ESTIMATION OF ENERGY CONTENTS

1. Carbon-Nitrogen Balance Technique:

• The main forms in which energy is stored by growing and fattening animal are protein and fat, carbohydrate is small in amount in body and are fairly constant.
• Objectives of this trial is to know the stored amount of proteins and fat in animal body.
• This can be estimated by carrying out C and N balance trial, i.e. by measuring these amounts of these elements entering and leaving the body and so, by difference, the amounts retained.
• The energy retained then calculated by multiplying quantities of nutrients stored by their calorific values.
• Both carbon and nitrogen enters the body through feed and N leaves it in feces and urine, however carbon leaves the body as CH₄ and CO₂. So, balance trial must be carried out in respiration chamber.
• The quantity of protein stored is calculated by multiplying the N balance by (1000/160 = 6.25), as body protein is assumed to contain 160 g N/kg
• It also contains 512 g C/kg and amount of C stored as protein can be calculated. The remaining carbon is stored as fat, which contains 746 C/kg.
• The energy present in the protein and fat stored is then calculated by using average calorific values for body tissues.

2. By Bomb Calorimeter:

• By Lavoisier’s classic experiment on origin of animal heat, it is known that food burned outside the body produce same amount of heat as food oxidized by slow process of intermediary metabolism.
• If foods are burned and heat produced is measured, the quantity of heat expressed in Kilocalories represents the gross energy value of combustion of food.
• The instrument used to determine heat of combustion is known as bomb calorimeter.
• Here the food are ignited and heat of combustion is calculated from rise in temperature of surrounding water placed in a Jacket inside calorimeter.
• To determine energy retention, intake of energy and losses of energy as heat in feces, urine and as combustible gas are determined.

3. By calculating TDN from digestibility trial:

• In number of countries including Nepal, energy value of feed is expressed in terms of TDN which is abbreviation of total digestible nutrients.
• It simply indicates the relative energy value of a feed to an animal. It is expressed either in kg or in percent.
• It is determined only by a digestion trial where % digestible nutrients are computed on fresh basis directly by multiplying the percentage of each nutrient present in the feed ingredient by their corresponding digestion coefficient.
• % TDN = Digestible protein % + Digestible fibre % + Digestible NFE % + Digestible EE x 2.25
• General accepted value of TDN is;

1 kg TDN = 4400 Kcal D.E

1 kg TDN = 3520 Kcal M.E

1 kg TDN = 0.869 SE (Starch equivalent)

4. From chemical composition:

• From gross-chemical composition of feed sample obtained either by proximate analysis or Van-Soest Method, amount of energy yielding from group of nutrients; carbohydrates, protein, fat are estimated.
• Once the amount of each components is known, estimation of expected amount of heat of combustion can be made easily by multiplying with appropriate factors.
• The heat of combustion of individual carbohydrate, protein, and fat differ with their composition but for practical use, individual figures were averaged to apply to major foodstuff as gross energy of food.
• While carbohydrates and fats are completely oxidized to CO₂ and water in body cells, protein are not completely oxidized by cell. Unoxidized protein matter is equivalent to 7.9 Kcal/gm of nitrogen, which in terms of protein is 1.25 Kcal/g of protein. This energy represents metabolic loss and must be subtracted from digestible protein.
• After considering this, Atwater has given factors for ME, which is also known as physiological fuel values.
• Atwater physiological fuel value factor

Carbohydrate- 4.15 Kcal/g

Fat- 9.4 Kcal/g

Protein- 5.65 Kcal/g

• Digestibility figures obtained by him

Carbohydrate = 98% digestible

Fats= 95% digestible

Protein = 92 % digestible

• The calorific values of three nutrients were multiplied by those corresponding digestible coefficients, to get physiological value as below:

1 gm carbohydrate = 4.15 x 0.98 = 4.0 Kcal

1 gm fat = 9.4 x 0.95 = 9.0 Kcal

1 gm protein= 5.65 x 0.92 = 5.20 Kcal

• These values are not suitable for calculating energy values of ruminant feed as gaseous loss also cost much of energy.
• Estimation of energy value of feeds obtained by multiplying % composition with appropriate Atwater’s fuel value factors are thus crude procedure for ruminants.