Modeling chemical and physical body composition of the growing pig

ABSTRACT In pig growth models, masses of body lipid (L) and body protein (P) are key state variables that can be related quantitatively to chemical and physical body composition for predicting growth response and carcass characteristics. The main chemical constituents in the empty body weight (EBW) are water (Wa), L, P, and ash. Within pig genotypes, Wa is independent of L and closely related to P (e.g., Wa = a × P^b). The scaling parameter (b) is remarkably constant across pig types, at about 0.855, and represents changes in distribution of P with increasing EBW and differences in Wa-to-P ratios among body pools. The parameter “a” ranges between 4.90 and 5.62 and seems to vary with pig genotype. The ash-to-P ratio, about 0.20, has little significance on estimates of EBW. Gut fill, the difference between live body weight (LBW) and EBW, ranges between 0.03 and 0.10 of LBW; it varies with LBW, feeding level, diet characteristics, and time off-feed. The distribution of P and L over the main physical body components (dissectible muscle and fat, viscera, blood, bone, integument) varies considerably among groups of pigs and appears influenced by EBW, pig genotype, feeding level, diet characteristics, and possibly thermal environment and health status. Except for extreme pig genotypes, the distribution of lean over the main carcass cuts is relatively constant; however, little is known about the observed variation in the distribution of L over body fat depots. Representing dynamic effects of animal and external factors on sizes of physical body components is an apparent weakness in pig growth models, further complicated by inconsistencies in defining some of the physical body components, and dissectible lean tissue in particular. Improved accuracy in representing physical body composition will provide more insight on manipulation of carcass value and efficiencies of converting diet nutrients into pork products.

Implications

In pig growth models, body composition is generally predicted from masses of body lipid and protein. Close attention should be paid to the relationship between body water mass and protein, which is remarkably constant across body weights and feeding regimens, but varies with pig genotype. The distribution of protein and lipid over the main physical body components (dissectible muscle and fat tissue, viscera, blood, bone, integument) varies considerably among groups of pigs and seems influenced by BW, pig genotype, feeding regimens, thermal environment, and health status. The dynamic representation of physical body components is an apparent weakness in pig growth models. This is further complicated by inconsistencies in defining physical body components, and dissectible lean tissue in particular. Improved accuracy in representing physical body composition will provide more insight on manipulation of carcass value and efficiencies of converting diet nutrients into pork products.

Key Words: Body Composition, Chemical Composition, Growth Models, Physical Properties, Pigs

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

J. Anim. Sci. 2003. 81(E. Suppl. 2):E159-E165