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Four experiments were conducted to investigate the effects of altering the standardized ileal digestible (SID) lysine to net energy (NE) ratio on the performance of weaned pigs housed under commercial conditions.Exp.1 and 2 utilized a total of 720 crossbred pigs ( Landrace × Yorkshire) weighing an average of 8.41 + 0.72 kg in Exp.1 and 6.94 ±0.63 kg in Exp.2.The pigs were allotted to one of three treatments with six pens of 20 pigs per treatment.The treatments consisted of diets with different NElevels (2.48,2.42,or 2.36 Mcal/kg) and 1.21％ SID-lysine.The diets were formulated using corn,soybean meal,wheat middlings,and broken rice.During both 14 day trials,the ADG of the pigs fed the diet providing 2.48 Mcal/kg NE was the highest among the three treatments (P ＜0.01 ).The ADG of pigs fed diets providing 2.42 and 2.36 Mcal/kg NE did not differ ( P ＞ 0.05 ).Exp.3 and 4 were conducted to study the effects of different SID-lysine levels on the performance of weaned pigs fed diets containing 2.48 Mcal/kg NE.Exp.3 and 4 utilized 720 crossbred pigs ( Landrace × Yorkshire) weighing 8.48±0.81 kg in Exp.3 and 6.83 ±0.74 kg in Exp.4.The pigs were allotted to three treatments with six pens of 20 pigs per treatment.The treatments consisted of three diets with different SID-lysine levels (1.31％,1.21％,or 1.11％ ) and the same NE level (2.48 Mcal/kg).Exp.3 and 4 showed that the ADG of pigs fed the diet with 1.31％ SID-lysine was higher than pigs fed the diet with 1.11％ SID-lysine (P ＜0.01).The ADG for pigs fed diets with 1.31％and 1.21％ SID-lysine did not differ (P ＞0.05).Based on the results of the present study,to maximize ADG of weaned pigs,the optimal level of NE is 2.48Mcal/kg and the optimum level of SID-lysine is 1.31％.