Four experiments were conducted to determine the effects of standardized ileal digestible (SID) Lys:calorie (Lys:Mcal) ratio on growth performance of 10- to 20-kg pigs of 2 different genotypes. Experiment 1 (360 pigs, average BW = 10.2 kg; source 1) and Exp. 2 (351 pigs; average BW = 9.3 kg; source 2), were both organized as a combination of 2 simultaneous experiments with the first set of diets consisting of 5 treatments with increasing SID Lys and the second set of diets consisting of 5 treatments with increasing energy density (Exp. 1: 9.9, 10.7, 11.5, 12.2, and 13.0 g/kg of Lys and 2.95, 3.09, 3.24, 3.38, and 3.52 Mcal/ kg of ME, respectively; Exp. 2: 11.1, 11.9, 12.6, 13.4, and 14.2 g/kg and 2.95, 3.10, 3.25, 3.40, and 3.55 Mcal of ME/kg, respectively). In Exp. 1, increasing dietary SID Lys increased (linear) ADG and G:F, and increasing dietary ME increased (quadratic) G:F. In Exp. 1 the optimal Lys:Mcal ratio was estimated to be at least 4.1 g of Lys/Mcal of ME based on G:F. In Exp. 2, increasing dietary SID Lys increased (linear) ADG and G:F. Increasing dietary ME increased (linear) G:F. Because of the linear responses in this experiment, optimal Lys:Mcal ratio was at least 4.0 g of Lys/Mcal of ME. In Exp. 3 (350 pigs; average BW = 9.4 kg; source 1) and Exp. 4 (350 pigs; average BW = 7.5 kg; source 2), Lys:Mcal ratios in Exp. 1 and 2 were compared by titrating Lys at 2 energy levels. Pigs were fed diets with 2.95 or 3.29 Mcal/kg of ME with SID Lys:Mcal ratios of 3.1 to 4.1 g/Mcal of ME (Exp. 3) and 3.5 to 4.5 g/Mcal of ME (Exp. 4). In Exp. 3, there was an ME × Lys:Mcal ratio interaction for ADG. The greatest ADG was a Lys:Mcal ratio of 3.60 for pigs fed low ME and a ratio of 3.35 for pigs fed high ME. Gain:feed ratio increased with increased ME concentration and as Lys:Mcal ratio increased (quadratic); the best G:F was observed at 3.67 g of Lys/Mcal of ME. In Exp. 4, there was a tendency for ME × Lys:Mcal ratio interaction for G:F. The greatest G:F was achieved with a Lys: Mcal ratio of at least 4.50 for pigs fed low ME and a ratio of 4.29 for pigs fed high ME. Results (i.e., lack of ADG response to high energy density) suggest pigs reared in this environment were not in energy dependent growth phases. These pigs needed approximately 11 g/d of SID Lys (19 g of Lys/kg of gain) to optimize ADG and G:F. Based on these results, optimal Lys:Mcal ratio may differ depending on daily caloric intake of the pig.

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