Excess Dietary Leucine Level Increases the Optimal Dietary Isoleucine to Lysine Ratio in 8 to 21 Kg Pigs

Feeding diets containing excess Leu can lead to branch-chained AA antagonisms, which may increase Ile requirement for pigs. A 28-d study was conducted to evaluate the effect of dietary Leu levels on the dietary standardized ileal digestible (SID) Ile:Lys ratio to optimize performance of 8 to 20 kg pigs. A total of 210 mixed-sex pigs (Danbred × Pietrain; initial BW 7.8 ± 0.20 kg) were assigned to 6 diet regimes using a 2 × 3 factorial arrangement with 2 levels of SID Leu:Lys ratio (110 and 165%) and 3 levels of SID Ile:Lys ratio (50, 54 and 58%) with 7 pen replicates (5 pigs per pen) per treatment. Diets were formulated based on corn, soybean meal, corn gluten meal, spray-dried blood cells using analyzed ingredient AA contents to meet requirements for AA except for Lys. Lysine was second limiting (1.21% SID Lys) in all diets. Pigs had ad libitum access to mash feed and water. Data were analyzed by ANOVA using the GLM procedure of SAS with initial BW and diets included in the model. Orthogonal-polynomial contrasts were used to determine the effect of SID Leu:Lys ratio (110 vs. 160%) and SID Ile:Lys ratios (50 vs. 54% and 54 vs. 58%) on performance responses. There was no Leu × Ile:Lys interaction (P > 0.05) for any measured parameter. During d 0-28, increasing SID Leu:Lys from 110 to 160% increased (P < 0.05) the final BW, ADG and G:F. When the diets contained 110% SID Leu:Lys, the final BW, ADG, ADFI and G:F increased (P < 0.05) by increasing the SID Ile:Lys from 50 to 54%; however, no further improvements were observed by increasing the dietary SID Ile:Lys from 54 to 60% (P < 0.05). Under excess dietary Leu (160% SID Leu:Lys), increasing the SID Ile:Lys from 50 to 54% increased (P < 0.05) the final BW, ADG and ADFI. The final BW, ADG and ADFI further increased (P < 0.01) by increasing the SID Ile:Lys from 54 to 58% in diets with excess Leu. However, the G:F was not affected by dietary Ile:Lys under excess dietary Leu condition. These results suggest that a dietary SID Ile:Lys ratio of 54% is optimal for 8 to 21 kg pigs when Leu is not in excess. However, pigs need a higher Ile (58% SID Ile:Lys) when the diets contain excess Leu (160% SID Leu:Lys) to maximize the ADFI and ADG.