A summary outline of digestion and absorption in the pig follows. This provides a basic understanding of how the feed is digested and the nutrients absorbed. Readers interested in a more detailed explanation of this topic should consult a recent text on pig nutrition.
Digestion is the preparation of feed for absorption, i.e. reduction of feed particles in size and solubility by mechanical and chemical means.
Mechanical breakdown of feedstuffs is performed by chewing in the mouth and contractions of the muscles of the gastrointestinal walls. Chemical breakdown is achieved by enzymes secreted in digestive juices and by gut microflora. The digestive process reduces feed particles to a size and solubility that allows for absorption of digested nutrients through the gut wall into blood and lymph.
© R. Blair 2007.Nutrition and Feeding of Organic Pigs(R. Blair) 23
Mouth
Digestion begins in the mouth. Here feed is chewed into smaller pieces, which increases its surface area. This aids subsequent chemical reactions with various digestive juices and enzymes. Saliva produced in the mouth by the salivary glands moistens the dry feed so that it is easier to swallow. At this point the feed is tasted and, if accepted, swallowed. The saliva is slightly acidic. Pigs produce about 15 l of saliva daily. Pigs are the only farm animals whose saliva contains the enzyme ptyalin, which has amylase activity, but it is doubtful whether much starch is digested in the mouth since the feed passes quickly to the stomach by means of a series of muscular contractions (peristalsis) in the oesophagus.
Stomach
The stomach capacity of a 90-kg pig is 6–8 l. A churning action here further softens and separates feed particles, exposing them to gastric juices secreted by the stomach. These juices contain several enzymes, principally pepsin, which act to break down protein. Pepsin can function only in an acid medium (pH less than 3.5), acidity being provided by hydrochloric acid produced by the stomach. Hydrochloric acid dissolves minerals ingested with the feed such as calcium salts and inactivates pathogenic bacteria present in the feed. Mucus is also released by the stomach to protect the stomach wall from acid damage. The small amount of lipase present in gastric juices initiates the digestion of fat in the stomach. In nursing pigs, the gastric juices also include the enzyme rennin, which breaks down the protein in milk. Thorough mixing of the feed with acidic gastric juices takes place only in the lower region of the stomach. The contents of the upper stomach remain at an alkaline pH. This enables digestion of starch in the slightly alkaline medium of the stomach by salivary amylase. One or two hours after eating, partially digested feed in a semi-fluid form known as chyme moves from the stomach into the small intestine.
Small intestine
The small intestine is a long tube-like structure connecting the stomach to the large intestine. This is where digestion is completed and absorption of nutrients takes place. Its capacity in the pig is about 9–10 l. Absorption includes various processes that allow the end-products of digestion to pass through the membranes of the intestine into the bloodstream for distribution throughout the body. Some absorption also takes place in the stomach.
Chyme is mixed with other fluids in the small intestine, the first part of which is known as the duodenum. Duodenal glands produce an alkaline secretion which acts as a lubricant and also protects the duodenal wall against hydrochloric acid entering from the stomach. The pancreas (which is attached to the small intestine) secretes fluid containing bicarbonate and
several enzymes (amylase, trypsin, chymotrypsin and lipase) that act on carbohydrates, proteins and fats. The duodenal wall also secretes enzymes which continue the breakdown process on sugars, protein fragments and fat particles. Bile synthesized by the liver passes into the duodenum via the bile duct. It contains bile salts which provide an alkaline pH in the small intestine and fulfil an important function in digesting and absorbing fats. As a result of these activities the ingested carbohydrates, protein and fats are broken down into small molecules. Muscles in the wall of the intestine regularly contract and relax, mixing the chyme and moving it towards the large intestine.
Newborn pigs for a short time after birth possess the ability to absorb large molecules in a manner similar to that by which an amoeba surrounds its food source (pinocytosis). This is important in that it allows newborn pigs to receive immunoglobulin from colostrum, which provides some immunity against diseases in the environment. Another feature of the young pig is its great ability to digest fat from sow’s milk, which is high in this component.
Jejunum and ileum
Absorption also takes place in the second section of the small intestine, known as the jejunum, and in the third section, known as the ileum.
Digestion and absorption are complete by the time the ingesta have reached the terminal end of the ileum. This area is therefore of interest to researchers studying nutrient bioavailability (relative absorption of a nutrient from the diet) since a comparison of dietary and ileal concentrations of a nutrient provides information on its removal from the gut during digestion and absorption.
Minerals and vitamins are not changed by enzymatic action. They dissolve in various digestive fluids and water, and are then absorbed.
Once the nutrients enter the bloodstream, they are transported to various parts of the body for vital body functions. Nutrients are used to maintain essential functions such as breathing, circulation of blood and muscle movement, replacement of worn-out cells (maintenance), growth, reproduction and secretion of milk (production).
The ingesta, consisting of undigested feed components, intestinal fluids and cellular material from the abraded wall of the intestine, then passes to the next section of the intestine, the large intestine.
Large intestine
The large intestine (lower gut) consists of two parts: a sac-like structure called the caecum and the last section, called the colon. The colon is attached to the rectum. The caecum is small, with a capacity of about 1.5–2 l.
Here the intestinal contents move slowly and no enzymes are added. Some microbial breakdown of fibre and undigested material occurs, but absorption is limited. Thus fibrous feeds, such as lucerne, have limited feed
value. Remaining nutrients, dissolved in water, are absorbed in the lower part of the colon (about 9 l capacity). The nutritional significance of certain water-soluble vitamins and proteins synthesized in the large intestine is doubtful because of limited absorption in this part of the gut. The large intestine absorbs much of the water from the intestinal contents into the body, leaving the undigested material which is formed into the faeces and later expelled through the anus.
The entire process of digestion requires about 24–36 h.
Digestibility
Only a fraction of each nutrient taken into the digestive system is absorbed.
This fraction can be measured as the digestibility coefficient. It is determined through animal digestibility experiments. Researchers measure both the amount of nutrient present in the feed and the amount of nutrient present in the faeces, or more exactly in the ileum. The difference between the two, commonly expressed as a percentage or in relation to 1 (1 indicating complete digestion), is the amount of the nutrient digested by the pig. Each feedstuff has its own unique set of digestibility coefficients for all nutrients present. The digestibility of a feedstuff or a complete feed can also be measured. Digestibility measured in this way is known as apparent digestibility since the faeces and ileal digesta contain substances originating in the fluids and mucin secreted by the gut and associated organs, as well as cellular material abraded from the gut wall as the digesta pass. Correction for these endogenous losses allows true digestibility to be measured.
Generally, the digestibility values listed in feed tables refer to apparent digestibility unless stated otherwise.
Some feed ingredients contain components that interfere with digestion.
This aspect is dealt with in Chapter 4.
Fig. 3.1.Diagram of the digestive system of the pig.