Two groups of 240 pigs (PIC 337 X 1050, PIC Genus, Hendersonville, TN) with initial body weight (BW) 13.20 ± 3.05 kg were used to investigate interactions between the branched-chain amino acids (BCAA). At weaning, pigs were placed into 40 pens with three barrows and three gilts per pen and fed a common diet for three weeks. On day 21 post-weaning, pens were randomly assigned to one of 15 dietary treatments arranged in a rotatable central composite design. Experimental diets were formulated to various levels of standardized ileal digestible (SID) leucine, isoleucine, and valine by replacing corn starch and glycine with L-leucine, L-isoleucine, and L-valine in the base diet. Levels of BCAA were expressed as ratios to SID lysine and ranged from 98 to 180 for Leu, 46 to 64 for Ile, and 51 to 78 for Val. Diets were formulated to be sub-limiting in lysine at 1.19% SID Lys and exceed all other essential amino acid requirements. Additionally, diets were formulated to be isocaloric and isonitrogenous and met or exceeded NRC (2012) vitamin and mineral requirements. Pen weights and feed intake were measured for the 21-day experiment to calculate daily gain (ADG), average daily feed intake (ADFI), and feed efficiency (G:F). Data were fit as a second-order response surface model according to the central composite design in R version 4.2.1 (R Core Team, 2022). The model included linear and quadratic effects of Leu, Ile, and Val, and their cross-products. Pen was the experimental unit, and parameters were considered significant at P ≤ 0.10. Valine linearly and quadratically impacted ADG and G:F, regardless of Leu and Ile levels. There was an interaction between Leu and Ile for ADG (P = 0.079) and G:F (P = 0.031), where, at increasing levels of Leu, decreasing Ile improved ADG and G:F. Conversely, ADG and G:F decreased as both Leu and Ile increased. Isoleucine did not impact ADFI (P ≥ 0.458); however, there was an interaction between Leu and Val (P = 0.088), where Leu negatively impacted ADFI at low levels of Val, but did not influence ADFI at higher levels of Val.
In conclusion, the response of nursery pig ADG and G:F to Val is independent of Leu and Ile levels, while ADG and G:F are reduced at high levels of Leu and Ile, which is resolved as either Leu or Ile are reduced. Furthermore, at deficient levels of Val, ADFI is negatively impacted by increased Leu; however, the negative impacts of increasing Leu on ADFI are negated when Val is adequate or above requirement. This experiment emphasizes the complexity of amino acid metabolism in nursery pigs and the importance of understanding potential interactions among amino acids when conducting research.