A Review of Dietary Metabolizable and Net Energy: Uncoupling Heat Production and Retained Energy

Abstract

SUMMARY The net energy (NE) system is often misunderstood. Dietary NE consists of 2 fractions: NE for maintenance (NEm) and NE for production (NEp). In practical terms, NE is usually thought of as retained energy (RE), or the fraction of dietary energy that ends up in body tissues or secretions, including feathers and eggs. In productive animals in a positive energy balance, NEp is technically equivalent to RE. When in a positive energy balance, the NEm fraction supports an animal's basal metabolic functions that keep it alive, and NEm is entirely lost as heat. However, NEm excludes all other sources of heat energy that cannot be retained. In the metabolizable energy (ME) system, ME for maintenance (MEm) is often estimated empirically as total heat production (HP). Thus, MEm includes NEm and all other sources of heat. All remaining ME that is not lost as heat is used for production (MEp); MEp is the energy retained as body tissues or secretions, including feathers and eggs. Thus NEp and MEp are identical. In a positive energy balance, NEm and MEm are both entirely lost as heat, but they include different heat fractions. Separating NEm from total heat production is difficult and therefore of limited practical use. Further, the component of dietary energy lost as heat depends on feed components, but also animal and environmental factors. The total amount of dietary energy lost as heat (MEm) is easier to estimate empirically, and it can be used to calculate RE (by subtraction). In poultry, feces and urine are excreted together, and as a result ME is particularly straightforward to measure. Ease of ME measurement in poultry, difficulty in parsing sources of heat production, and RE equivalence (i.e., MEp = NEp) suggest that for the poultry industry the NE system provides no advantage over the ME system.