digestive systems based on how they digest components of feedstuffs (Cassady, 1997).  Both horses and ruminants have developed digestive tracts which are suited to digest and utilise high fibre diets.  Pigs, which are omnivores, also have the capacity to digest a fibrous diet.  The basic mechanism for each system is identical, the type of digestive tract which each has developed, however, is quite different (Pagan, 1991).

Non-ruminant systems, such as the pig, are characterised by enzymatic digestion of carbohydrates, proteins, and fats in the foregut (mouth to ileum), with limited fibre digestion in the hindgut (large intestine).  Ruminants, such as cattle, have more complex digestive systems that allow fibre digestion in the rumen, enzymatic digestion in the small intestine, and relatively minimal digestion of fibre in the hindgut.  The equine digestive system is somewhere between non-ruminants and ruminants with high rates of enzymatic digestion occurring in the foregut; plus, high rates of fermentive microbial digestion occurring in the hindgut (caecum to rectum). The horse is classified as a non-ruminant herbivore, a roughage eater (Cassady, 1997).

Comparison of digestive systems and processes

The mouth

The mouth is the initial opening of the alimentary canal.  Chewing is the first step in the processing and mechanical breakdown of food.  For both the cow and the horse the chewing action is very similar, the cow however, has no upper incisors and tears rather than cuts its food.  Being an herbivore means their diet consists largely of hay and pasture, both containing cellulose, which is particularly hard to digest. Each animal chews and grinds its food into very small particles which are more easily digestible by the stomach, both making up to 60,000 jaw movements per day.  The pig is somewhat different in that it will simply chew and grind its food into small enough pieces to be swallowed.

In the mouth, saliva is mixed with the food and acts to soften and moisten the food particles. Saliva contains salivary amylase that begins the breakdown of starch. This process is identical for the pig, cow and horse.

The oesophagus

This is the tube which carries food from the mouth to the stomach. In the pig and horse a series of muscular contractions called peristalsis push the food towards the stomach.  At the end of the oesophagus is the cardiac sphincter, a one-way valve which prevents food from passing from the stomach back into the oesophagus.