The amino acids requirements of growing-finishing pigs are met by utilizing a variety of protein rich ingredients, including soybean meal (SBM) and distillers dried grains with solubles, as well as synthetic amino acids. As nutritionists have become more comfortable with synthetics, this has obviously reduced the demand for SBM. However, there is growing suspicion that SBM may provide protective benefits to pigs experiencing disease challenges, especially in mid-late finishing phase of production. Anecdotal and unpublished data suggest that limiting the levels of synthetic amino acids in disease-challenged pigs can result in improved growth and financial performance. Therefore, our objectives were: 1) To determine whether dietary soybean meal displacement by crystalline amino acids increases the severity of a health challenge in finisher pigs; and 2) To determine the effect of a late breaking health challenge on finisher pig growth and carcass performance. To test these objectives, two experiments where conducted using two groups of 96 and 90 finisher pigs. In both experiments, pigs were stratified by BW across either a high SBM (HSBM) or low SBM (high synthetic Trp; LSBM) diet. Both diets were formulated to contain 0.70% TID Lys and were isocaloric; only crude protein was different between HSBM and LSBM diets (15.9 vs. 13.5%, respectively). In both experiments, all pigs were co-inoculated intratracheally with Mycoplasma hyopneumonia (MHP) and intramuscularly with a field strain of PRRSV, weekly pig performance and serology, and subsequent carcass characteristics were assessed over a 28 or 49 day test period (Exp1 and Exp2, respectively). For each experiment, pen was considered the experimental unit with Exp1 having 8 pens, 6 pigs per pen per treatment and Exp2 having 9 pens, 5 pigs per pen per treatment. Summary of results:

During the pre-challenge period, no performance differences were detected between the LSBM and HSBM fed pigs.
• As expected, antibody titers for PRRSV and MHP increased post inoculation. However, change in PRRSV antibody titers from 0 to 28 days post inoculation tended to be increased in LSBM verses HSBM pigs.
• MHP antibody titers and lung lesion scores did not differ between dietary treatments.
• No difference in ADG, ADFI or G:F between pigs challenges with PRRSV and MHP fed either a LSBM or HSBM diets.
• There was also no difference in hot carcass weight, yield percentage or muscle depth. However, the LSBM pigs had an increase in carcass fat depth compared to the HSBM pigs.
• Altogether, these data indicate that diets with increased synthetic AA or decreased SBM do not alter pig performance during a late breaking respiratory health challenge.