Effects of dietary tryptophan and threonine supplementation above nutritional recommendations on performance, diarrhoea, and intestinal health parameters in weaned piglets

•20% extra Thr and Trp above requirement stimulated goblet cell area in the colon.•20% extra Thr and Trp reduced the number of antibiotic treatment days.•Extra amino acids did not reduce the probability of diarrhoea.•Piglet performance was unaffected by 20% extra Thr and trp. Pigs may have an enhanced need for the amino acids (AA) tryptophan (Trp) and threonine (Thr) around weaning. This study aimed to evaluate the effect of additional dietary Trp and Thr above nutrient requirements; on performance, the probability of diarrhoea, intestinal permeability, and morphology in weaned pigs. A total of 120 piglets (7.20 ± 0.26 kg), weaned at day 28, were randomly allocated to five dietary treatments and individually housed. A standard diet with maximum crude protein (CP) according to Danish nutrient recommendations was used as control diet (HP). The other dietary treatments consisted of the HP diet with additional 20% Thr (THR), 20% Trp (TRP) or 20% of both (THR+TRP) relative to the recommendations for these AA (standardised ileal digestible (SID); 7.8 g SID Thr/kg and 2.6 g SID Trp/kg). Finally, a low-protein diet according to recommendations was included as a negative control (203 g CP/kg DM; LP). Diets were designed to be isoenergetic and fed ad libitum from 0 to 4 weeks post-weaning. Faecal consistency was evaluated daily using a 4-scale visual scoring system. Blood was collected on day 4, 14, 21, and 28 and small intestinal and colonic tissue were obtained from HP, LP, and THR+THR pigs at day 28. Average daily gain and feed intake were unaffected by diet (P > 0.10), whereas the feed conversion ratio was increased with 11% in LP compared with HP pigs (P 0.10), neither in separate weeks nor across the entire period. The number of antibiotic treatment days were 3.92 days higher for TRP compared with THR+TRP pigs (P = 0.01), with the other three diets being intermediate. The concentration of the intestinal permeability markers diamine oxidase and d-lactate in blood was unaffected by the diets. The systemic inflammatory markers haptoglobin and C-reactive protein did not differ between feeding groups. The THR+TRP diet increased the area of neutral mucin-producing cells in the colon with 83% compared with the HP diet (P = 0.02), but other intestinal morphology measurements were unaffected by the diets. In conclusion, supplementation with Thr and Trp together stimulated neutral mucin