Prediction of optimal slaughter age in four commercial pig genetic lines using GAM growth and backfat curves combined with a desirability optimization approach

In commercial pig production, slaughter age and week are often defined by fixed criteria. This can lead to suboptimal decisions and increased feed costs. Thus, quantitative tools are needed to identify an optimal slaughter window. This study aimed to determine the optimal slaughter age for different commercial pig genetic lines and sexes by modeling growth dynamics in body weight and backfat thickness. Growth curves for live weight, backfat, and feed intake were fitted with Generalized Additive Models (GAMs). A composite desirability function was applied to estimate the optimal slaughter week under multiple criteria. Operating ranges were defined for live weight, cumulative feed intake, and age (weeks of life). Two metrics were defined:1) the biological optimum (tD), based on live weight, backfat thickness, and feed intake to prioritize carcass quality within the defined operating ranges; and 2) the economic optimum (tP), which additionally incorporates carcass price and feed cost to maximize profitability. Combining both metrics allows proposing slaughter windows differentiated by genetic line and sex. The results provide practical insights for maximizing carcass quality and production efficiency in commercial pig farming. Multi-criteria optimization placed the optimal-desirability week between 20 and 21 weeks across line and sex groups. When economic variables were included, the optimal-profit slaughter week consistently shifted later in age, clustering between 22 and 23 weeks. GAMs consistently captured nonlinear growth phases, showing earlier peaks in the commercial line, intermediate timing in F1, and later peaks in ML1/ML2. Within each line, females reached their peaks earlier than males, consistent with shifts in slopes and breakpoints. Concordance between GAM peak ages and optimization function outputs supports the biological plausibility of muscle and fat deposition trajectories. Together, these patterns offer practical value for optimizing slaughter timing and guiding selection decisions under commercial conditions. Nonlinear growth and tissue deposition in four commercial pig genetic lines were characterized using GAMs. Peak ages followed a consistent maturity gradient, with commercial lines reaching maxima earlier, F1 showing intermediate timing, and ML1/ML2 peaking later; females consistently reached peak values earlier than males. These patterns provide actionable criteria to optimize slaughter timing, tailor feeding and management by line and sex, guide selection for carcass yield and quality, and motivate multi-site validation and standardized ultrasonographic measurements.