Systems for Expressing Energy Value

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

The expression of the energy value of foods for ruminants, pigs, and poultry is crucial in animal nutrition to formulate balanced diets and optimize animal performance. Here are some of the commonly used systems:

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

1. Gross Energy (GE):

GE measure of the total energy content of a given feed.

The amount of heat arising from the complete oxidation of a unit weight of food in a bomb calorimeter at 25-30 atmosphere of Oxygen is  known as its gross energy (GE) value.

It is expressed in units of energy, such as calories or joules, per unit weight of the food (e.g., calories per gram). Not all the GE is useful to animals because energy losses in the animal takes place due to digestion and metabolism.

2. Digestible Energy (DE):

DEtakes into account the energy lost in the feces during digestion and determined by digestibility trials. It is typically measured in kilocalories per kilogram of feed or megajoules per kilogram of dry matter.

DE = GE – FE   —-> DE can be. apparent and true

True DE  > Apparent DE , as in True DE we subtract endogenous wastes from feces 

Endogenous losses- intestinal cells, enzymes, and microbes. Generally, digestion   trials measure apparent digestibility.

In ruminants: Roughages faecal energy losses can range between 40-50%. In the case of concentrates, energy loss is between 20-30%. In horses, faecal energy loss is 40 % whereas in pigs this loss is only 20%.

3. Metabolizable Energy (ME):

ME is that portion of feed energy that is actually available for metabolic transformation in the animals left after the metabolic loss (through urine and gases).

  ME = DE – Energy loss through urine and gases.

 ME= DE x 0.82

• For poultry, ME is measured more easily than DE, because the feces and urine are  voided together.
• Energy loss through urine is 4-5% of GE in cattle and 2-3% of GE in pigs. 
• Energy loss through gases: high in ruminants (fermentation) through methanogenesis: 7-8% of GE.

4. Net Energy (NE):

NE represents the energy available to an organism after accounting for all energy losses, including those in feces, urine, and heat production through digestive and metabolic processes. NE is the actual energy available for physiological processes.

             NE = GE – FE – UE – HI or ME – HI

Heat increment: HI is the amount of energy lost as a result of chemical and physical processes associated with digestion and metabolism. HI is greater in ruminants compared to monogastric. HI is also called as specific dynamic effect/ calorigenic effect/ thermogenic action it consists of the following:

• Heat of nutrient metabolism (HNM)
• Heat of fermentation (HF)
• Heat production from increased muscular activity due to nutrient metabolism.

5. Total Digestible Nutrients (TDN):

TDN measure of the energy content of a feed, expressed in terms of the weight of digestible nutrients present in the feed. The total digestible nutrient content of a food was calculated as the combined weight in 100 kg of food of digestible crude protein and digestible carbohydrate (crude fiber plus nitrogen-free extractives), plus 2.25 times the weight of digestible ether extract. The ether extract is multiplied by 2.25 because the energy value of fat is approximately 2.25 times higher than that of carbohydrate. The formula for calculating TDN is as follows:

TDN (%) = % digestible crude protein + % digestible crude fiber + % digestible N-free extract + (2.25 x % digestible ether extract)

  Factors affecting TDN value of feed:

1. Dry matter (DM)% of feed: More the moisture level, the less there are other nutrients that contribute to TDN (based on fed basis not on DM basis).
2. DM digestibility: TDN value is the sum of the digestible components of the DM. so, the more the digestibility of total dry matter, the higher will be the TDN value.
3. Mineral matter content of feed: more the inorganic/minerals level, lesser will be the organic component and TDN value of feed.
4. Digestible fat level: more the level of digestible fat in the feed, higher will be its TDN level.

Merits

1. TDN is a measure of apparent DE but is expressed in units of weight or percent rather than energy per se.
2. TDN value provides a relative measure of the DE content of feed.
3. It is easy to determine the TDN content of feedstuffs; proximate composition of feeds and faces and digestion trials are to be done.
4. Digestible energy can readily be determined by using a bomb calorimeter to measure the GE of feed and faces. No chemical analyses are required.

Demerits

1. TDN system takes into account only the losses of nutrients in the faces but not the other losses from the body.
2. TDN system overestimate the energy value of poor quality roughages in relation to concentrates specially in hot environment because TDN does not consider large amounts of energy wasted in the digestion of fibrous feeds in the form of gases and heat increment and Ether extract of forages largely comprise other than true fat. Therefore, a kg of TDN in roughage has less value for productive purposes than a kg of TDN in concentrate.
3. Certain species of forage were found to have high gross energy and high TDN values due to essential oils but low ME values.
4. The measurement of DE takes into account the losses only through feces.
5. The TDN of oil seeds may exceed 100%.
6. TDN is based on proximate analysis of feed that does not represent the chemical composition of the feed in well-defined constituents. For example, CF shall contain all the cellulose, hemicellulose and lignin. But, for certain feeds, some part of CF comes in the NFE portion also.
7.  It does not measure energy in energy units like calories.
8. It attempts to measure what feed contains rather than what it actually accomplishes in the body system. 

6. Starch Equivalent (SE):

SE concept used to express the energy value of a feedstuff relative to its fat-producing ability compared to that of pure starch. 

SE is defined as the amount of kilograms of starch required to produce the same amount of fat as produced by 100 kg of the respective feed. SE is typically expressed in kilograms.

SE = (Weight of fat stored per unit of food / Weight of fat stored per unit weight of starch)

When we say that the SE of wheat bran is 45, it means that 100 kg of wheat bran can produce as much animal fat as 45 kg of pure starch when fed in addition to maintenance ration or in other words 100 kg of wheat bran contains as much net/productive energy as 45 kg of the starch

Atwater Physiological Fuel Values: energy that can be derived from feedstuffs after accounting for losses during digestion and metabolism. While carbohydrates and fats are completely oxidized to CO2 and water in body cells after digestion and absorption, proteins are not completely oxidized by the cell. 

• Unoxidised protein matter is equivalent =7.9 Kcal/gram of nitrogen, which in terms of protein is 1.25 Kcal/g of protein: This energy represents metabolic loss and must be subtracted from the ‘digestible protein’.

 • After considering this Atwater has given factors for ME, which is also known as physiological fuel values. 

Atwater physiological fuel value factors 

Carbohydrate – 4.15 x 0.98 = 4 Kcal/g 

Fat – 9.40 x 0.95 = 9 Kcal/g 

Protein – (5.65 – 1.25) x 0.92 = 4 Kcal/g

In ruminants gaseous loss also costs much of energy thus these physiological fuel values are not applicable in the case of ruminants. 

Starch Equivalent (SE) vs Net Energy: 

Starch Equivalent (SE)	Net Energy (NE)
Measure of the energy value of feeds expressed as the amount of pure starch that would produce the same fattening effect.	Measure of the energy available for maintenance and production after accounting for losses in digestion and metabolism.
Based on the digestible organic matter content of the feed.	Based on metabolizable energy (ME) with specific partitions for maintenance (NEm) and production (NEp).
Expressed in kg of starch.	Expressed in Mcal per unit of feed.
Does not explicitly separate energy into maintenance and production.	Clearly separates energy into net energy for maintenance and net energy for production.
Commonly used in European feeding systems.	Widely used in the United States and other regions for precise energy evaluation.
Focuses on digestible organic matter without detailed metabolic losses.	Accounts for losses in digestion, fermentation, and metabolism.
Used for evaluating energy content in ruminant diets.	Used for formulating diets for various animal species, including ruminants and non-ruminants

DE v/s ME:

DE:

• The energy that remains after subtracting the energy lost in feces from the gross energy of the feed
• DE = Gross Energy (GE) – Fecal Energy (FE)
• Indicates the energy absorbed by the animal from the feed
• Commonly used for formulating diets, especially in ruminants
• DE is often calculated as a percentage of gross energy

ME:

• The energy available to the animal after subtracting energy losses in urine and gases from digestible energy
• ME = DE – Energy lost in urine and gases
• Represents the energy available for maintenance, growth, and production after accounting for metabolic losses
• More relevant for assessing energy available for physiological functions and production.
• ME is often estimated using a conversion factor, traditionally around 0.82 × DE, though this can vary based on diet type.

ME v/s NE:

ME v/s NE:
Metabolizable Energy (ME)	Net Energy (NE)
The energy available to the animal after subtracting energy losses in urine and gases from Digestible Energy (DE).	The energy available for maintenance and production after accounting for all energy losses, including heat increment.
ME = DE – Energy lost in urine and gases	NE = ME – Heat increment (HI)
Represents the energy available for maintenance, growth, and production.	Represents the energy that can be used for physiological processes, including maintenance and production.
Commonly used for formulating diets, especially in poultry and swine.	Used for more precise energy evaluations in various animal species, including ruminants and non-ruminants.
ME is often estimated from DE, typically around 0.82 × DE.	NE is calculated from ME, accounting for heat losses, making it theoretically more accurate.
crucial for understanding energy available for growth and production.	for evaluating the efficiency of energy used for maintenance and production.

For Ruminants:

1. Total Digestible Nutrients (TDN)
2. Net Energy (NE)

For Pigs and Poultry:

1. Metabolizable Energy (ME)
2. Net Energy (NE)

Practice Questions

1. Explain the concept of Gross Energy (GE) and discuss the various forms of energy losses that occur in animals. How do these losses affect the calculation of Digestible Energy (DE)?
   Marks: 10
2. Describe the process of energy partitioning from Gross Energy (GE) to Net Energy (NE). Include a discussion on the significance of each energy type (DE, ME, NE) in animal nutrition.
   Marks: 10