Livestock Farm

It has to do with the energy transferred via trophic levels.
The general statement used is that every time you eat something, you only gain 10% of the energy it used/stored/had.
Most of the energy absorbed/synthesised by organisms are released as heat (especially in mammals) and used in other bodily processes.
So basically, when a plant uses photosynthesis [...]

It is unfortunate that we do not have a rapid system for determining the metabolizable energy content of feeds or ingredients. In all situations, energy is the most expensive nutrient in a diet, and so it is somewhat surprising that we do not have a measure or guarantee of energy content of feeds or energy [...]

The three major objectives of nutritionists are to define the bird’s needs, define the content of these same nutrients within available ingredients and then to integrate these two data sets in the form of a least-cost formulation. In this instance, “least-cost” refers to the minimal cost to achieve the diet specifications rather than the lowest [...]

Dietary Allowances for Swine
Energy| Gestating sows differ from pigs in most other stages of production in needing a restriction of feed intake. That creates substantial challenges in the physical management of feed delivery, to ensure that each animal consumes the target amount of feed. Sows that consume too little during gestation enter the challenging lactation [...]

The National Research Council (1998) provides estimates of nutrient requirements for all phases of pig production. It is important to remember that these estimates do not contain safety factors to allow for variation in pig requirements, variation in level of availability of nutrients or loss of vitamin potency during processing and storage, so it is [...]