SID Trp–Lys Ratio on Pig Performance and Immune Response After LPS Challenge

This study aimed to evaluate the effects of the standardized ileal digestible tryptophan-to-lysine (SID Trp-Lys) ratio through the supplementation of different levels of L-tryptophan on pig performance and immune response following an LPS challenge. A total of 120 entire male pigs, with an average body weight of 16.5 ± 0.50 kg, were allocated in a randomized block design with four treatments, ten replicates per treatment, and three animals per experimental unit. The experimental treatments consisted of SID Trp-Lys ratios of 16%, 18%, 21%, and 24%, achieved through L-tryptophan supplementation. The evaluated performance parameters included the final body weight (BW), average daily gain (ADG), average daily feed intake (ADFI), and feed conversion ratio (FCR). Blood samples were collected on day 21 to determine serum serotonin levels, and on day 26, pigs were inoculated with LPS to induce an immune challenge, followed by blood sampling to assess cytokine responses. The results showed that pigs fed the 16% SID Trp-Lys ratio exhibited a lower FBW (<i>p</i> < 0.05). The SID Trp-Lys ratios influenced performance parameters, with quadratic responses (<i>p</i> < 0.05) observed for the FBW and FCR, where the highest FBW and lowest FCR were recorded at 22.05% and 21% SID Trp-Lys, respectively. A linear increase (<i>p</i> < 0.05) was observed for ADG, with a trend for a linear increase (<i>p</i> = 0.056) in ADFI. No effects (<i>p</i> > 0.10) of the SID Trp-Lys ratios were detected on serum serotonin levels. An increase in cytokine levels (GM-CSF, IFN-γ, IL-1α, IL-1β, IL-1ra, IL-4, IL-6, IL-8, IL-10, IL-12, IL-18, and TNF-α) was observed in pigs challenged with LPS (<i>p</i> < 0.10) compared to non-challenged animals. An interaction effect (<i>p</i> < 0.10) was detected for IL-2 and IL-18. SID Trp-Lys ratios between 21% and 24% optimize growth performance in pigs from 16 to 33 kg and modulate the immune response under LPS-induced challenge conditions.