Effects of dietary standardized ileal digestible tryptophan:lysine ratio on performance, plasma urea nitrogen, ileal histomorphology and immune responses in weaned pigs challenged with Escherichia coli K88

A study was conducted to determine the optimal standardized ileal digestible (SID) tryptophan:lysine (Trp:Lys) ratio in piglets challenged with Escherichia coil K88 (E.coli K88) and fed antibiotic -free diets. Thirty individually housed mixed -sex pigs (Duroc x [Yorkshire x Landrace]) with an initial body weight (BW) of 6.4 +/- 0.18 kg and weaned at 21 +/- 1 day (d) were randomly assigned to 5 dietary treatments each with 6 pig replicates. Dietary treatments consisted of increasing levels of SID Trp:Lys ratios (16.1%, 18.6%, 20.3%, 22.9%, and 24.6%). Diets were corn-wheat-soybean meal-based with a constant SID Lys of 1.18% that was set to be the second limiting amino acid (AA) but adequate in other AA. Pigs had ad libitum access to feed and water for 13 d. After feeding the experimental diets for 6 d, pigs were orally challenged with 6 mL of E. coil K88 (2 x 10(11) colony forming unit/mL) on d 7. Body weights and pen feed disappearance were recorded weekly to determine ADG, ADFI and G:F. On d 13, all pigs were euthanized and ileal tissue samples were collected to measure mRNA expression of tumour necrosis factor-alpha (TNF-alpha), interleukin-10 (IL-10) and interferon-gamma (IFN-gamma) using qRT-PCR. During the pre -challenge period (d 0-6), increasing dietary SID Trp:Lys ratio increased (linear, P < 0.05) ADG (157, 162, 173, 179 and 201 g/d) and G:F (0.71, 0.73, 0.74, 0.81 and 0.84). During the post-challenge period, increasing SID Trp:Lys ratios tended (linear, P = 0.08) to increase ADG (177, 180, 208, 210 and 213 g/d), whereas there was no effect on G:F (0.58, 0.63, 0.67, 0.66, 0.65) (P > 0.10). The optimal SID Trp:Lys ratio determined using the broken-line regression analysis for ADG and G:F in piglets subjected to E.coli K88 challenge was 21.7% and 20.1%, respectively. Expression of IL-10 mRNA, increased (linear and quadratic, P < 0.01) with increasing SID Trp:Lys ratio, however, expression of TNF-alpha and IFN-gamma mRNAs were not affected. In conclusion, in antibiotic free starter diets, an average optimal SID Trp:Lys ratio of 21% optimized performance of piglets under an E. colt K88 challenge.