Individual responses of growing pigs to diets with valine and isoleucine to lysine ratios

• SID Val:Lys requirement was found to be 0.70 (ranged from 0.62 and 0.68) for optimal N retention. • SID Ile:Lys requirement was found to be 0.62 (ranged from 0.52 and 0.60) for optimal N retention. • Variability between pigs’ requirements can be used for the development of cost-effective amino acid-balanced feeds. Two nitrogen balance (NB) studies were performed to estimate individual responses and the ideal standardized ileal digestible (SID) of valine (Val) and isoleucine (Ile) to lysine (Lys) ratios, as well as to examine the individual variability among pigs of 20 to-30 kg. Eight castrated male pigs with an initial average body weight of 20.80 ± 1.30 kg were used to evaluate SID Val:Lys in experiment 1 (Exp 1), and eight males weighing 21.90 ± 1.66 kg were used to assess SID Ile:Lys in experiment 2 (Exp 2). The experiments lasted 28 days, consisting of 7 days of adaptation and 21 days of collection. The SID Val:Lys and SID Ile:Lys in diets increasing by a total of seven equal increments (SID Val:Lys from 0.56 to 0.74 and SID Ile:Lys from 0.43 to 0.61) were sequential fed every 3 days for each pig. Nitrogen retention (NR) as a function of SID Val:Lys and SID Ile:Lys was estimated per individual and per group using a linear broken-line (LBL) and was also estimated per group using a quadratic broken-line (QBL). In the LBL adjusted for each individual in Exp 1, the highest breakpoint was 0.68 and the lowest was 0.62. In terms of NR, the highest plateau in the LBL was 52.03 g and the lowest was 34.78 g. In the LBL adjusted for each individual in Exp 2, the highest breakpoint was 0.60 and the lowest was 0.52. The highest plateau for NR in the LBL was 56.04 g and the lowest was 36.44 g. In the LBL model including all animals, the optimal ratio in Exp 1 was estimated to be 0.67 (breakpoint), and the NR plateau was 43.25 g. In the QBL model, the optimal ratio estimated was 0.70 (breakpoint) and the NR plateau was 42.75 g. In the LBL model including all animals, the optimal ratio in Exp 2 was estimated to be 0.55 (breakpoint) and the NR plateau was 40.89 g. In the QBL model, the optimal ratio estimated was 0.62 (breakpoint) and the NR plateau was 42.32 g. The individual ideal ratios for pigs of 20 to-30 kg varied from 0.62 to 0.68 for SID Val:Lys and from 0.52 to 0.60 for SID Ile:Lys. The estimated ideal ratios to optimize NR were 0.70 for SID Val:Lys and 0.62 for SID Ile:Lys.