Energy value of pig feeds: effect of pig body weight and energy evaluation system

Ad libitum energy intake and performance in pigs depends on many animal and environmental factors in which feed energy density plays an important role. In addition, feed represents an important cost in pig production, and energy represents the greatest proportion of this cost. It is therefore important to express the feed energy value on an appropriate basis, and both energy supply (a dietary characteristic) and energy requirement (an animal characteristic) should be expressed using the same system. Energy content depends first on the nutrient composition; the constituents differ markedly in GE content (23.0, 39.0, 17.4, and 18.4 kJ/g for CP, fat, starch, and dietary fiber, respectively). Due to differences in digestibility and associated endogenous energy losses, the actual contribution of nutrients to apparent DE supply in growing pigs is even more variable and ranges from 31.7 kJ/g for fat, 22.4 kJ/g for CP, 17.2 kJ/g for starch, to only 3.2 kJ/g for dietary fiber. Nutrient composition also affects the efficiency of conversion of ME to NE, which varies from 90% for fat to 82% for starch and 60% for CP. Consequently, the energy values (relative to a conventional diet containing 14.2, 13.6, and 10.3 MJ/kg of DE, ME, and NE, respectively) of corn, soybean meal, and animal fat are 100, 104, and 235 on a DE basis; 102, 99, and 244 on a ME basis; and 107, 79, and 289 on a NE basis. Energy value thus depends on the system of evaluation. The energy density of pig feeds can also be affected by feed processing. For example, pelleting markedly increases fat and energy digestibilities in corn or full-fat rapeseed. Also the animal itself can affect the energy value of nutrients; digestion of dietary fiber becomes more efficient with increasing BW, with subsequent differences in energy content of feeds according to BW. In conclusion, a satisfactory characterization of the energy value of feeds should be based on their NE content. Factors affecting nutrients digestibility (e.g., BW and feed processing) should also be taken into account.