Effect of increasing the dietary tryptophan to lysine ratio on plasma levels of tryptophan, kynurenine and urea and on production traits in weaner pigs experimentally infected with an enterotoxigenic strain of Escherichia coli.

ABSTRACT

This experiment examined if immune system stimulation of weaner pigs, initiated by inoculation an enterotoxigenic strain of Escherichia coli (ETEC), increased the requirement for dietary tryptophan (Trp), modulated the inflammatory response, altered plasma levels of Trp and its metabolite kynurenine (Kyn) and effected post-weaning diarrhoea. Individually housed pigs (n = 72) weaned at 21 d of age were allocated to one of six treatments (n = 12) according to a two by three factorial arrangement of (1) with or without ETEC infection and (2) three dietary ratios of standardised ileal digestible (SID) Trp to lysine (Lys) (SID TrpLys) of 0.16, 0.20 or 0.24, in a completely randomised block design. Pigs had ad libitum access to diets (per kg 14.13 MJ ME, 12.4 g SID Lys, 195 g crude protein) for 3 weeks after weaning. Pigs were infected with ETEC (O149K98K88) at 72, 96 and 120 h after weaning and then bled on day 3, 11 and 19. An increased dietary TrpLys ratio increased plasma Trp and Kyn (p < 0.001) without effect of infection. On day 3, pigs fed 0.24 SID TrpLys had lower levels of plasma urea than at 0.20 TrpLys (p = 0.047) and on day 11, plasma urea was lower at 0.20 than at 0.16 SID TrpLys (p = 0.007). Infection increased (p = 0.039) the diarrhoea index and deteriorated faecal consistency from day 4-10 (p < 0.05). Treatments did not affect haptoglobin and acid soluble glycoprotein levels or daily gain and feed intake. However, 0.24 SID TrpLys improved (p = 0.021) feed efficiency without an effect of infection. In conclusion, in the absence of dietary antibiotic growth promotants, increasing the dietary SID TrpLys ratio to 0.24 improved feed conversion ratio after weaning and increased plasma levels of Trp and Kyn regardless of infection with E. coli.