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An experiment was conducted to determine the optimum lysine (lys):Net energy (NE) ratio of low-crude protein (CP) amino acid (AA)-supplemented diets needed in conjunction with ractopamine (RAC) to improve growth performance of late-finishing barrows from the University of Nebraska-Lincoln (UNL) herd. Treatments consisted of jive low CP, AA-supplemented diets with addition of ractopamine (16% CP; 4.5g/ton), formulated to contain 3.35, 3.95,4.57, 5.2 and 5.83 g of lys/Mcal of NE. A corn-soybean meal diet with no RAC supplementation served as negative control (20% CP; 5.24 g of Lys/Mcal of NE). Treatment did not affect growth performance (P > 0.05). Despite the lack of treatment efect (P = 0.09), increasing dietary lys/NE concentration resulted in a linear decrease in final backfat (P = 0.01). Treatments did not affect final longissimus muscle area (P = 0.69). Results indicate that the optimum lys/NE for late-finishing pigs from the UNL herd fed low-CPAA supplemented diets containing 4.5 g of RAC is between 4.57, and 5.2 g of lys/ Mcal of NE.