Piglet birth weight predicts grow–finish performance despite dietary lysine adjustment

Piglet birth weight is a key determinant of postnatal growth performance and carcass quality in commercial pig production. This study evaluated whether piglet birth weight predicts grow–finish performance and whether low-birth-weight (LBW) pigs respond differently to dietary standard ileal digestible (SID) lysine levels, compared with heavy-birth-weight (HBW) pigs. A total of 144 pigs (72 LBW: 0.9–1.2 kg; 72 HBW: 1.7–2.0 kg at birth) were selected at 70 days of age and allocated to a 3 × 2 × 2 factorial design, including three dietary SID lysine levels (control, +10%, and −10%), two birth weight categories, and two sexes. Pigs were fed isocaloric maize-soybean-based diets during a 77-day grow–finish period. Growth performance was recorded weekly, and carcass traits were measured at slaughter (147 days of age). Birth weight significantly influenced body weight and average daily gain, with HBW pigs remaining heavier throughout the experimental period up to slaughter. Dietary SID lysine did not influence growth performance or carcass traits within birth weight groups. During the finisher phase, HBW pigs increased feed intake when fed the low-lysine diet, whereas LBW pigs reduced intake and exhibited improved feed conversion ratio without affecting growth rate. HBW pigs produced heavier carcasses with greater backfat thickness, while lean meat percentage was unaffected by birth weight or dietary lysine level. Sex effects on carcass traits were observed primarily under low-lysine conditions. These findings confirm that piglet birth weight remains a strong predictor of finishing performance, indicating that moderate adjustments in dietary lysine supply from 70 days of age are insufficient to overcome growth limitations associated with low birth weight.