Optimum proportion of standardized ileal digestible sulfur amino acid to lysine to maximize the performance of 25-50 kg growing pigs fed reduced crude protein diets fortified with amino acids

The study was conducted to determine the standardized ileal digestible (SID) sulfur amino acid (SAA) to lysine (Lys) ratio required to maximize the performance of 25-50 kg pigs fed reduced crude protein (CP) diets fortified with crystalline amino acids. A total of 360 crossbred (Duroc × (Landrace × Large White)) pigs, weighing 25.6 ± 2.7 kg, were blocked by gender, litter, and initial body weight (BW) and allotted to 1 of 5 dietary treatments with 6 pens per treatment and 12 pigs per pen for a 35-day performance trial. The basal diet was based on corn, soybean meal, and wheat bran and was formulated to be deficient in SAA (50% proportion of SID SAA to Lys). Graded levels of dl-methionine were added to the basal diet at the expense of wheat bran in order to provide 55.6, 60.0, 65.6, or 70.0% proportion of SID SAA to Lys, respectively. A constant SID Lys level of 0.90% was set so that Lys was the second limiting amino acid (AA) in all diets. Average daily gain (ADG) and feed conversion ratio (FCR) improved (linear and quadratic, P < 0.05) with increasing dietary proportion of SID SAA to Lys. Increasing the dietary proportion of SID SAA to Lys decreased the serum urea nitrogen (SUN) level (quadratic, P < 0.05). A two-slope broken-line model estimated the optimum proportion of SID SAA to Lys to be 62.2, 61.5, and 62.3% for maximum ADG and minimum FCR and SUN, respectively, whereas a curvilinear-plateau model yielded an optimum proportion of SID SAA to Lys level of 63.8, 62.5, and 61.5% for maximum ADG and minimum FCR and SUN, respectively. Based on an average of these estimates, we conclude that the proportion of SID SAA to Lys required for 25-50 kg pigs fed low CP diets is 62.3%. This estimate is higher than the NRC (2012) recommendation of 56.1% for 25-50 kg pigs fed normal CP diets.