• Overview
• Animal Nutrition
• Digestion in non ruminants and ruminants
• All about feeding and flow planning
• Topic questions

Overview

Animal Nutrition

The alimentary canal
The alimentary canal is the passage along which food passes through the body.

• The canal contains a series of organs of the body involved in digestion.
• The alimentary canal also absorbs water and excretes parts of food that cannot be digested.
• The external structure of the alimentary canal of ruminants and non-ruminants is different.

DIGESTION IS THE PROCESS IN WHICH FOOD IS TAKEN IN AND PROCESSED TO TURN IT INTO BASIC NUTRIENTS THAT CAN BE ABSORBED INTO THE BLOODSTREAM.
Alimentary canal of a ruminant: External structure Cows and sheep are examples of ruminants. Their alimentary canal consists of the following organs: mouth, oesophagus, forestomach, the abomasum and the large intestine.

Alimentary canal of non-ruminants: External structure
The main difference between the structure of the alimentary canal of ruminants and non-ruminants is that non-ruminants do not have forestomachs.

The alimentary canal of a pig
The pig is a typical non-ruminant. It has a true stomach and no forestomach (i.e. it has no rumen, reticulum or omasum). The alimentary tract contains these organs: mouth, oesophagus, stomach, small intestine and the small intestine (it can be sub-divided into the duodenum, jejunum and ileum).

The alimentary canal of a fowl
The chicken is a non-ruminant so it does not have a forestomach. Instead, the oesophagus and stomach are modified to process the kind of food that fowls consume. The digestive tract of the chicken contains these organs: mouth, oesophagus, crop, stomach or gastric complex (divided into two distinct compartments, called the proventriculus and the gizzard), small intestine and large intestine.
The differences in the external structure of the alimentary canal of non-ruminants and ruminants is summarised in the table on the next page.

Internal structure of the ruminant alimentary tract

The alimentary tract of a ruminant is made up of the rumen, the reticulum, the omasum, the abomasum and the small intestine. This tract has some special internal modifications to assist digestion. These modifications can be seen in the structure of each organ.

• Internal sof the small intestine: The small intestine of ruminants is moist. It appears to be smooth, but the small intestine actually contains microscopic, finger-like projections called villi. The villi increase the absorptive ability of the intestines.

Digestion in non-ruminants and ruminants

Digestion in non-ruminants
Digestion in non-ruminants is a combination of mechanical and chemical actions:

• mechanical action breaks food down into smaller pieces
• chemical action breaks the components of feed into their basic chemical constituents.

Intake of feed
Pigs take in their feed with their lips and they grip the food with their canine teeth.
The tongue moves the food into the mouth and it is then chewed by the large molars.

The process of digestion

• Mouth
• Mechanical: Mechanical breakdown of food into finer particles is called chewing. The tongue moves the food around in the mouth and then to the back of the mouth or throat where it can be swallowed.
• Chemical: Saliva is secreted into the mouth in response to the presence of food. The saliva softens the food. The enzyme known as salivary amylase then begins the chemical breakdown of starch into the sugar called maltose.
• Stomach
• Mechanical: Food passes from the oesophagus into the stomach. The stomach contracts and moves the ingesta around by muscular force.
• Chemical: The mechanical action of the stomach mixes the hydrochloric acid (HCl) and digestive enzymes in the stomach with the food so that its chemical breakdown can begin. The hydrochloric acid produced reacts with the enzyme pepsinogen and forms pepsin, which breaks down proteins into smaller components called peptides. The digestive enzyme rennin reacts with the protein in milk, called caseinogen, and causes it to curdle or clot. The enzyme reaction forms the protein casein, which can be digested.
• Small intestine
• Mechanical: The main function of the small intestine is to absorb nutrients that have been broken down into their basic components. The nutrients are absorbed through the villi by the processes of osmosis and diffusion, which you will learn about in detail later on in this unit. The absorption of nutrients is assisted by the mechanical contraction of intestinal walls.
• Chemical: Various enzymes are released into the duodenum from the liver (bile) and the pancreas. They break down fats, protein and starches in the small intestine and some chemical digestion takes place.
• Large intestine
• Mechanical: Most of the nutrients have been extracted from the ingesta once it reaches the large intestine. This organ re-absorbs water by the process of osmosis. The ingesta become harder and drier as it passes through the large intestine. The contents start to resemble the faeces that will pass out through the rectum. The mechanical contraction of the large intestine assists with the re absorption of water and the movement of its contents.
• Chemical: No chemical processes take place in the large intestine.

Functions of the accessory glands
Accessory glands play a role in digestion:

• Salivary glands
• Found in the mouth and neck of animals. They are very important because they secrete saliva (spit), which contains water and mucous. These soften and lubricate the food, and allow it to move smoothly down the oesophagus. Saliva also contains an enzyme called salivary amylase, which breaks down starch to a sugar called maltose. Saliva dissolves the food components and allows the animal to taste the food and decide if it is suitable to eat or not.
• The liver
• A large gland found in the abdomen. It has many functions in the body, including digestive, storage and metabolic functions.
• The pancreas
• Lies in the curve of the duodenum in most animals. It produces the hormone insulin, which regulates blood sugar. It also has a function in digestion because it secretes pancreatic enzymes that break down proteins into peptides, starch into maltose, and fats into glycerol and fatty acids. The pancreatic juice also contains sodium bicarbonate, which neutralises stomach acids.
• Duodenal or intestinal glands
• Glands that are found in the wall of the duodenum of the small intestine secrete various enzymes. The enzymes break down fats, sugars and proteins.
• Duodenum of the small intestine: This is where bile and the digestive enzymes of the pancreas are secreted, to assist chemical digestion of fats,
  proteins and carbohydrates. Has glands which produce mucous to help neutralise and protect the intestine from the acidic gastric contents.
• Jejunum and ileum of the small intestine: Here digestion and absorption take place.
  → Contains glands that secrete enzymes that break down some sugars and peptides.
  → Products of digestion are also absorbed here, through the micro-villi into the villi, where they are then taken up in the blood stream. Fats that are broken down are absorbed by the villi and then enter the lymphatic system.

Digestion in ruminants
The forestomach of the ruminant is designed to contain microbes and it provides them with a large storage vat (container) and a large amount of water.

Intake of food and chewing of the cud
Cattle use their long, mobile, muscular tongues to grasp their food. This is made possible by the rough surface of the tongue.

• The sharp, lower incisors help to cut the grass as it is pulled into the mouth.
• The food forms a loose mass, or bolus, in the mouth, but it is hardly chewed at all before it is moved by the tongue to the back of the throat and swallowed.
  
  • It moves down the oesophagus by the process of peristalsis, and this process is assisted by the large amount of saliva secreted into the mouth.
  • The food then enters the forestomach where the grass is mixed with water and coated in microorganisms by the contractions of the rumen.
  • The honeycomb surface of the reticulum trap the coarse, long fibres of the grass and squeeze them into a bolus, or lump.
  • The bolus is moved up the oesophagus and into the mouth by reverse peristalsis.
  • The food is then chewed again. Cows chew the cud, or ruminate, for up to eight hours each day. They make up to 40 000 chewing movements a day.

Peristalsis: The wave-like motion of the circular muscles in the oesophagus and the small intestine. These circular muscles constrict behind the food mass that the animal has swallowed, pushing it forward, along the oesophagus or small intestine.
Rumination: Refers to the process in which food is returned to the mouth after it has been swallowed. This takes place so that ruminants can use their large, flat molars and premolars to grind food more effectively. Rumination is the result of regurgitation.
Regurgitation: The process by which food in the forestomach is returned to the mouth in the form of a lump of cud. It takes place by reverse peristalsis. It In this case, the muscular wave-like motion is upwards instead of downwards. As a result, the swallowed food is moved upwards, through the oesophagus, into the mouth.

Differences in the digestive tract of mature and young ruminants

• The rumen, reticulum and omasum are all underdeveloped.
• The forestomach of the newborn ruminant is designed to digest milk initially.
• As the young ruminant grows older, it begins to graze and the forestomach also begins to develop. There is therefore a difference in size and functionality between young and mature (weaned) ruminants.

Size of the stomach compartments of mature and young ruminants
A major difference between mature and young ruminants = the size of their stomach compartments.

• At birth the rumen, reticulum and omasum of the calf are small and underdeveloped and the abomasum is the largest stomach.
• The rumen starts to enlarge in size when the calf begins to ruminate and it is functional when the calf is three months old.
• The rumen is the largest stomach compartment in a mature ruminant.
• It is important to provide the young ruminant with concentrate and hay so that the rumen develops as soon as possible.

Functionality of the four stomach compartments in mature and young ruminants
The milk a newborn calf drinks bypasses the forestomach and enters directly into the abomasum, where it can be digested.

• The milk bypasses the forestomach due to the contraction of a muscular fold of the reticulorumen to form a groove, in response to neural stimuli.
• This closure is stimulated by the process of suckling, the presence of milk and the anticipation of drinking. This reflex can also be triggered by feeding newborn calves with a bottle.
• It is therefore important to bottle-feed calves until they can be taught to drink from a bucket.

Digestion in the rumen
The rumen is the main part of the forestomach where digestion takes place in ruminants.

• Rumen microbes are microorganism that live in the rumen → able to digest the cellulose of plants into basic nutrient components.
• Different types of rumen microbes: rumen bacteria, protozoa, and fungi and yeasts.

Functions of the rumen microbes
Rumen bacteria are divided into various groups according to the substance that they break down, utilise or produce:

• the cellulolytic group (that break down cellulose)
• the methanogenic group (that produce methane).

Digestion of cellulose
Ruminants cannot break down cellulose.

• But rumen bacteria and protozoa produce the enzyme cellulase, which breaks down the cellulose in grass and other plants.
• Cellulose is broken down into volatile fatty acids.

Synthesis of protein
Unlike ruminants, microbial organisms can synthesise amino acids from simple nitrogen containing substances such as urea.

• The microorganisms use the non-protein nitrogen sources, like urea, to build their bodies.
• The microbial protein is digested when the organisms die and used by the ruminant.

Breakdown of dietary protein
The microbes in the rumen break down the dietary protein of the ruminant.
This product is then further digested in the abomasum and the small intestine.

Vitamin production
The rumen microbes can produce B vitamins and a small amount of vitamin K.
Because of this, the ruminant does not need to obtain these in the feed under normal conditions.

Absorption of food in the rumen
Ruminants can absorb breakdown products of the digestive process, and other substances such as water, directly into the bloodstream through the papillae.
Two processes are used:

• osmosis
• diffusion.

Absorption of water in the rumen
The rumen papillae absorb water by the process of osmosis.
Water moves from the rumen across the membrane into the blood vessels when the water concentration in the blood vessels of the papillae is low.

Absorption of volatile fatty acids in the rumen
The ruminant’s main source of energy is VFAs. VFAs are the end product of rumen fermentation.
As the VFA concentration in the rumen increases, these molecules move across the membrane of the rumen papillae into the bloodstream.

Requirements for rumen microbes to function normally
Important requirements for normal functioning of rumen microbes or microorganisms.

• Water A large amount of water is needed in the rumen in which to suspend the microbial soup: about 50 litres (beef cattle) to 70 litres (dairy cows) of water per day.
• pH The rumen functions between pH 6 and pH 7.
• Muscular contractions The function of the rumen depends on regular contraction of the rumen wall muscles, which mixes the contents. The rumen wall contracts once or twice per minute.
• Fibre intake Regular and sufficient fibre intake by animals will keep the rumen microorganisms alive. Even if animals are fed concentrates such as maize, they still need to be fed hay to allow the rumen to function.
• Other nutrients Rumen organisms need a source of nitrogen such as proteins or urea. They also need some dietary carbohydrate.
• Temperature The rumen microbes grow at the body temperature of the ruminant host (37–39 °C.)

All about feed and feed flow planning

Components of feed
The basic nutrients contained in animal feed: water, proteins, carbohydrates, fats, vitamins and minerals. Animals may develop certain diseases if their feed lacks one of these essential nutrients.

Water in feed
All feed contain a certain amount of water. It is not really a nutrient but water is essential for the function of the animal’s body because it makes up 50–70% of its total volume. The body needs this high percentage of water to perform certain functions.

Important functions of water in animal feed

• It dissolves and transports chemicals to the cells of the body via the bloodstream.
• It is the medium in which all chemical reactions take place in the body.
• It controls the body temperature through sweating and panting.
• It moistens and lubricates joints and mucous membranes to allow them to function easily.
• It provides a shock-absorbing fluid around the brain to protect it from injuries.
• It allows the excretion of waste products in the urine.
• It provides tensile strength to the cells of the body.
• It allows dairy cows to produce large amounts of milk.

Animals become dehydrated if they do not drink enough water, and then they cannot function normally. Dairy cattle need a particularly high daily water intake.

Water requirements of various farm animals

Dry matter
Although water is vital, high moisture levels in feed can limit nutrient intake. The feed that remains when the water has been removed is referred to as dry matter(DM)

Formula for calculating DM
DM (g) = Total weight of feed (g) – Moisture content of feed (g)
It can also be expressed as a percentage using the formula:
% DM =             DM (g)             × 100
             Total weight of feed (g) 

Functions and importance of proteins in feed
Animals need to take in proteins as a source of amino acids. There are about 20 different types of amino acids which are required by various species. Animals can use these amino acids to synthesise their own proteins.

Functions of proteins in feed
Animals produce thousands of different proteins, each with their own specific function.

• Mechanical strength: Keratin gives strength to the tissues in the body. It is found in hair, nails, hooves, tendons and horns. Myosin is found in skeletal muscle which makes up the meat of the body. Myosin is also found in smooth muscle where it allows the movement of the internal organs such as in the alimentary tract. 
• Oxygen transport: Haemoglobin provides the pigment found in red blood cells. It transports oxygen around the body and removes toxic carbon dioxide from the cells.
• Regulation of bodily functions: Hormones regulate or control certain functions. . 
• Chemical reactions: A large number of chemical reactions take place in the body.
  Most of these reactions are catalysed by a special group of proteins called enzymes.

Importance of proteins in feed
Proteins, as well as fats and carbohydrates, can be broken down to provide energy, which powers the functions of the body. Proteins are also a source of amino acids for most animals.
Each farm animal species needs particular essential amino acids to stay healthy and these are usually provided by the diet.

It is important to note that the rumen flora of ruminants can synthesise the amino acids that they need. This means that ruminants seldom need high quality protein unless they are very high producers like dairy cattle.

Functions and importance of carbohydrates in feed
Carbohydrates contain carbon, hydrogen and oxygen atoms in a 1:2:1 ratio. There are three main groups of carbohydrates.

Main groups of carbohydrates

 
Functions and importance of sugar

• Simple sugars are used as building blocks for other substances in the body.
• Sugars are also an essential source of energy for animals.
• Chemical energy is released when the chemicals bonds between the atoms of sugars are broken.
• This chemical energy is used to perform all the functions in the body.

Functions and importance of starch

• Starch is a polysaccharide stored in plants.
• Animals eat starch as part of their diet.
• It is then broken down into the simple glucose rings which are used for energy.
• Animals store the excess sugar in their liver as glycogen.

Functions and importance of crude fibre
Crude fibre is the main component of plant materials. It contains the polysaccharide cellulose.

• Cellulose is made up of long chains of simple glucose sugars which are held together by hydrogen bonds. These bonds make the chains very stiff and give grass and plant their structural strength.
• Ruminants need the help of other organisms to break down cellulose into its glucose components.
• The rumen microbes produce the enzyme cellulase which breaks cellulose down into its glucose subunits.
  → These are rapidly converted into volatile fatty acids.
  → These volatile fatty acids are the main source of energy for ruminants.

Fats and oils (ether extract)
Fats and oils belong to a group of compounds called lipids.

• Lipids contain mainly carbon and hydrogen atoms, but also some oxygen atoms.
• Their subunits are called fatty acids which are long chain carboxylic acids. 
• Three fatty acid chains are bonded to a molecule of glycerol to form a triglyceride.

Oils are lipids found in plants and they are made up of saturated triglycerides. Fats are lipids found in animals and they are made of unsaturated triglyceride chains.

Functions and importance of fats and oils in animal production and growth
Fats and oils are better sources of energy than carbohydrates and proteins because they contain more energy per gram.
Non-ruminants obtain essential fatty acids such as linoleic and linolenic acid from fats and oils.

Steroids are a special group of lipids and they can function as hormones.

Related Items

• Fats form part of the structure of cell membranes and nerve cells.
• Lipids such as lanolin or wool fat protect the skin from the damaging effects of bacteria by waterproofing it.
• Fat is laid down in animal tissues when there is an excess of energy in the diet.
• Fat can be broken down and used when the animal’s diet is low in energy.

Bio-chemical functions of macro-elements
Macro-elements are minerals required in large amounts by the animal body to ensure good health. They play a very important role in animal metabolism.
Summary of macro-elements

Bio-chemical functions of trace elements
Trace elements are inorganic minerals or micro-elements animals require in very small amounts to perform essential bodily functions. A lack of trace elements in the diet may affect animal health and production, while an excess of some may have toxic effects.

Summary of micro-elements

Vitamins
Vitamins are organic substances needed by living organisms in very small amounts to regulate various body functions.

• Farm animals either make vitamins in their bodies, obtain them from their feed or they can be produced by their microbial symbionts.
• Non-ruminants obtain most of their vitamins from their diet.
• Usually ruminants only need vitamin A from their diet, because their rumen microbes produce the other vitamins they need.

Water-soluble vitamins in feed
Water-soluble vitamins include the B complex vitamins (B1, 2, 6, 12) and vitamin C. Water-soluble vitamins cannot be stored in the body. They must be produced continually or taken in daily to maintain the good health of the animal.

• Vitamin B1(also known as thiamine): Found in grains, green forage, hay and milk.
• Functions of vitamin B1: It is a co-enzyme (helper to an enzyme) in the metabolism of energy, and so it is needed for the growth of the animal.
• Deficiencies of vitamin B1: It causes poor appetite, slow growth and weakening of muscles in chicks. Pigs show poor appetite and growth, and may have lung problems.
• Vitamin B2 (also known as riboflavin): It is found in the same foods as vitamin B1.
• Functions of vitamin B2: It is needed for various enzyme systems for energy as well as protein metabolism.
• Deficiencies of vitamin B2: It causes curled toe paralysis in chicks. They walk on their hocks with their toes curled inwards due to nerve degeneration in their feet. Young piglets show poor appetite, retarded growth, vomiting, diarrhoea, and skin and eye problems.
• Vitamin B6 (also known as pyridoxal): It is found in cereal grains (e.g. wheat), legumes, liver and yeast.
• Functions of vitamin B6: It is essential for amino acid metabolism and the release of glucose from glycogen.
• Deficiencies of vitamin B6: It causes poor appetite, slow growth and excitability, and convulsions in some cases, in chicks and pigs. There is loss of appetite and reduced egg production in adult birds.
• Vitamin B12 (Cyanocobalamin is a synthetic form of vitamin B12): Good natural sources of vitamin B12 include protein-rich foods such as lucerne (also known as alfalfa), fishmeal and carcass-meal, and fermented products such as silage.
• Functions of vitamin B12: It is an important co-enzyme in several bio-chemical processes. It is involved in the metabolism of propionic acid in the rumen of ruminants. It also plays an important role in the function of red cell maturation. 
• Deficiencies of vitamin B12: It leads to a lack of hind leg coordination and unsteadiness in pigs. The main symptom in hens is poor hatchability of eggs.
• Vitamin C (or ascorbic acid): Needed to keep blood vessels healthy. It can be synthesised by all farm animals and is readily available in green feed.

Fat-soluble vitamins in feed
The second main group of vitamins is called the fat-soluble vitamins. They can be stored in the liver when there is an excess intake.

• Vitamin A (sometimes called beta-carotene): The vitamin must be ingested, even in ruminants. It is found in green pastures, yellow maize, yellow vegetables, hay and silage.
• Functions of vitamin A: It is essential for the health of the mucous membranes. This includes the eyes and the reproductive system. Cells also need vitamin A to function normally.
• Deficiencies of vitamin A: It leads to a rough and dry hair coat, as well as eye problems in cattle. Night blindness can occur in severe cases. Chickens that are deficient in vitamin A can die from bacterial infections.
• Vitamin D (also known as alpha-tocopherol): The action of sunlight on the skin synthesises vitamin D, but it is also available in hay. Young animals raised in houses will become vitamin D deficient if they do not get enough of this vitamin from their feed.
• Functions of vitamin D: It is essential for normal absorption of calcium and phosphorus from the gut. Vitamin D is also needed for calcium and phosphorus metabolism.
• Deficiencies of vitamin D: It could lead to rickets in young animals and osteomalacia in adult animals.
• Vitamin E: Contained in whole grains, green forage, good quality hay, and oil seeds.
• Functions of vitamin E: It is essential for the metabolic regulation of the cell nucleus. It protects phospholipids from oxidative damage.
• Deficiencies of vitamin E: It can complicate a selenium deficiency in young animals because vitamin E and selenium have similar functions. A vitamin E deficiency causes a muscle problem called white muscle disease which results in muscle stiffness and sometimes heart failure. Deficiency of this vitamin in adult animals may lead to reproductive failure.
• Vitamin K: Some farm animals produce their own vitamin K. But others need to obtain it from green pastures and good quality hay.
• Functions of vitamin K: It is essential for the formation of prothrombin. The protein prothrombin is essential for blood clotting.
• Deficiencies of vitamin K: It is seldom found in ruminants as they use microbial organisms to produce their own vitamin K. But a shortage may arise when cattle eat a toxin called dicoumarol found in spoiled sweet clover. Dicoumarol destroys vitamin K which results in slower blood clotting after injuries, and even fatal bleeding in some cases. Chickens suffer from this deficiency if they do not get enough vitamin K in their diet. In birds the signs of deficiency are anaemia which is noticeable by their pale combs. They may also bleed under the skin.

Digestibility of feed
Farmers need to know whether feeds available will meet the nutritional requirements of the animals being fed. There are different ways to measure this.

Digestibility and digestibility coefficient of feeds

• Digestibility of feed refers to the amount of the feed which is not excreted in the faeces and is therefore assumed to be absorbed by the animal.
• It is expressed in terms of dry matter, either as a coefficient or as a percentage.
• High quality feeds have greater digestibility values which mean that more of their nutrients can be absorbed.

Digestibility coefficient of feeds The digestibility coefficient is expressed as the amount of dry matter contained in the feed minus the amount excreted in the faeces, as a fraction of the dry matter.
The answer can be expressed as a coefficient or it can be converted to a percentage.

Factors affecting feed digestibility

• Animal species: Ruminants digest high fibre feeds better than monogastrics. This is because ruminants have rumen microbes to assist digestion. Both groups digest low-fibre feeds well. Sheep digest whole cereal grains better than cattle do, because sheep chew the grain rather than swallow it whole.
• Feed composition: The presence of the woody substance, lignin, decreases feed digestibility. Straw has poor digestibility because it has a high percentage of lignin.
• Ration composition: The other foods included in the ration may affect the digestibility of a feed component. For example, the presence of a large amount of carbohydrate in the ration may decrease the digestion of cellulose because of changes to the rumen environment.
• Processing: Processing can improve the digestibility of feed. For example, processes which break up the feed into smaller pieces can increase digestibility.
• Size of the meal: Large meals pass rapidly through the digestive tract. This will reduce the amount of digestion and therefore lower the digestibility.
• Age of plants fed to the animal: Young plants are usually more digestible. This is because older plants contain more indigestible lignin.

Methods to improve or increase feed digestibility
Methods to improve the digestibility of feed involve processes that change the physical nature of a feed.

• Mechanical breaking of feeds
• Grinding, crushing and rolling are ways of increasing digestibility of grain feed. This makes it easier for older cattle and calves to chew the feed and it improves the taste. Birds raised in intensive systems perform better when their feed is ground. However, feed must not be ground too fine. Grinding roughage reduces the digestibility because it passes through the gastrointestinal tract more rapidly.
• Pelleting
• Pelleting is another method of improving or increasing the digestibility of feeds, such as lucerne hay. The intake of pigs and poultry improves when they are fed pelleted meal. This is because pelleted meal is easier to eat.
• Heating
• Another method of improving digestibility of feeds is heating. For example, grain can be boiled or roasted to soften it and expand the germ. This makes the grain more digestible. It is also more palatable than raw grain so animals will eat more. Cooking improves feeds that contain digestive enzyme inhibitors, like potatoes and root vegetables, which contain trypsin inhibitors. This is because the heat of the cooking process destroys these inhibiting factors.
• Additives
• Additives can be added to feed to improve or increase their digestibility. For example, a NPN source such as urea can be added to carbohydrate-rich feed like grain to increase digestibility in ruminants. This occurs because it provides a nitrogen source for the rumen microbes. Poorly digestible feeds like straw can be treated with an alkali such as ammonia. This improves their digestibility coefficient by separating the lignin and cellulose, allowing microbes to digest the cellulose compound.

Calculating and interpreting the digestibility coefficient of a feed
The formula for measuring the digestibility coefficient (DC) is shown below, with an example of how to calculate DC.
DC = DM (g) – DM excreted in faeces (g)
                             DM (g)
Note that the moisture content of the feed and faeces must be known or estimated to do the calculation. This information can be found in agriculture handbooks and on the Internet.

Interpreting of the DC of a feed
A higher the DC or percentage of digestibility means that more nutrients can be absorbed from the feed. It therefore allows us to compare the value of various feeds for a particular species.
This is important because each species digests feed differently. It also allows us to evaluate how various processes or additions affect the digestibility of feed.

Calculating the digestibility coefficient of a feed
A cow eats 15 kg of hay which contains 10% water and passes 5 kg of dry matter in the faeces. Calculate the DC of the feed and express it as a coefficient and a percentage.
The hay contains 1,5 kg water (10% of 15 kg). So the amount of dry matter (DM) eaten is 15 kg – 1,5 kg = 13,5 kg. Now you can apply the formula.
DC = 13,5 – 5 = 0,63
            13,5
You can multiply the coefficient by 100 to express it as a percentage. So the feed digestibility is 63%.

Quality of feed: Biological value of proteins
A rough measure of the protein content called the crude protein (CP) content, is used to work out the amount of protein in an animal’s ration.

• CP is calculated using the Kjeldahl method which measures the total amount of nitrogen, including non-protein nitrogens (NPN).
• This method is only relevant for ruminants since they can utilise NPNs. We need other ways to evaluate the protein content for non ruminants in terms of the amino acid content.

Importance of animal proteins in rations
Animal proteins in rations are needed for three processes in the body.

• Growth: Young animals need protein for tissue growth.
• Production: The production of milk, eggs and meat requires an extra intake of protein in the diet.
• Reproduction: Animals require additional proteins to produce and feed their young.

Evaluating quality of protein in feeds

• Biological value (BV):
• BV refers to the ability of a specific feed protein to fulfil the nutritional needs of an animal. It is a measure of how much nitrogen is available for metabolism and growth. A feed with a high BV provides all of the amino acids needed by the animal, whereas a feed with a lower BV does not.
• The BV is determined by measuring the amount of nitrogen retained by the body. This is equal to the amount of nitrogen in the food ingested minus the amount of nitrogen excreted.
• Essential amino acid index
• This index is the ratio of the amount of the 10 essential amino acids contained in a feed relative to the amount of amino acids in egg protein.
• The ratio is calculated relative to egg protein because eggs have the ideal amino acid content.
• Ideal proteins
• An ideal protein supplies all essential amino acids in the right amounts. This means that the protein has the optimal nutritional quality.
• An example of an ideal protein is the protein found in eggs. It is used as a standard for comparison with other proteins.

Evaluation of biological value of feed protein
The BV of egg protein is considered to be 100. This is because it contains all 10 essential amino acids, in the right proportions. The BV of milk as a feed protein is also relatively high, namely 80. It is richer in the amino acid lysine than egg protein, but its nutritional quality is limited by a deficiency of the sulphur-containing amino acids methionine and cysteine.

Energy value of feed
Most of the ration is used to provide energy for the animal.

• Energy can be obtained from carbohydrates (including fibre in the case of ruminants), fats and proteins.
• Food contains chemical energy which can be converted into mechanical energy.
• The mechanical energy is used to power muscular action (movement), as well as for the chemical powering of basal metabolism (maintenance), and growth and production of meat, milk and eggs.
• Excess energy can be stored in the body as glycogen in the liver or as fat in and around the muscles and organs of the body.
• Some food energy is lost in the form of radiant heat (into the environment).
  Energy is also lost when urine and faeces are excreted.

Units in which energy value is expressed
Energy contained in feed is expressed in joules (J). One joule of energy is defined as the amount of energy needed to perform the work needed to exert a force of one newton over a distance of one metre.

To understand the flow of energy from feed through the animal body you need to understand some terminology.

• Gross energy
• Gross energy (GE) is the total amount of chemical energy contained in feed. It is determined in a laboratory using an apparatus called a calorimeter.
• The gross energy of food is not the total amount of energy available to the animal because energy can be lost in various ways.
• Digestible energy
• Digestible energy (DE) is the energy available after subtracting the energy lost in the faeces from the gross energy contained in the feed.
• Metabolic energy
• Metabolic (or metabolisable) energy (ME) is the energy available in feed after the energy lost in the excretion of urine and methane gas production has been subtracted from the digestible energy.
• Metabolisable energy is measured in megajoules (MJ). It used to be referred to as total digestible nutrients or TDN.
• Nett energy
• The nett energy value of feed is the quantity of energy that remains after the energy lost in the form of heat has been subtracted from the ME.
• It is the proportion of energy from the feed that is available for the animal to do work, grow, fatten, reproduce, lay an egg, produce milk and keep itself warm.

Purpose of calculating energy value of feed
The energy value of feed is calculated:

• to ensure that animals are given a balanced feeding programme
• that provides them with sufficient ME so that
• they have adequate nett energy to carry out functions, such as body maintenance and reproduction.
• The flow of food energy through the animal can be represented schematically, as shown below.

Calculation of feed energy flow and interpretation of the results
The formulae below, which used gross energy (GE) as total energey intake, can be used to calculate the:

• digestible energy (DE)
• metabolisable energy (ME)
• nett energy (NE).
  DE = GE – Energy in faeces
  ME = DE – Energy in urine and methane gases
  NE = ME –Heat energy lost

Nett energy represents most accurately the energy that is available to the animal for maintenance growth, production and warming.

Nutritive value and nutritive ratio
Animal feeds vary in the amount nutrients they contain. Nutritive needs of livestock differ, depending on their production cycle.

• It is therefore important to know the nutritive value of a feed and the requirements for each domestic animal species in each of their production cycle phases, such as:
• reproduction, lactation and growth.

Nutritive value of fishmeal

• A nutritive ratio (NR) is used to express the relationship between various components in a ration.
• The most commonly used ratio is the relationship between digestible protein and the total non-protein energy in the feed or ration.
• It is used to determine whether or not a feed is suitable for:
• a specific animal species during a specific phase of its production cycle
  OR
• the fattening phase during the production cycle of a specific animal.

Calculation of the nutritive ratio of a feed
NR = Percentage digestible carbohydrate + percentage of fat – % digestible protein in the feed 
                                               Percentage digestible protein
e.g. 1: 63% + 9% – 8%
                   8%
= 1:64%
       8%
= 1: 8

Interpretation of nutritive ratio in a feed
Ration or feed that contains a protein: Total non-protein NR of 4 to 5 is usually suitable for growth as it has high protein while a feed with an NR ratio of 7 to 8 will be best suited for the fattening phase.

Other ways of determining and expressing this important ratio are:

NR = 1: carbohydrates +    fats   , or
                                       proteins 

NR = 1: %Total Digestible Nutrients (TDN) – % Digestible Protein(DP), or
                                                      %DP

NR = 1: non-nitrogenous energy
                  digestible protein .

Types of feed
Animal feeds can be subdivided into roughages and concentrates based on their origin and fibre content.

• Roughages are bulky feed of plant origin with a high weight to volume ratio, because of its relatively high fibre content.
• Concentrates have a low weight to volume ratio because they are low in fibre and weigh less.
• They have a high percentage of either protein or carbohydrates.

The characteristics of roughages and concentrates

• Roughages (often called forage or fodder). Roughages are mainly used to feed ruminants as they contain cellulose, which only rumen microbes can digest.
• Contain various amounts of plant fibre, which depends on the type and age of the plant. Roughage is high in young pasture grass and legume pastures, but low in straw and hulls.
• Some roughages contain as much as 50% crude fibre.
• Their protein and mineral content also varies.
• Roughages can be rich in protein, such as legume pastures like lucerne, but they are usually low in energy.
• Examples of roughages include veld grass, browse (trees or shrubs), planted grass pastures, hay, plant residues and silage. Concentrates
• Concentrates can be of plant or animal origin. Both ruminants and non-ruminants can be fed concentrate diets.
• They have high levels of protein or carbohydrates, depending on their type. 
• Carbohydrate-rich concentrates include grains such as maize, barley and sorghum.
• Oil seed cakes and seeds like groundnuts and soybeans are protein-rich concentrates.

Forage refers to plants that are eaten where they grow, whereas fodder refers to plants which are harvested and taken to animals.

Description of types of roughages

• Forage
• Veld grass: It is the most commonly grazed roughage. Young grass is softer and contains less fibre than old grass which becomes hard and dry towards the end of the season.
• Veld grass can also be mowed to dry it and used as a hay. It is then classified as fodder.
• Trees for browsing: Cattle browse the leaves of trees and shrubs in the veld.
  Some exotic trees like leuceana, carob and mesquite have been planted to feed livestock because their leaves are rich in nutrients.
• Fodder
• Planted grass pastures: Kikuyu grass can be grazed or it can be cut to use as a hay feed.
• Legumes like lucernes are pastures with very high protein levels.
• Acacia pods: Acacia trees produce protein-rich pods which can be used to feed animals in the winter.
• The pods must be collected when they are dry as some green pods can be poisonous.
• Crop residues: Examples of crop residues include maize cobs and stover (leaves and stalks), husks of peanuts and peanut hay which is made from the plant after the peanuts have been harvested.
• Most crop residues are high in fibre and low in energy.
• Vegetable residues: Examples of vegetable residues include rape, pumpkins, sweet potatoes and cassava. These can be used to feed non-ruminants such as pigs and poultry. Free-range hens, in particular, need some green feed to supply them with vitamin A. This will give their yolks a deep yellow colour.
• Kitchen waste can also be fed to free range non-ruminants like pigs and chickens.
• Silage: This is plant material which is placed in a silo where it undergoes bacterial fermentation.
• Various plant materials such as grasses, legumes, maize, and fruit and vegetable residues can be used to produce silage.
• The advantage of silage is that it can be stored for long periods and then used for winter feeding when other feeds are less available.

Description of types of concentrates
Feed concentrates are dried forms of animal feed that have had their water removed.

• Plant protein concentrates: They are derived from legume seeds such as soybean and chickpeas.
• Oil seed crops such as groundnuts and sunflowers are an important source of protein for animal feeds.
• The oil is extracted from these crops and the protein-rich remains are made into a type of cake to feed animals.
• Animal protein concentrates: They contain a higher percentage of proteins than plant concentrates. For example:
• fishmeal contains 60% protein whereas the plant protein concentrate soybean meal only contains 45% protein.
• Carbohydrate concentrates: They come from cereal grains such as maize, wheat, sorghum and oats. They are usually mixed with other feed to make rations.
• They contain less than 10% crude protein but contain a high percentage of digestible carbohydrates.
• Cereal grains have to be crushed so that animals can digest them.
• Mixed feeds: They are the main type of feed that is used for non-ruminants. Farmers either make mixtures or feed companies sell them.
• Different formulations are made according to the animal species, age and stage of production.
• Mixed feeds are available as meal, cubes, pellets, cakes, mash or crumbs.

The schematic representation of different types of animal feeds
Animal feeds are derived from various sources. The schematic representation below shows the origins and derivations of the various types of feed.

The functions (importance) of roughages and concentrates
The two categories of feed, roughages and concentrates, have different roles to play in the nutrition of farm animals.

Subdivision of feeds based on nutritive content
Feeds can be subdivided into two groups based on their main source of energy, namely carbohydrate-rich and protein-rich type feeds.
Comparison between protein-rich and carbohydrate-rich type feeds

Feed supplements
A supplement refers to a nutrient (e.g. mineral; vitamin) which is added to the ration to improve its quality. Supplementation of rations is required under certain conditions.

Mineral supplements
Mineral supplements are needed when soils and thus the plants that animals eat are deficient in certain minerals. Here are some examples of mineral supplements used in livestock farming.

• Calcium and phosphorous supplementation:
• Given to cattle in late summer and winter to supplement the low phosphorus in veld grass at this time of the year.
• It can be supplied in the form of a lick, to which salt is added to make it attractive to animals. The salt component of the lick also supplements the sodium and chlorine levels which are usually low in veld and pastures.
• Licks must always be kept dry, and animals must have access to drinking water at all times to prevent salt poisoning.
• Iron injections:
• These are given routinely to day old piglets in piggeries to prevent the development of anaemia.
• Mineral supplements:
• They can be given to correct rare cases of copper, cobalt and iodine deficiencies.
• This must be done with caution to prevent poisoning.

Vitamin supplementation of rations
Non-ruminants on a poorly balanced diet may need supplementation when vitamin deficiencies occur. Cattle and sheep fed on dry hay or winter forage may need vitamin A supplementation. Add vitamin A to their feed or, preferably, give them regular vitamin A injections as directed by the manufacturer.

Commercial feeds for non-ruminants like pigs and poultry contain all the essential vitamins needed to ensure a balanced diet.

Non-protein nitrogen (NPN) as supplements
The protein content of forage like veld grass drops is too low to sustain production during the winter months.

• So ruminants must be given NPN like urea because their rumen microbes can use this to synthesise amino acids.
• Urea is a white crystalline powder that is made by combining air with water and it contains about 46% nitrogen.
• It can be given in the form of a solid commercial lick block or as a mixture made up by the farmer. The mixture contains the energy sources molasses or maize meal, as well as urea, salt and di-calcium phosphate.
• The urea mixture must be made up carefully because urea poisoning can occur if an excess of urea is given.

Growth stimulants
Growth stimulants are chemical compounds which can be added to feed or given by injection to improve the growth of various farm animals.

• Antibiotic growth stimulants
• These are antimicrobial substances used to suppress microorganisms which cause disease or which may interfere with the uptake of nutrients in the gastro-intestinal tract.
  Note that all growth stimulants must be used with care and in consultation with a veterinarian.
• The product must be chosen based on the animal species to be treated.
  For example:
• tetracyclines are used for feedlot cattle, ionophores are used for pigs and poultry, and flavophospholipol is used for cattle on winter veld to improve fibre digestion.
• Repartitioning agent (beta agonist)
• The substance clenbuterol is used in feedlot cattle to promote muscle growth rather than the deposition of fat.
• This will provide the leaner carcasses which are demanded by the commercial meat market.
• Hormones and hormone mimics
• The hormones testosterone and oestrogen cause retention of nitrogen which results in better muscle development. These products can be used in feedlot cattle but not in breeding animals.
• The hormone BST is used to increase the milk production of dairy cattle.

Planning a feed flow programme
A feed flow programme is a plan for the year to ensure continual and acceptable nutritional levels for animals in all stages of growth and production. Feed flow planning requires knowledge about the basic nutritional requirements of the farm animals and whether they need additional nutrients to meet growth and production demands.

The single Pearson Square method for feed formulation
It is a simple and effective method used to calculate the proportions of two different feeds required to achieve a desired nutrient percentage. It can be used to calculate the percentage of any desired ingredient, for example crude protein or fibre content.

How to formulate a ration which contains 12% crude protein (CP)
Two available feeds are combined to provide a source of grain (maize, CP 8%) and protein (soya bean meal, CP 36%).

1. Draw a square and place the desired CP content, in this case 12%, in the centre of the square.
   

   12

2. Write the % CP of the two feeds near the left hand corners of the square. soya 36%
   

   12

   maize 8%
3. Subtract the value inside the square from each value near the corner of the square. Write each answer near the corner which is diagonally opposite in the square. Always subtract the smaller value from the larger one (for soya: 36 – 12 = 24; for maize: 12 – 8 = 4). These answers on the right hand side of the square represent units or measures of the two feeds.
   soya 36% 4
   

   12

   maize 8% 24
4. So we need to add 24 parts of maize to 4 parts of soya. There are a total of 28 parts of feed. To convert these values to percentages, calculate the proportion which each feed makes up of the whole (28) and multiply the answer by 100.
   Maize (²⁴/₂₉)× 100 = 14,3%
   Soya(⁴/₂₈)× 100 = 85,7%

Interpretation of the results (in step 4) If you want to make 100 kg of mixed feed which contains 12% CP, then you need to add 14,3 kg soya (14,3% of 100 kg) to 85,7 kg of maize (85,7% of 100 kg).

Topic questions

• Answer the questions below. Check your answers afterwards and do corrections.
• Give yourself one hour.
• Marks: 100

1. State the three types of digestive processes that take place in ruminants. (3)
2. Describe the function of saliva in the digestive process. (3)
3. The bovine stomach consists of four compartments.
   3.1 Name the compartments. (4)
   3.2 Explain the role of each one briefly. (8)
4. All ruminants regurgitate their feed.
   4.1 Outline the process of regurgitation in a ruminant. (4)
   4.2 Explain the importance of regurgitation in the digestive process of a ruminant. (3)
5. Which components of feed can animals use for energy? (3)
6. Give ONE word/term/phrase for each of the following descriptions:
   6.1 Roughage with a high moisture content which is mostly used as a feed source for dairy cattle
   6.2 A nutrient supplement that is placed in a pasture field to provide the grazing animals with additional nutrients (2)
7. Name five main categories of nutrients in feed. (5)
8. The______ is responsible for the grinding of food by means of small stones found in it. 8.1 crop
   8.2 ventriculus
   8.3 proventriculus
   8.4 caecum (2)
9. Trace elements are important for the production of healthy red blood cells.
   9.1 Name these trace elements. (2)
   9.2 What condition results from a deficiency in these elements? (1)
10. What nutrient deficiency causes each condition below?
    10.1 parakeratosis;
    10.2 curled toe paralysis;
    10.3 night blindness;
    10.4 milk fever;
    10.5 white muscle disease (5)
11. Name five essential functions of proteins in the body. (5)
12. Food supplies animals with energy for metabolic processes and to grow and reproduce.
    12.1 What kind of energy does food contain? (1)
    12.2 What other kinds of energy can it be converted into by animals? (2)
13. Cellulase is an enzyme that is found in the rumen of a______.
    13.1 horse.
    13.2 fowl.
    13.3 pig.
    13.4 goat. (2)
14. Name FOUR methods of processing that can be used to improve the digestibility of feed. (4)
15. Name three main characteristics of concentrates. (3)
16. Give four examples of feed concentrates. (4)
17. Non-protein nitrogen sources can be used to supplement protein in the ruminant diet.
    17.1 Briefly explain why. (2)
    17.2 Briefly explain how this is achieved. (3)
18. What is the single Pearson Square method? (1)
19. Explain the difference between a maintenance and production ration. (4)
20. Examine the table below and answer the questions that follow.
    

    Feed	Crude protein (%)	Crude fibre (%)	Metabolisable energy (MJ/kg)
    Lucerne pasture	22,5	25,8	9,4
    Wheat pasture	12,3	15,5	10,5
    Veld grass (summer)	7,0	36,0	8,0
    Lucerne hay	14,1	30,1	7,5
    Groundnut hay	9,2	24,1	8,7
    Maize stover	5,8	27,1	6,8
    Oat hay	8,2	28,1	7,1
    Soybean meal	45,8	5,8	11,6
    Milk powder (skim)	33,5	0,0	12,0
    Maize meal	8,9	2,0	12,0
    Blood meal	82,2	0,0	9,1
    Fishmeal	60,9	0,0	10,6
    Ground nut oilcake	45,3	12,6	11,4
    Sorghum grain	11,0	1,7	12,2

    20.1 Name the four feeds with the highest crude protein content. (4)
    20.2 Which feeds identified above are examples of protein-rich animal concentrates? (2)
    20.3 Which of the feeds named above are examples of legumes? (2)
    20.4 Which feed has the highest crude fibre content? (1)
    20.5 Is the feed you named above usually eaten by non-ruminants? Why or why not? (3)
    20.6 Which four feeds have the highest ME content? (5)
    20.7 Classify the feeds you identified above as roughage or concentrates. (3)
    20.8 For each feed that you identified above, name the feed component that gives it a high metabolisable energy. (4)