Energy Requirements for Maintenance, Growth, Pregnancy, Lactation
Energy Requirements for Maintenance
Why Feeding for Maintenance ?
Certain amount of energy, protein and other nutrients is required for life sustaining activities of the body such as for the heart to pump blood, for respiration, for the nervous system to maintain its own activity and muscle tone, for temperature regulation, for repair of damaged or worn tissues, protein turnover and for the production of hormones and enzymes.
Factors affecting the maintenance energy requirement
The animal’s size and physiological state are two of the biggest factors affecting the maintenance energy requirement.
Other factors include, cold, heat, wind speed, humidity, hair length, hair color, floor surface, stall comfort, and activity level.
What is Maintenance Requirement ?
The maintenance requirement of a nutrient can be defined as the quantity which must be supplied in the diet so that the animal experiences neither net gain nor net loss of the nutrient.
Energy Required for Maintenance
The maintenance energy requirements include three components,
Energy to maintain the animal’s body temperature
Energy for voluntary activity, protein turnover etc.
MEm is always considered to include the energy for basal metabolism (cell survival) plus that needed by the animal to combat adverse climatic conditions (energy to maintain core temperature when the weather is cold, or energy to dissipate heat by panting when it is hot). The energy used for walking and grazing is also often included in MEm. If these components are included, MEm is substantially more than fasting heat production (basal metabolic rate).
Total ME requirement = ME (maintenance) +ME (growth) + ME (lactation) + ME (pregnancy)
Energy Metabolism of Fasting Animal
If an animal is not fed, the energy, protein and other nutrients required for the above functions will be drawn from the animal’s body reserves of fats, proteins etc. leading to negative energy or protein balance and the animal will lose body weight over a period of time. The destruction of body tissue is referred to as fasting catabolism.
The energy expended in the fasting animal is represented by the fasting animal heat production and this can be measured in therespiration calorimeter (Direct Calorimetry)or can be obtained by one of the methods of indirect calorimetry.
The quantity of heat arising in this way is known as the basal metabolism and measuring this heat produced gives a direct estimate of the net energy the animal should get from its feed to meet the maintenance energy requirement.
Methods to Determine the Energy Required for Maintenance of Animals (Very Important)
Measuring basal or fasting metabolic rate.
Conducting short and long-term trials with mature, non-producing animals fed at the maintenance level (if the energy content of their food is known).
By conducting feeding trials with different levels of feed intakes and by extrapolation of intake of feed towards zero level of production.
Regression method by conducting slaughter experiments.
Energy Requirements for Growth, Pregnancy & Lactation
Animal body is made up of polymers such as protein and fat. These macro–molecules are stores of chemical energy.
During their synthesis, energy is required for the formation of the bonds between the monomers (building units) and ME is incorporated to enable these macro–molecules to be synthesised.
Because many different polymers are produced by growing cells, the net energy costs associated with growth depend on the composition of the tissues laid down. This composition changes as the animal matures and deposits more fat in the tissues relative to protein and water
Composition of gain simply means whether animals are putting onmore muscle or more fat. For example, protein requirements will be higher for young animals because they are gaining more muscle than fat. As animals reach maturity, gain then has a larger percentage of fat, and requires relatively more energy.
Feed conversion efficiency (gain/unit feed): FCE will be greatest in animals laying down lean tissues (predominantly protein and water) and lowest in those in the fattening phase of growth. At higher feeding levels, the amounts of grain required to produce 1 kg gain (referred to as FCR) vary from about 3 kg in young animals depositing mainly lean tissue to about 10 kg in older ruminants laying down mainly fat. Note that FCR is commonly used in the poultry industry.
Adjustments to requirements for reproduction are based on stage of gestation. Requirements include development of maternal tissue as well as the fetus. Usually, pregnancy does not significantly affect requirements until the last one-third of pregnancy when the fetus is growing rapidly.
Nutrient deficiencies prior to breeding may result in low fertility or failure to maintain pregnancy. Underfeeding during growth can result in delayed sexual maturity. Fetal tissues have priority for nutrients over maternal tissues. Body reserves may be depleted.
Energy is needed to synthesise the macro–molecules in milk such as lactose (milk sugar) and casein (milk protein). Heavy lactation has greater nutrient demands than any other production state.
Nutrient requirements for lactation are based on the amount of milk at peak lactation and the composition of the milk. Animals that produce more milk, and milk with more fat and protein, will have higher nutrient requirements.
Because the energy requirement for milk production varies with the fat content of the milk, the energy requirement is usually determined for ‘fat corrected milk’ using 4% as a standard