Threonine (Thr) is important for mucin and immunoglobulin production. We studied the effect of added dietary Thr on growth performance, health, immunity and gastrointestinal function of weaning pigs with differing genetic susceptibility to E. coli K88ac (ETEC) infection and challenged with ETEC. Forty-eight 24-day-old weaned pigs were divided into two groups by their ETEC susceptibility using mucin 4 (MUC4) gene as a marker (2 MUC4(-/-) , not-susceptible, and 2 MUC4(+/+) , susceptible, pigs per litter). Within genotype, pigs were fed two different diets: 8.5 (LThr) or 9.0 (HThr) g Thr/kg. Pigs were orally challenged on day 7 after weaning and slaughtered on day 12 or 13 after weaning. Before ETEC challenge, HThr pigs ate more (p < 0.05). The diet did not affect post-challenge growth, but HThr tended to increase post-challenge feed efficiency (p = 0.087) and overall growth (p = 0.087) and feed efficiency (p = 0.055). Before challenge, HThr pigs excreted less E. coli (p < 0.05), while after challenge, diet did not affect the number of days with diarrhoea and ETEC excretion. MUC4(+/+) pigs responded to the challenge with more diarrhoea, ETEC excretion and anti-K88 IgA in blood and jejunal secretion (p < 0.001). HThr pigs had a higher increase of anti-K88 IgA values in jejunal secretion (p = 0.089) and in blood (p = 0.089, in MUC4(+/+) pigs only). Thr did not affect total IgA and IgM values, morphometry of jejunum, goblet cells count in colon, total mucin from jejunum and colon, but varied jejunal goblet cells counts (p < 0.05). In the first two post-weaning weeks, 8.5 g Thr/kg diet may be not sufficient to optimize initial feed intake, overall feed efficiency and intestinal IgA secretion and to control the gut microbiota in the first post-weaning week, irrespective of the pig genetic susceptibility to ETEC infection.

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An experiment was conducted to test the hypothesis that excess dietary Leu affects metabolism of branched-chain amino acids (BCAA) in growing pigs. Forty barrows (initial body weight [BW]: 30.0 ± 2.7 kg) were housed individually in metabolism crates and allotted to 5 dietary treatments (8 replicates per treatment) in a randomized complete block design. The 5 diets were based on identical quantities of corn, soybean meal, wheat, and barley and designed to contain 100%, 150%, 200%, 250%, or 300% of the requirement for standardized ileal digestible Leu. Initial and final (day 15) BW of pigs were recorded. Daily feed consumption was also recorded. Urine and fecal samples were collected for 5 d following 7 d of adaptation to the diets. At the end of the experiment, blood and tissue samples were collected to analyze plasma urea N (PUN), plasma and hypothalamic serotonin, tissue BCAA, serum and tissue branched-chain α-keto acids, and messenger ribonucleic acid abundance of genes involved in BCAA metabolism. Results indicated that acid detergent fiber, average daily feed intake, and gain-to-feed ratio decreased (linear, P < 0.05) as dietary Leu increased. A trend (linear, P = 0.082) for decreased N retention and decreased (linear, P < 0.05) biological value of dietary protein was also observed, and PUN increased (linear, P < 0.05) as dietary Leu increased. A quadratic reduction (P < 0.05) in plasma serotonin and a linear reduction (P < 0.05) in hypothalamic serotonin were observed with increasing dietary Leu. Concentrations of BCAA in liver increased (linear, P < 0.001), whereas concentrations of BCAA in skeletal muscle decreased (linear, P < 0.05) as dietary Leu increased. Concentration of α-ketoisovalerate was reduced (linear and quadratic, P < 0.001) in liver, skeletal muscle, and serum, and α-keto-β-methylvalerate was reduced (linear, P < 0.001; quadratic, P < 0.001) in skeletal muscle and serum. In contrast, α-keto isocaproate increased (linear, P < 0.05) in liver and skeletal muscle and also in serum (linear and quadratic, P < 0.001) with increasing dietary Leu. Expression of mitochondrial BCAA transaminase and of the E1α subunit of branched-chain α-keto acid dehydrogenase increased (linear, P < 0.05) in skeletal muscle as dietary Leu increased. In conclusion, excess dietary Leu impaired growth performance and nitrogen retention, which is likely a result of increased catabolism of Ile and Val, which in turn reduces availability of these amino acids resulting in reduced protein retention, and excess dietary Leu also reduced hypothalamic serotonin synthesis.

There is no consensus concerning the Trp requirement for piglets expressed relative to Lys on a standardized ileal digestible basis (SID Trp : Lys). A meta-analysis was performed to estimate the SID Trp : Lys ratio that maximizes performance of weaned piglets between 7 and 25 kg of BW. A database comprising 130 experiments on the Trp requirement in piglets was established. The nutritional values of the diets were calculated from the composition of feed ingredients. Among all experiments, 37 experiments were selected to be used in the meta-analysis because they were designed to express the Trp requirement relative to Lys (e.g. Lys was the second-limiting amino acid in the diet) while testing at least three levels of Trp. The linear-plateau (LP), curvilinear-plateau (CLP) and asymptotic (ASY) models were tested to estimate the SID Trp : Lys requirement using average daily gain (ADG), average daily feed intake (ADFI) and gain-to-feed ratio (G : F) as response criteria. A multiplicative trial effect was included in the models on the plateau value, assuming that the experimental conditions affected only this parameter and not the requirement or the shape of the response to Trp. Model choice appeared to have an important impact on the estimated requirement. Using ADG and ADFI as response criteria, the SID Trp : Lys requirement was estimated at 17% with the LP model, at 22% with the CLP model and at 26% with the ASY model. Requirement estimates were slightly lower when G : F was used as response criterion. The Trp requirement was not affected by the composition of the diet (corn v. a mixture of cereals). The CLP model appeared to be the best-adapted model to describe the response curve of a population. This model predicted that increasing the SID Trp : Lys ratio from 17% to 22% resulted in an increase in ADG by 8%.

Dose-response studies of dietary leucine (Leu) in weaners are needed for a proper diet formulation. Dietary Leu effect was assessed in a 3-weeks dose-response trial with a 2 (genotype) x 5 (diets) factorial arrangement on one-hundred weaned pigs (9 to 20 kg body weight (BW)). Pigs differed for a polymorphism at the aminoadipate-semialdehyde synthase (AASS) gene, involved in lysine (Lys) metabolism. Pigs received experimental diets (d7 to d28) differing for the standardized ileal digestible (SID) Leu:Lys: 70%, 85%, 100%, 115%, 130%. Daily feed intake (ADFI), daily gain (ADG) and feed:gain (F:G) in all pigs and ADG and F:G in two classes of BW were analyzed using regression analysis with curvilinear-plateau (CLP) and linear quadratic function (LQ) models. Amino acid (AA) concentrations in plasma, liver, muscle and urine were determined. AASS genotype did not affect the parameters. Dietary Leu affected performance parameters, with a maximum response for ADG and F:G between 100.5% and 110.7% SID Leu:Lys, higher than the usually recommended one, and between 110.5% and 115.4% and between 94.9% and 110.2% SID Leu:Lys for ADG for light and heavy pigs respectively. AA variations in tissues highlighted Leu role in protein synthesis and its influence on the other branched chain AAs.