ANN 121: Applied Animal Nutrition-I (2+1)

METHODS ADOPTED TO ESTIMATE ENERGY REQUIREMENT FOR MAINTENANCE - INDIRECT CALORIMETRY

Indirect Calorimetry

• Most of the work on energy requirement in India, was conducted using the indirect calorimetry  method.
• The fasting metabolism is only a portion of the energy required for maintenance, since it is only the energy required in a fasting animal, in a comfortable temperature, without voluntary activity.
• Energy required for consumption and digestion of food, energy required for the increased respiration and heart rate due to walking and other movements, varying environmental temperatures are not accounted for in the determinantion of basal heat production.
• The amount needed for activity is known as activity increment.
• Cattle under feedlot require less than those under grazing or range system.
• In case of cattle, sheep and swine the activity increments may be of the order of 20 to 30%.
• Adding factors such as activity increment to the fasting metabolism to obtain the maintenance energy requirement is called the factorial method of estimating requirements.
• Values obtained in this way are not as reliable as those determined under practical conditions in feeding trials. 

Devices used for estimating gas exchange

• Because the animal body ultimately derives all of its energy from oxidation, the magnitude of energy metabolism can be estimated from the exchange of respiratory gases.
• A variety of techniques is available for measuring the respiratory exchange; all ultimately seek to measure oxygen consumption and CO₂ production per unit of time.

Open circuit system

• Devices allow the animal to breath atmospheric air of determined composition; the exhaust air from a chamber or expired air from a mask or cannula, is either collected or else metered and sampled and then analysed for O₂ and CO₂ content.
• Analysis of gases has been accomplished with chemical and volumetric or manometric techniques.

Closed circuit system

• Devices require the animal to rebreathe the same air. CO₂ is removed with a suitable absorber which may be weighed before and after use to determine its rate of production.
• The use of oxygen by the animal body decreases the volume of the respiratory gas mixture, and this change in volume is used as a measure of the rate of oxygen consumption.
• Oxygen used by the animal is then replaced by a metered supply of the pure gas.
• Both O₂ consumption and CO₂ production must be corrected for any differences in the amounts present in the circuit air at the beginning and end of the experiment.
• Methane is allowed to accumulate in the circuit air, and the amount present is determined at the end of the experiment.