Characterization of growth parameters needed as inputs for pig growth models

Swine growth models have the potential to evaluate alternative management decisions and optimize production systems. However, the lack of economical, yet accurate methods to obtain the growth parameters required to characterize pig genotypes, and which are required by growth models, limits their widespread implementation. The four primary parameters required are 1) daily whole-body protein accretion potential, 2) partitioning of energy, intake over maintenance between protein and lipid accretion, 3) maintenance requirements for energy, and 4) daily feed intake. Estimation of daily protein accretion rates requires that serial estimates of composition and growth be fitted to flexible nonlinear functions. Serial dissection and chemical analysis are too expensive to be routinely conducted on an adequate number of pigs for precise daily protein accretion rates at different live weights. Three alternate methods include 1) serial slaughter and double sampling; 2) use of serial live measurements to estimate composition, i.e., serial ultrasonic measurements; and 3) use of generalized functions that estimate daily protein accretion as a function of mean daily fat-free lean gain over a specified weight interval. The energy partitioning between lipid and protein accretion can be expressed as two interchangeable measurements, either as the slope of protein accretion or the change in the lipid: protein gain ratio as a function of energy intake at each live weight. Both methods require serial estimates of composition and scale feeding of pigs to specified energy intake levels. Maintenance requirements for energy are better expressed as a function of protein mass than body weight. However, differences in body protein mass do not fully explain difference in maintenance requirements between various pig genotypes. Daily feed intakes at each live weight can be estimated by accurately collecting feed intake data at least three live weight ranges and fitting the data to nonlinear functions. An alternative method to estimate daily feed intake is to develop daily lipid and protein accretion curves. On the basis of their energetic costs of lipid and protein deposition and assumed maintenance requirements, daily energy intakes can be estimated. Genetic selection changes the underlying growth parameters. The selection criteria and testing environment direct the relative genetic change for each growth parameter. The different sexes may also be affected differently by selection. For this reason, each closed uniformly selected population must be evaluated for each parameter for each sex.