Effect of Standardized Ileal Digestible Lysine to Crude Protein Ratio on Growth Performance and Blood Urea Nitrogen of 45 to 125 kg Pigs

Four experiments were conducted to determine the effect of standardized ileal digestible (SID) Lys to crude protein (CP) ratio on growth performance and blood urea nitrogen (BUN) of 45 to 125 kg pigs. In each experiment, pens of pig were blocked by body weight (BW) and randomly assigned to one of five dietary treatments in a randomized complete block design. There were 8 to 10 pigs per pen and 13 or 14 replications per treatment in each experiment. A total of 621, 663, 624, and 609 pigs were used in Exp. 1 through 4, respectively. Pigs were DNA 600 × 241 and had initial BW of 45.5±0.53 kg, 55.1±0.49 kg, 84.8±0.85 kg, and 103.8±0.61 kg for Exp. 1 to 4, respectively. Pens were mixed gender, and each experiment lasted 21 d. Dietary treatments were formulated to contain 0.94, 0.83, 0.71, and 0.63% SID Lys for Exp. 1 to 4, respectively, which corresponded to 90% of their estimated Lys requirements. Within each experiment, diets provided SID Lys:CP ratios of 85.0, 92.5, 100.0, 107.5, and 115.0% of requirement estimates calculated from NRC (2012). Intermediate SID Lys:CP levels were achieved by blending the extreme diets. Blood samples were collected on d 14 of each experiment for determination of BUN. Dose response curves were evaluated using linear, quadratic polynomial, cubic polynomial, and broken-line linear (BLL) models. In Exp. 1, increasing SID Lys:CP had no effect on final BW and ADG, but increased (linear, P < 0.001) ADFI and decreased (linear, P < 0.001) G:F. In Exp. 2, final BW was unaffected, ADG tended to decrease (linear, P = 0.053), ADFI increased (quadratic, P = 0.025), and G:F decreased (linear, P < 0.001) with increasing SID Lys:CP. In Exp. 3, increasing SID Lys:CP ratio decreased (linear, P ≤ 0.046) final BW, ADG, and G:F. In Exp. 4, no effects were observed on BW or ADG, but ADFI tended to increase (linear, P = 0.069) and G:F decreased (linear, P = 0.009) with increasing SID Lys:CP. Across experiments, increasing SID Lys:CP ratio reduced N intake and BUN (linear, P ≤ 0.001). Cubic models provided the best fit for Exp. 1, 2 and 4, while a linear model best fit Exp. 3. In conclusion, increasing SID Lys:CP worsened G:F and decreased BUN in 45 to 125 kg pigs. Based on the local minima determined using cubic models, estimated SID Lys:CP ratios to optimize G:F were 5.55% (45 to 70 kg), 5.47% (55 to 78 kg), and 5.27% (103 to 125 kg). For 84 to 108 kg pigs, requirements were estimated below 5.13% (linear model).