Evaluation of Different Mixed Model Nonlinear Functions to Describe the Feed Intakes of Pigs of Different Sire and Dam Lines1

Daily individual feed intakes (DFI) of 1,932 barrows and gilts from 3 sire and 2 dam lines were evaluated over 2 replicates. Five functions relating DFI to age or BW were evaluated: linear-quadratic, exponential, generalized Michaelis-Menten, nonlinear, and Bridges functions. The Bridges function provided the best fit to DFI data. Two pig-specific random effects were included. The barrow and gilt data were fitted separately because their values for one parameter of the Bridge function were different. For DFI, there were overall significant effects (P < 0.05) for replicate, sex, and the sire line × dam line interaction. Repeated measures analyses indicated significant (P < 0.001) interactions of sire line, sex, and replicate, with age and BW indicating that the shapes of the DFI curves were different for the pigs of the different sire lines, sexes, and replicates. The research indicates that separate DFI curves are needed for each sire line and sex, and potentially for season. Daily feed:gain values were estimated for each pig by using a generalized Michaelis-Menten function previously fitted to the BW data. The analyses of feed:gain values indicated significant (P < 0.05) effects for sire line, dam line, sex, sire line × sex, and sire line × replicate.