Energy is expressed as digestible (DE), metabolizable (ME), or net energy (NE) by considering the loss of energy during digestion and metabolism from gross energy (GE) in the feed, as follows:
One calorie is the energy required to raise the temperature of 1g of water to 15.5°C from 14.5°C. 1000 calories = 1Kcal (amount of heat required to raise 1kg of water to 1°C). 1000 Kcal = 1Mcal
• Gross energy is the total heat of combustion of a material as determined with a bomb calorimeter-ordinarily expressed as kilocalories per kilogram of feed or mega joule/kg dry matter.
• Roughages have gross energy values comparable to concentrates, but the two differ greatly in digestible, metabolizable and net energy values.
• Fat, because of their greater proportion of carbon and hydrogen, yield 2.25 times more gross energy per kg than carbohydrates and protein
• Energy supplied by the food in excess of that needed for maintenance is used for the various forms of production. A young growing animal will store energy principally in the protein of its new tissues, a fattening animal stores energy in fat, and a lactating animal will transfer food energy into milk.
(DE) is the amount of energy in the feed minus the amount of energy lost in the feces
It is the digestible energy less the energy lost in urine and combustible gases leaving the digestive tract, chiefly methane. It is the portion of energy available for metabolism.
Metabolizable energy = Energy in the food – (Energy lost in faeces + energy lost in combustible gases + energy lost in urine).
• Normally about 8 per cent of the gross energy intake is lost through the methane production. Metabolizable energy can also be calculated from the digestible energy by multiplying with 0.82 which means roughly about 18 per cent of the energy is lost through urine and methane.
ME = DE * 0.82
ME is commonly used to evaluate feedstuffs for poultry because the birds void urinary and faecal losses together. Urinary losses of energy are quite stable in a given species and is usually 2-3% of GE. The losses are more in ruminants.
Factors Affecting the Metabolizable Energy Values of Foods
• Species of animals
• Composition of feed
• Processing of food
• Level of feeding
This is that portion of metabolizable energy which may be used as needed by the animals for work, growth, fattening, fetal development, milk production, and/or heat production. It differs from metabolizable energy in that net energy does not include the heat of fermentation and nutrient metabolism or the heat increment.
Net energy is obtained from ME by subtraction of heat increment. NE is that portion of energy that is completely useful to the animal for maintenance and production purpose. The portion of NE used for maintenance is the energy required to sustain life processes. The other portion of NE is used for tissue gain or milk or egg production.
Heat increment is the amount of energy lost as a result of chemical and physical processes associated with digestion and metabolism. HI increases with the amount of feed consumed and may be used in animals reared in cold environment to warm the body otherwise. HI is also called as specific dynamic effect it consists of the following.
Food evaluation systems are based on digestible, metabolic and net energy.
TDN is simply a figure which indicates the relative energy values of a feed to an animals. It is ordinarily expressed in kilogram’s or in percent ( kg of TDN per 100 kg of feed). TDN is the sum of digestible crude fiber, digestible crude protein, digestible fat multiplied by the factor 2.25, and digestible nitrogen-free-extract.
1. It over estimates the value of roughages because more energy spent in chewing of such feeds remains unaccounted.
2. Only the loss in faeces is accounted for in this method.
3. If feeds are high in fat content, TDN will some time exceed 100 in percentage of TDN