CHARACTERIZATION OF GROWTH-PARAMETERS NEEDED AS INPUTS FOR PIG GROWTH-MODELS

Swine growth models have the potential to evaluate alternative managem ent decisions and optimize production systems. However, the lack of ec onomical, yet accurate methods to obtain the growth parameters require d to characterize pig genotypes, and which are required by growth mode ls, limits their widespread implementation. The four primary parameter s required are 1) daily whole-body protein accretion potential, 2) par titioning of energy intake over maintenance between protein and lipid accretion, 3) maintenance requirements for energy, and 4) daily feed i ntake. Estimation of daily protein accretion rates requires that seria l estimates of composition and growth be fitted to flexible nonlinear functions. Serial dissection and chemical analysis are too expensive t o be routinely conducted on an adequate number of pigs for precise dai ly protein accretion rates at different live weights. Three alternate methods include 1) serial slaughter and double sampling; 2) use of ser ial 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 an d protein accretion can be expressed as two interchangeable measuremen ts, either as the slope of protein accretion or the change in the lipi d: protein gain ratio as a function of energy intake at each live weig ht. Both methods require serial estimates of composition and scale fee ding of pigs to specified energy intake levels. Maintenance requiremen ts for energy are better expressed as a function of protein mass than body weight. However, differences in body protein mass do not fully ex plain difference in maintenance requirements between various pig genot ypes. Daily feed intakes at each live weight can be estimated by accur ately collecting feed intake data at least three live weight ranges an d fitting the data to nonlinear functions. An alternative method to es timate daily feed intake is to develop daily lipid and protein accreti on 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 para meters. The selection criteria and testing environment direct the rela tive genetic change for each growth parameter. The different sexes may also be affected differently by selection. For this reason, each clos ed uniformly selected population must be evaluated for each parameter for each sex.