EXAMINATION OF THE EFFECTIVENESS OF PROVISION OF FUNCTIONAL AMINO ACIDS TO ENHANCE ROBUSTNESS IN ENTERIC CHALLENGED PIGS

The predisposition of weaned pigs to enteric pathogens may increase if high protein (HP) diets, particularly from plant based (PB) sources, are fed. Growth performance and immune status of diseased pigs may be improved by supplementation with functional amino acids (FAA), including Threonine (Thr), Methionine (Met) and Tryptophan (Trp). The beneficial effects of FAA supplementation may be more advantageous in susceptible categories such as low birth weight (LBW) pigs. Therefore, this thesis evaluated the independent effect of FAA supplementation in Salmonella Typhimurium (ST)-challenged pigs and its interaction with dietary protein content and source, and with pig birth weight. The impact of adaptation period to FAA before a ST challenge was also evaluated. A disease challenge study revealed improved growth performance and immune status of ST-challenged pigs fed diets with supplemented with FAA, specifically Thr, Met, and Trp at 120% of requirements for growth, regardless of dietary protein content (low protein; LP [16%] vs. HP [20%]). The beneficial effects of FAA were associated with improved intestinal health and antioxidant defense, despite a lack of effect on ST presence in lymphoid tissues. In a subsequent ST-challenge study, a longer adaptation period (i.e., 2 weeks) to the same FAA-supplemented diets further enhanced gut health and suppressed ST intestinal colonization, despite no effect on ST presence in lymphoid tissues. Furthermore, antioxidant defense measurements were improved by FAA intake, but not adaptation period, which may be attributed to the dynamic sulfur amino acid (SAA) metabolism. Using intestinal samples from the first two studies, a third study provided evidence that the effects of FAA supplementation in ST-challenged pigs previously reported may be associated with or mediated by intestinal alkaline phosphatase (IAP) activity. Interestingly, there was a lack of effect of FAA adaptation period on IAP activity, which may suggest that amino acid (AA) intake, rather than length of feeding, is more important to the regulation of antioxidant balance. To investigate possible carryover effects of FAA supplementation, two follow-up disease challenge studies were conducted. The first one showed that FAA supplementation during nursery (e.g., 31 d post-weaning) improved growth performance, suppressed ST shedding, and reduced bacterial translocation to spleen in pigs placed onto a common grower diet and subsequently challenged with ST. However, the positive effects were observed particularly in normal birth weight (NBW), but not LBW pigs, which had aggravated gastrointestinal dysfunction, oxidative stress, and systemic commitment of the immune system. In the second study, pigs previously fed a PB feeding program in the nursery period (e.g., 31 days) had increased susceptibility to a subsequent ST challenge, particularly with increased bacterial colonization in the intestine, shedding, and diarrhea, compared to pigs fed animal-based (AB) protein sources. Conversely, supplementing PB, but not AB, nursery diets with FAA attenuated the negative effects of the subsequent ST challenge, despite a lack of effect of FAA supplementation on systemic markers of acute-phase responses and antioxidant balance. In summary, FAA supplementation and adaptation period is a valuable strategy to improve growth performance and immune status of ST-challenged pigs. The positive effects are more pronounced in NBW pigs and PB diets, regardless of dietary protein content