Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most economically significant pathogens in the U.S. today. The disease associated with this virus is Porcine reproductive and respiratory syndrome (PRRS), which antagonizes all stages of production by causing increased morbidity, mortality and reduced growth. Controlling the spread and eradication of this endemic pathogen has remained challenging due to the ease of transmission between animals, the various viral strains that exist, and varying vaccine efficacies. Thus, interest in nutritional strategies to help mitigate the negative growth performance phenotypes typically associated with a PRRSV challenge in growing pigs is rising. One nutritional strategy is to modulate the ratio of dietary amino acids (AA) to energy. Increasing standardized ileal digestible Lys per Mcal metabolizable energy (SID Lys:ME) above the requirement of healthy pigs has been reported to help mitigate reduced growth performance during a PRRSV challenge. Thus, the overall objective of this thesis was to evaluate the importance of increasing dietary SID Lys:ME above a growing pig requirement (i.e. targeting 120% of requirement) in pathogen challenged pigs to improve growth performance. Further, we also evaluated the formulation approaches used to achieve this increased ratio in PRRSV challenged pigs. To address this overarching thesis objective, a series of experiments were conducted and are outlined in two research chapters (Chapters 2 and 3).

In Chapter 2, the first experiment was conducted to evaluate the effects of increasing SID Lys:ME on growth performance in PRRSV vaccinated or nonvaccinated pigs facing a subsequent PRRSV challenge. Additionally, we evaluated the formulation approach used to achieve an increased SID Lys:ME (i.e. 120%), by either increasing SID Lys and other essential AA relative to energy, or by diluting energy relative to Lys. In Chapter 3, a second experiment was conducted to further evaluate the formulation approach used to achieve a 120% Lys:ME ratio in PRRSV challenged grower pigs, by utilizing an industry applicable feedstuff to reduce dietary energy. Although an increase in Lys:ME has been shown to be beneficial in PRRSV challenged pigs, a third experiment (Chapter 2) was conducted to assess if an increased Lys:ME ratio would be beneficial to growth performance in Mycoplasma hyopneumoniae (MHP) challenged late-finishing pigs.

The results from this research validates previous work that increasing dietary SID Lys:ME to 120% of grower pig's requirement during a PRRSV challenge aided in mitigating negative growth performance associated with a PRRSV challenge (Chapter 2 and 3). Additionally, irrespective of PRRSV vaccination status, diluting ME by ~20% with sand (inert feed ingredient) to achieve a 120% Lys:ME, resulted in increased feed intake throughout the PRRSV challenge period, which translated to an increased ADG and end BW compared to the 100% Lys:ME control (Chapter 2). Thus, these data highlighted that pigs attempt to eat to their energy needs, even while undergoing a severe health-challenge. Utilizing dietary fiber to achieve the targeted 120% Lys:ME in the diets (Chapter 3), an increase in feed intake was observed in pigs fed the reduced ME diet, which was achieved via soybean hulls inclusion. However, the increase in feed intake did not translate into increased overall ADG or end BW compared to 100% Lys:ME control (Chapter 3).

Results of the third experiment, also outlined in Chapter 3, reported that increasing Lys:ME to 120% of requirement had no effect on growth performance in late finishing pigs challenged with MHP. However, in non-MHP challenged pigs, increasing Lys:ME also had no bearing on late-finishing growth performance. These data suggest that pigs undergoing a bacterial MHP challenge may not benefit similarly to increased Lys:ME ratios as previously reported in virally challenged pigs or that increased Lys:ME is less important to support lean tissue growth in late-finishing.

In summary, this thesis concludes that both viral and bacterial health-challenges alter nutrient requirements of growing pigs as apparent by reduced pig performance during health-challenges. In PRRSV challenged pigs, this work herein validates that increasing SID Lys:ME to 120% of requirement augmented growth performance during the peak disease period of a PRRSV. Additionally, if pigs are fed a diet diluted in energy, these pigs will eat to their energy needs, even while undergoing a health-challenge. This increase in ADFI then translated to increased ADG and end BW in comparison to 100% Lys:ME fed pigs. However, increasing the Lys:ME ratio in bacterial challenged late finishing pigs resulted in no improvement in growth performance. Thus, indicating that feeding a diet containing Lys above requirement during a bacterial challenge in late finishing may not hold the same beneficial effects as previously seen in virally challenged grower pigs.