Applied Animal Nutrition-I

METHODS ADOPTED TO ESTIMATE ENERGY REQUIREMENTS FOR MAINTENANCE - DIRECT CALORIMETRY

• Energy requirements are best determined by measurement of energy expenditure.
• Energy expended for maintenance of an animal is converted into heat and leaves the body.Thus an intake sufficient to offset the loss represented by the fasting metabolism would be the requirement if the animal is maintained under basal conditions.
• Data on maintenance  requirements of energy have mainly been obtained in three ways.
  • Fasting metabolism as a basis for estimating maintenance requirement.
  • By short and long-term trials  with mature, non producing animals fed at the maintenance level.
  • Data on maintenance requirements are obtained by extrapolation of intake of feed towards zero level production. 

Fasting metabolism as a basis for estimating maintenance requirement:

• Dry non-producing, mature animals were fasted, kept in a thermoneutral environment and their heat production was determined (fasting catabolism). This gives an estimate about the minimum quantitity of net nergy which must be supplied to the animal to keep it in energy equilibrium. This can be estimated by both direct and indirect calorimetry.

Direct calorimetry 

• This is simple in theory, difficult in practice; sensible heat loss (heat of radiation conduction) from the animal body can be measured with two general types of calorimeters, adiabatic and gradient.
• The insensible heat (latent heat of water vapourized from the skin and the respiratory passages) is estimated by determining in some way the amount of water vapour added to the air, which flows through the calorimeter. For this, rate of airflow and change in humidity is measured.

Adiabatic calorimeters

• In this type an animal is confined in a chamber constructed in such a way that heat loss through the walls of the chamber is reduced to near zero. This is attained by a box within a box.
• When the outer box or wall is electrically heated to the same temperature as the inner wall, heat loss from the inner wall to the outer wall is impossible.
• Water circulating in a coil in such a chamber absorbs the heat collected by the inner wall; the volume and change in temperature of the water can be used to calculate sensible heat loss from animal body.
• The construction and operation are complicated and very expensive.

Gradient calorimeters

• Calorimeters of this type allow the loss of heat through the walls of the animal chamber.
• The outer surface of the wall of the calorimeter is maintained at a constant temperature with a water jacket; the temperature gradient is measured with thermocouples, which line the inner and outer surfaces of the wall.
• By the use of appropriate techniques it is possible to measure separately the radiation component of the sensible heat loss.