Feed is the greatest single cost in pig production. Energy accounts for the largest proportion of the cost. The amount of energy that can be derived from diet differs between protein, fat, and carbohydrates. Common systems to describe dietary energy are gross energy (released by burning a sample of feed in excess oxygen; useless for pig production), digestible energy (gross energy of feed minus the gross energy of feces), metabolizable energy (digestible energy minus energy excreted in urine and as combustible gases such as methane), and net energy (metabolizable energy minus the heat produced during digestion, metabolism, and excretion). Current net energy systems are based on the digestible nutrient content in feedstuffs and mixed diets. Net energy systems have the advantage that they express useable energy rather than potential energy and include the efficiency with which nutrients can be utilized. The net energy system provides a more realistic estimate of dietary energy. This will allow a better estimate of the effects of diet on performance. The lower energy value for protein and fibre, and the greater value for starch and fat in the net energy system affect diet formulation. Greater use of cheap feedstuffs and limited use of expensive protein-rich feedstuffs would lead to reduced feed costs and lower protein contents, thus reducing nitrogen excretion and the environmental impact of pig production. The different valuation of feeds will favour domestic crops such as peas. Together, a switch to the net energy system will improve the economics of both Canadian pig and crop production. Therefore, a shift to net energy will help the prairie provinces regain traditional feeding advantage relative to major competitors.