What does the animal need from its feed?

Water

Water is not strictly a nutrient but it is essential to life.  Muscle for example contains 80% water, fat 20-30%, and bone in a young animal 70-80%.  Animals will survive for a long time without a feed but will soon dehydrate without water.

The blood which transports all the digested nutrients around the body is largely water, as is the lymph which finally bathes the cells in nutrients.  Blood and lymph then carry the waste products away from the body cells and tissues for excretion.

Water is the main constituent of all digestive juices and gland secretions.  It is also a product of digestion when foodstuffs are broken down in the body to produce heat and energy.

Carbohydrates

These are compounds made up of carbon, hydrogen, and oxygen and provide the "energy" part of the diet.  They contain such things as sugars, starch, and cellulose - the things that make up about 75% of the dry matter of most plants.  In a laboratory feed analysis, you actually explode or burn a feed sample to measure the energy released as heat. 

Sugars

These provide energy in a readily digestible form.  Glucose is a good example.  So most complex carbohydrates have to end up as sugars before final absorption.

Starch

This is the most common form of energy storage in plants.  It is stored in seed kernels, tubers, and stems.  Starch is insoluble in water and is broken down into many different sugars during digestion and before absorption.

Cellulose

This is a structural material of plants and is a very complex carbohydrate.  It is not water-soluble and cannot be digested by man, so we rely on ruminants to convert it into animal products we can digest.  The rumen bacteria break it down to a range of sugars that can be absorbed.

Lignin

This is a completely indigestible substance even to a ruminant and is laid down with cellulose in the later stages of plant growth.  It gives the plant great strength like the reinforcing in concrete and often protects the cellulose.  It passes through the animal undigested.

Glycogen

This is the carbohydrate stored in the muscle ready for action.  Marathon runners have to build this up days before the race so they don't have to wait for digestion to take place before energy can be used.

To get the energy out of the carbohydrates the animal needs oxygen from the lungs via the bloodstream.  This burns up the carbohydrate releasing carbon dioxide and water which is then excreted.  Excess carbohydrates are stored in the body as fat.

This building up process is called "anabolism" whereas the breaking down process is called "catabolism".  They are both parts of "metabolism".

Fats and oils

These are made up of chains of chemicals called "fatty acids".  They contain carbon, hydrogen, and oxygen just like carbohydrates, but they differ by having a greater proportion of carbon in them.  They also contain more than twice as much energy on an equal weight basis.

Nature designed fat depots to be highly effective energy stores for times of need.  You get a lot of energy into a small bulk.  They also serve as important body insulation.

It takes 4 kg of starch to provide sufficient energy to be stored as 1kg of fat..  But getting the energy back out of fat is less efficient.  You only get 2kg of starch energy from 1kg of fat.  This is worth remembering when considering putting on and using up cow condition later.

Proteins

Proteins are used for muscle building and we obtain them from both animal and plant sources.   The building blocks of these proteins are over 20 "amino acids" some of which are essential to the animal and some not.  The best way to make sure you have covered all the essential amino acids in a diet is to use both animal (eg milk or meat) or plant (eg. grass) proteins.

Proteins like carbohydrates and fats are made up of carbon, hydrogen, and oxygen.  But they have one important difference - they contain nitrogen.  Proteins are insoluble in water so have to be broken down by enzyme action into amino acids which are soluble.  These are then easily transported around the body.

Animals can break down protein for energy but this is not a very efficient process.  Surplus nitrogen in the process is excreted as urea in the urine.

In New Zealand, we're not concerned about a protein shortage in mature animals eating plenty of protein-rich green pasture.  The only concern would be diets for young growing calves once they were weaned off milk, and before they were fully developed ruminants.

The protein needs of different classes of stock vary.  For example here is the protein needed as a percentage of the total diet:

Vitamins

These are essential to body functions such as reproduction and health.  Under New Zealand's pasture conditions vitamin deficiencies are rare.

Vitamin D is important to help absorb calcium and phosphorus in the gut.  Sunshine is involved in vitamin D synthesis in the skin and deficiencies would be rare in stock kept outside.  Vitamin D deficiency causes rickets (bent legs) which you might see in starved pups.

Some vitamin deficiencies may be triggered by the lack of another component of the diet.  An example is where there is a lack of selenium which then causes a failure of the animal to use vitamin E.

Ruminants can synthesise most of their own vitamins, one exception being vitamin B12.  It needs cobalt to help its synthesis in the rumen.  Cobalt may be added to a drench.

Minerals & Trace Elements

There is a range of minerals needed for various functions of the body.  They are used in the skeleton (bones and teeth), muscles, glands, body fluids, and for the correct functioning of the cells.

These are:

If you get a feed analysis done, you will see a component called "ash".  This is what is left after the feed has been burned to measure the heat or energy released.  In the ash are all the mineral components.

Some of these minerals are only needed in very small or "trace" amounts - hence the name trace elements.  Excess intake of some of these elements can cause poisoning, or too much of one can affect the absorption of another.  Too much zinc can for example cause a copper deficiency.