Determination of the standardised ileal digestible requirements of tryptophan and sulphur amino acids in weaner pigs under conditions of enterotoxigenic Escherichia coli challenge

A series of six experiments was conducted to determine if supplementing the diet with additonal tryptophan (Trp) and/or sulphur amino acids (SAA) would improve production and markers of inflammation in the gastrointestinal tract (GIT) of weaner pigs and particularly in pigs not provided with in-feed antimicrobial compounds (AMC) and suffering from infection with Escherichia coli. The current recommended level of standardised ileal digestible (SID) Trp:Lysine (Lys) and SAA:Lys ratios are 0.16 and 0.55 for pigs between 5-11 kg (weaner pigs), respectively. Chapter 3 investigated the effects of Trp compared to AMC under conditions of experimental infection with enterotoxigenic E. coli (ETEC). Experiment 1 Part A, was a 3 x 2 design with (i) 3 levels of SID Trp:Lys (no AMC in diets) and (ii) infection or non-infection with ETEC. Part A of Experiment 1 found only a low-grade inflammatory response to infection with ETEC however did find that pigs fedv a SID Trp:lys ratio of 0.24 improved feed conversion ratio (FCR) and increased plasma Trp and its metabolite kynurenine levels regardless of infection with ETEC. Experiment 1 Part B was a 2 x 2 x 2 design used (i) 2 levels of SID Trp:Lys (0.17 and 0.25) and (ii) with or without AMC in the diet and (iii) sacrifice on d 4 or 11 after weaning with all pigs infected with ETEC to determine gut function, structure and gene expression. Part B of Experiment 1 found that additional Trp and addition of AMC increased expression of intestinal alkaline phosphatase on d 11. Experiment 1B also found that additional Trp decreased activity of amino peptidase N on d 11 regardless of inclusion of AMC in the diet. Chapter 4 was conducted at a commercial facility using 6 levels of dietary SID Trp:Lys. Experiment 2 included AMC in the diet and found pigs had a low level of inflammation but did not fit any broken-line models. Nevertheless, Experiment 2 found that a ratio of 0.234 SID Trp:Lys had the best performance in terms of both daily gain and FCR. Experiment 3 examined the effects of 6 levels of SID SAA:Lys compared to AMC under conditions of experimental infection with ETEC. Chapter 5 consisted of 2 parts, similar to Chapter 3. Part A was a 6 x 2 experimental design of (i) 6 SID SAA:Lys ratios (0.51, 0.55, 0.61, 0.62, 0.67 and 0.70) and (ii) with or without AMC in the diet to determine effects on production performance and Part B was a 2 x 2 experimental design where pigs from the (i) 2 extreme SID SAA:Lys ratios (0.51 and 0.69) and (ii) with or without AMC were sacrificed on d 11 to determine gut function, structure and gene expression. Unfortunately, in the second replicate of pigs could not be used due to large variations from expected SAA levels in the diet. In Part A of Chapter 5, no differences in production performance was found. However it is likely that the cessation of the experiment meant that sample size was not large enough to determine differences between treatments. Chapter 5 Part B found trends of increasing SAA to decrease activity of leucyl amino peptidase activity and expression of intestinal alkaline phosphatase and glutathione reductase. Chapter 5B also found a trend for pigs fed AMC to have higher feed intake and growth when fed lower levels of SAA. Chapter 6 consisted of feeding 5 SAA:Lys ratios and infection with ETEC with no AMC in the diet. This study was successful in eliciting an inflammatory response and found optimum ratios of 0.73, 0.71 and 0.68 SID SAA:Lys for daily gain, feed intake and feed efficiency, respectively. Chapter 7 was conducted in a commercial facility and used 7 levels of SID SAA:Lys. This experiment included AMC in the diet and pigs showed low levels of inflammation, an optimum ratio of SID SAA:Lys of 0.61 was found for ADG in the first week and a ratio equivalent to the recommended level for FCR was found for first three weeks after weaning. Chapter 8 compared two levels of SID Trp:Lys (0.16 and 0.24) and 2 levels of SID SAA:Lys (0.52 and 0.60) under experimental infection with ETEC and found that supplementation above the recommended level of either Trp or SAA improved FCR while supplementation with both tended to further improve FCR in the first 2 weeks after weaning. In conclusion, the studies found that supplementation of Trp above the recommended level consistently improved FCR (regardless of inclusion of AMC) while the requirement of SAA was higher than the recommended level when piglets were not supplied with AMC, but adequate for pigs fed AMC.