Lesson 6, Topic 3
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Systems for expressing energy value of foods in ruminants, pigs and poultry

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Systems for Expressing Energy Value of Foods in Ruminants, Pigs and Poultry

Food evaluation systems are based on digestible, metabolic and net energy.

  1. Gross energy.
  2. Digestible energy.
  3. Metabolisable energy.
  4. Net energy.
  5. TDN.
  6. Starch equivalent

Basic Unit of Energy

Calorie: 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 or Therm.

Gross Energy

It is defined as the energy liberated as heat when feed, faeces or any other substance is fully oxidised by burning a sample completely in a bomb calorimeter.

Digestible Energy

It is the energy of the feed less the faecal energy. Energy lost in faeces accounts for the largest loss of energy, which ranges between 20 to 40%.

Metabolisable Energy

  • It is the digestible energy less the energy lost in urine and combustible gases leaving the digestive tract, chiefly methane.

  • It can also be defined as ingested gross energy minus faecal energy minus urinary energy minus gaseous energy.

  • It is the portion of energy available for metabolism.

  • 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.

Heat Increment (Hi)

  • 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 a wasteful process.

  • HI is also called as specific dynamic effect it consists of the following.

  • Heat of nutrient metabolism.

  • Heat of fermentation.

  • Heat production from work by the kidney.

  • Heat production from increased muscular activity due to nutrient metabolism.

  • HI is greater in ruminants compared to monogastric.

Net Energy (Ne)

  • 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.

Total Digestible Nutrients

This is the simplest form of energy evaluation wherein the animal requirements and the value of feeds in meeting these requirements are expressed in terms of the weight of digestible material in the feed.

  • The digestibility of nutrients is determined by digestibility trials.

  • TDN is simply a figure, which indicates the relative energy value of a feed to animals.

  • It is ordinarily expressed in Kg or in percent.

  • It can be calculated by the formula.
  • TDN (%) = % digestible crude protein + % digestible crude fibre + % digestible N-free extract + (2.25 x % digestible ether extract).

  • The digestible ether extract is multiplied by 2.25 because on oxidation fat provides 2.25 times more energy as compared to carbohydrates.

Merits

  • It is easiest to determine the digestible values through digestive trials unlike the ME and NE, which require complicated equipments and procedures.

  • The TDN values for most of the feedstuffs are obtained from carefully conducted digestion trial and are available in standard books.

  • The energy requirements of the ruminant were in TDN values.

Factors affecting the Total Digestible Nutrients (TDN) value of feed

  • Percentage of dry matter
  • Digestibility of dry matter
  • Amount of mineral matter in the dry matter
  • Digestibility of fat in the dry matter

Weakness of the TDN System

  • It is based on proximate analysis of the feed, which does not partition the feed into well-defined chemical constituents. Almost all proximate principles are composed of more than one chemical compound.

  • The factor, 2.25 used in case of fat to equalise its high energy content with that of carbohydrate and protein is not always a constant.

  • It is also based on human and dog experimental data. The ether extract of various feeds differs in the true fat content.

  • It does not measure energy in energy units.

  • It attempts to measure what feed ‘contains’ rather than what they accomplish or produce.

Calculation of Starch Equivalent

  • The percentages of the digestible nutrients are multiplied by the respective starch equivalent factors

  • The arithmetic sum of these products is called as production value/starch value.

  • Concentrate – Golden number (0.95)

  • For concentrates the actual starch value is obtained from the production value by multiplying with the ‘golden number’ or ‘value number’

  • The value number expresses the ratio between the starch value of a feedstuff and that of the pure nutrients contained in the feedstuff

  • Actual SE of concentrates =Calculated production value x 0.95 Golden number

  • Roughages (Correction factor): The production value of a roughage would be reduced by 0.58 units for every 1 per cent crude fibre present in the roughages.

  • Actual SE of roughages = Calculated production value x (CF% x 0.58) Correction factor.

Limitations

  • The starch equivalent system suffers from the same weaknesses as other net energy systems namely,

  • The starch value of the ration is not constant at different levels of feeding, but decreases with increasing levels.

  • The starch value differs considerably for different productive purposes, even at the same level of feeding.

  • For fattening the efficiency is lower than for other functions like growth, lactation, etc.