Gross energy (GE) determination

The intake of food is determined by the energy content. This means an animal will eat to satisfy its energy requirements, all other nutritional requirements need to be balanced with energy. The most accurate picture is given by looking at Metabolisable Energy (ME).


Gross-energy is the quantity of heat resolution from the complete oxidation of a substance (Pond et al., 1995). By determining temperature (heat is a form of energy) inside the oxygen bomb calorimeter (IKA calorimeter C4000) before and after burning the sample with pure oxygen, the gross-energy content can be calculated. The ground foodstuff- and faeces samples were weighed before they were ignited with pure oxygen in the oxygen bomb calorimeter. The oxygen bomb calorimeter records the ambient temperature before and after the burning process. With these two temperatures the gross energy content was calculated.


To calculate gross energy content on a DM base, the following formula can be used;

Gross energy(Kj /kg)(DM) = (C x ∆T - QF)/ (Mp x DM of AD)

Were C is the warmth capacity of the calorimeter (J/k); ∆T is the measured temperature rise; QF is the sum of all foreign energies (wire and fibre, (J)); Mp is the weight of the sample (g) and DM of AD is the percentage of DM in the air-dried sample.

Digestible energy (DE)

The digestible energy (DE) content of a feedstuff is determined by subtracting the amount of energy in the feed from the energy lost in the faeces.

Determining digestible energy


the DE value of a feed is normally determined with pigs kept in metabolism cages. The pigs are first allowed to adjust to the test diets for a period of 10 to 14 days. The feed intake is then held constant and the faeces are collected over a 5-7 day period. Test samples are either fed as the whole diet, e.g. wheat or barley, or as additions or substitutions to a basal diet. The latter approach is used in the case of protein concentrates which cannot form the entire diet.

At the end of the collection period, the faeces are mixed, sampled and analysed for GE. The DE in the feed is then calculated as follows:

DE (MJ/kg) = (GE in feed (MJ/kg) - GE in faeces (MJ/kg)) / Total feed intake (kg)

Factors affecting digestible energy content of feeds

DE is relatively independent of breed, sex and feed intake although there is a slight trend for an increase in digestibility with age, but this is so small over the grower-finisher phase that it is usually ignored. High levels of fibre in the diet can depress the digestibility of energy and other nutrients. The fineness of grinding of cereals may also have an influence.

The major limitation of DE is that the relationship between DE and NE is not constant. Fibre is not digested in the small intestine but passes to the large intestine. There, micro-organisms convert part of the fibre to volatile fatty acids which are then absorbed. This form of digestion is less efficient than direct absorption of energy from the small intestine. As a consequence, DE can over-estimate the NE content of high-fibre diets. Apart from fibre, there are differences in the efficiency of NE usage even with nutrients that are absorbed in the small intestine. For example, energy in the form of fats and oils is more available to the pig than energy from carbohydrates.

This limitation of DE values is generally ignored in formulating diets. However, it is possible to account for these discrepancies using computer models that simulate metabolic processes. Thus, in the future, it should be possible to give greater emphasis to the NE values of feeds.

Metabolizable energy (ME)

Metabolizable energy (ME) is the digestible energy minus the energy lost in urine and gases (Figure 4). The energy lost as gas (in the gastrointestinal tract) is usually negligible (less than 1% of total energy intake).

Net energy (NE)

Net energy is the part of the total energy from the feed, available for body maintenance and productive functions. It½s the part, that remains after reducing the total energy intake with fecal energy, urine and gaseous energy losses and heat increment.

Basal metabolic rate (BMR)

Basal metabolic rate (BMR) is the number of calories used by the body when it is at rest. BMR accounts for most of a person's calorie use. A person's basal metabolic rate is based on body functions such as respiration, digestion, heartbeat, and brain function. One's age, sex, body weight, and level of physical activity impact the basal metabolic rate. Basal metabolic rate increases with the amount of muscle tissue a person has, and it reduces with age.

Factors affecting the BMR at a short term are: illnesses such as a fever, high levels of stress hormones in the body and either an increase or decrease in the environmental temperature will result in an increase in BMR. Fasting, starving or malnutrition all result in a lowering of BMR. This lowering of BMR can be one side effect of following a diet and nothing else. Solely dieting , i.e. reducing the amount of calories the body takes on, will not be as affective as dieting and increased exercise. The negative effect of dieting on BMR can be offset with a positive effect from increased exercise.

Thyroxin (the body's BMR controller)is a hormone that is produced by the thyroid - a small gland in the neck, just under the Adam's apple - and is a key BMR-regulator which controls the metabolic activity within the body.
When the thyroid is not working properly (called thyroid disorder), it can affect, body weight, energy level, muscle strength, skin health, menstrual cycle (periods), memory, heart rate, and cholesterol level.


Field metabolic rate (FMR)

Field metabolic rate (FMR) is the average rate of energy utilization as the animal goes about its normal activities.