1.1 Partitioning of food energy within the animal
Energy gained from food is partitioned for different uses. Gross energy, digestible energy, metabolisable energy and the net energy are all used for different functions.
• 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 concentrates, but the two differ greatly in digestible, metabolizableand 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.
• This is that portion of the gross energy of a feed which does not appear in the faeces. It include metabolizable energy as well as the energy of the urine and methane.
• The undigested food nutrient present in faeces can burnt in bomb calorimeter produced enough heat. This means that considerable quantity of heat of the digested food is eliminated in the faeces.
METABOLIZABLE ENERGY (ME)
• It is that portion of gross energy not appearing in the faeces, urine and gases of fermentation (Principally methane).
• It is digestible energy minus energy of the urine and methane. It is comparable energy of TDN minus the energy of the fermentation gases.
• 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
Factors Affecting the Metabolizable Energy Values of Foods
• Species of animals
• Composition of feed
• Processing of food
• Level of feeding
NET ENERGY (NE)
• 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 not used for heat production unless however and above that from other sources is required to keep the animal warm.
• It is important to understand that of the heat lost by the animal only a part, the heat increment of the food, is truly waste energy which can be regarded as a direct tax on the food energy.
Evaluation of Energy Value of Feed
For expressing the energy value of feeds and requirements of animals, various systems are followed in different countries are as follows :
• Total digestible nutrients (TDN)
• Starch equivalent (SE)
• Gross energy (GE)
• Digestible energy (DE)
• Metabolizable energy (ME)
• Net energy (NE)
TOTAL DIGESTIBLE NUTRIENTS (TDN)
• TDN is simply a figure which indicates the relative energy values of a feed to an animals.
• It is ordinarily expressed in pounds or kilogram’s or in percent (pound or kg of TDN per 100 pound or kg of feed).
• It is arrived at by adding together the following:
% TDN = % DCP + % DCF + % DNFE + (% DEE* 2.25)
The percentage TDN content of any feed represents energy of heat value of that particular feed.
• Since fat on oxidation provides 2.25 time more energy as compared to carbohydrates, hence the figure is multiple by 2.25.
• The protein in this equation has been included because of the fact that excess of protein eaten by the animals serve as a source of energy to the body.
Limitation of the TDN System
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 will some time exceed 100 in percentage of TDN
Factors Affecting the TDN Value of a Feed
• The percentage of the dry matter
• The digestibility of the dry matter
• The amount of mineral matter in the digestible dry matter.
• The amount of fat in the digestible dry matter
• The percentage of the dry matter.
THE STARCH EQUIVALENT
• KelIner, in Germany with steers in a respiration apparatus, measured the values of feeds for productive purposes in terms of starch values, instead of net energy values.
• In this system 1 pound of digestible starch is taken as the net energy unit
• Using the nitrogen-carbon-balance method the starch equivalent can be calculated from the following equation :
SE = (Weight of fat stored per unit of food / Weight of fat stored per unit weight of starch ) *100
• Kellner added pure carbohydrate, protein and fat to a basal maintenance ration to determined the relative amounts of these pure digestible nutrients required to produce a unit of body fat using the nitrogen-carbon-balance method.
• One kg of digestible proteins produces 235 grams of fat
• One kg of digestible starch and cellulose produces 248 grams of fat
• One kg of digestible cane sugar produces 188 grams of fat
• One kg of digestible fat produces 474 to 598 grams of fat
• Taking starch as the unit, the fat producing power of proton, f fats and carbohydrates was then calculated by him as follows :
• One part digestible protein = 235/248 =0.95 starch equivalent (SE)
• One part of digestible fat = 474/248 to 598/248 = 1.91 to 2.41 (SE)
• One part of digestible starch = 248 / 248 = 1.00(SE)