138 Effect of dietary leucine on the tryptophan requirement in diets for 25-50 kg growing pigs fed soybean meal or corn fermented protein

An experiment was conducted to test the hypothesis that the optimum Trp:Lys ratio in diets for growing pigs is greater if corn fermented protein (CFP) rather than soybean meal (SBM) is used as the protein source in the diets. Two basal diets based on corn and SBM or corn and CFP were formulated. Both diets contained 0.90% SID Lys and both diets had a standardized ileal digestible (SID) Trp:Lys ratio of 10% (i.e., 60% of the requirement). All other indispensable amino acids were included at or above the requirement in both basal diets. Eight additional diets were formulated by adding crystalline L-Trp to both basal diets to generate diets with 14, 17, 20, or 24% SID Trp:Lys ratio for each protein source. Therefore, there were five SID Trp:Lys ratios for each protein source. A total of 320 growing pigs with an initial body weight (BW) of 25.53 ± 2.85 kg were randomly allotted to one of the 10 dietary treatments and fed experimental diets for 28 d. On day 1 and 28, a blood sample, which was analyzed for blood urea nitrogen (BUN), was collected from the jugular vein of one barrow in each pen. Data were analyzed by linear and quadratic contrasts in SAS (Inst. Inc. Cary, NC, USA). The optimal SID Trp:Lys ratio was estimated using linear broken-line and quadratic broken-line regressions for average daily gain (ADG) and BUN, using the NLIN procedure in SAS. A Wilcoxon rank-sum test was used to determine if the requirement estimates for CFP or SBM were different. Results indicated that pigs fed the corn-SBM diets had greater (P < 0.05) final BW, ADG, average daily feed intake, gain to feed ratio, and daily Trp intake than pigs fed corn-CFP diets (Table 1). On d 28, BUN decreased (linear and quadratic, P < 0.05) as the SID Trp:Lys ratio increased regardless of protein source. For pigs fed the SBM diets, combined broken-line quadratic analyses estimated the optimum SID Trp:Lys requirement as 18.95 and 15.54% for ADG and BUN, respectively, but for pigs fed the CFP-based diets, the optimum SID Trp:Lys requirement were 20.51 and 15.65% for ADG and BUN, respectively. The average SID Trp:Lys ratio for the two measurements was 18.08 and 17.24% for CFP and SBM, respectively. These values were not different (Table 2), and under the conditions of this experiment, the requirement for Trp was not different in diets based on CFP compared with diets based on SBM. Therefore, the hypothesis for the experiment was rejected. In conclusion, pigs fed diets based on CFP rather than SBM have reduced growth performance, but the main reason for this does not appear to be a deficiency of Trp.