An increased ratio of dietary tryptophan to lysine improves feed efficiency and elevates plasma tryptophan and kynurenine in the absence of antimicrobials and regardless of infection with enterotoxigenic Escherichia coli in weaned pigs

This experiment examined if a higher ratio of dietary Trp:Lys in the absence of antimicrobials improves production indices and modulates diarrhea in weaned pigs infected with enterotoxigenic Escherichia coli (ETEC). Effects of the Trp:Lys ratio on plasma levels of Trp and its metabolite kynurenine (Kyn) were also examined. Individually housed mixed-sex pigs (n = 72) weaned at 21 d of age (Landrace × Large White; initial BW of 6.3 ± 0.32 kg) were stratified into 1 of 6 treatments (n = 12) according to a 2 × 3 factorial arrangement of (i) infection or without infection with ETEC and (ii) 3 dietary standardized ileal digestible (SID) Trp:Lys ratios of 0.17, 0.21, or 0.26 in a randomized complete block design. Pigs were fed diets (10.4 MJ NE; 1.24% SID Lys; 19.5% CP) ad libitum for 3 wk after weaning. Pigs were infected with ETEC (O149:K98:K88) at 72, 96, and 120 h after weaning and then bled on day 11. A Trp:Lys ratio of 0.26 improved (P = 0.021) G:F over the study period compared to other ratios, without an infection effect (P > 0.05). Treatments did not affect ADG or ADFI (P > 0.05). Infection increased (P = 0.039) the diarrhea index and increased fecal consistency scores (P = 0.010). Plasma Trp and Kyn were lower (P < 0.001) in pigs fed 0.17 Trp:Lys than those fed ratios of 0.21 and 0.26 and were not affected (P > 0.05) by infection. In conclusion, in the absence of antimicrobials, increasing the dietary Trp:Lys ratio to 0.26 improved G:F after weaning and increased plasma levels of Trp and Kyn.