Partitioning of energy intake to heat, protein, and fat in growing pigs

ABSTRACT Modeling aspects of energy metabolism in growing pigs involves establishing “rules” on the partitioning of dietary energy between protein deposition (PD), lipid deposition (LD), and heat production (HP) at a given point in time, as well as the changes that occur during growth. Growing pigs rarely retain more than 50% of their ME intake; the remainder is lost as heat. Part of the heat loss is due to the heat increment, which includes the transformation of dietary nutrients to PD and LD, and to the associated energy (ATP) cost. Consequently, different nutrients are used with different efficiencies and, due to the ATP cost associated with protein synthesis and turnover, PD is energetically less efficient than LD. Different modeling approaches have been adopted to represent partitioning of energy between PD and LD (e.g., by assuming minimal ratios of LD:PD, marginal LD:PD, and lipid:protein mass or the existence of an upper limit to PD). Most of the HP is associated with biophysical processes (e.g., “maintenance,” physical activity) requiring ATP, which are not directly related to PD and LD. Since it is virtually impossible to obtain direct estimates of these ATP requirements, indirect methods must be used. For example, the cost of maintenance may be estimated by measuring the fasting HP. Estimates of the fasting HP typically range from 700 to 800 kJ/(kg of BW^0.60•d), which corresponds to 50 to 60% of the total HP. Also, HP associated with physical activity is an important component of HP (15%), but can be variable between individual animals. Feed intake in nonproducing, mature mammals theoretically equals maintenance energy requirements. This implies that, while maturing, maintenance will become an increasingly important component of energy intake. In addition, while maturing, a decreasing fraction of the energy intake above maintenance is used for PD. The result is that PD typically reaches a maximum at 60 to 80 kg in growing pigs and decreases thereafter. In contrast, with aging, an increasing fraction of the available energy is used for LD, and maximal LD may not be reached before slaughter (110 to 130 kg). In modeling, this has been represented by assuming that the aforementioned energy partitioning rules (e.g., minimal LD:PD ratio, upper limit to PD) change with BW and/or age.

Implications

Feed intake capacity and the partitioning of metabolizable energy between protein deposition, lipid deposition, and heat production in pigs change considerably during the growing and finishing periods. Although this review only addresses the energy aspects during the course of life, these changes also affect other aspects of nutrition. For example, the partitioning of energy between protein deposition and lipid deposition determines amino acid requirements. As this partitioning changes during growth, so will the optimal ratio between amino acids and energy. Traditionally, nutrient requirements originated from feed recommendation tables, and feeds were formulated to meet or exceed these requirements. As systems of energy and amino acid utilization become more refined, the classical notion of “requirement” and “feed value” becomes less clear. The use of mathematical models then becomes essential to quantify the growth response of an animal to changes in nutrient supply.

Key Words: Energy, Pigs, Heat Production, Lipids, Models, Pigs, Protein

© 2003, by the American Society of Animal Science. All rights reserved.

J. Anim. Sci. 2003. 81(E. Suppl. 2):E86-E93