A number of systems exist for calculation of requirements for nutrients and energy for different categories of horses. Not only are the predominant breeds and the climates they live in different, but the way nutritionists express the requirements also differ greatly. This can be confusing to the horse owner interested in trying to calculate the requirements of energy, protein and minerals for his horse, especially when trying to compare requirements across national borders or to communicate standards of energy and protein requirements under circumstances where the basis for calculation of requirements differ. Different methods may also express the same requirement in different ways and with different units. For example: Requirements of energy may be expressed as digestible energy, metabolisable energy or net energy, and for the sake of further confusion, the expression of the unit for energy may be given as megacalories, megajoules or feed units. The rules (equations) for calculation of energy requirements are derived from large sets of digestive trials with horses and other farm animals. Fortunately, it is possible to interconvert requirments expressed as digestible, metabolisable or net energy from one to the other, and to change megacalories into feed units or megajoules. The PC-Horse programme provide the user with the possibility to select between the two most important systems presently used throughout the world to calculate protein and energy requirements:

1: "European standard" based on the system for energy and protein requirements for horses developed in France over the last 20 years, and further adjusted by the Dutch. This system is presently recommended by a Nordic group of scientists as a common base for the Nordic countries. The "European standard" is the default choice for versions of the programme compiled for sale in European countries.

2: National Research Council recommendations (NRC 2007) which are used in the USA and many other countries worldwide. The NRC system expressess protein requirements as crude protein, but we have chosen to express the requirements for protein as digestible protein in the european countries. When NRC is selected as the mode of calculation of protein requirements in the programme, it is important for the user to ascertain that feed composition and nutrient requirements are given in the same units. Information on units are given in the "configure page" of the programme.

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