Effects of Increasing Lysine to Calorie Ratio and Added Fat for Growing-Finishing Pigs Reared in a Commercial Environment: I. Growth Performance and Carcass Characteristics1

Two experiments were conducted to determine the effects of none or 6.0% added fat (choice white grease) and different Lys:calorie ratios on growth performance and carcass characteristics of growing-finishing pigs (PIC C22 × 337) reared in a commercial environment. Gilts (n = 1,200, initially 24 kg) were used in Exp. 1 and barrows (n = 1,200, initially 34 kg) in Exp. 2. The corn-soybean meal-based diets were arranged in a 2 × 4 factorial in a completely randomized design with 25 pigs per pen and 6 pens per treatment. Diets were fed in 4 phases: in Exp.1, phases were 27 to 45, 45 to 75, 75 to 100, and 100 to 120 kg; In Exp.2, phases were 34 to 60, 60 to 80, 80 to 100, and 100 to 120 kg. Lysine:calorie ratios (g Lys/Mcal ME) for Exp. 1 were 2.96, 3.26, 3.56, and 3.86 in phase 1; 2.25, 2.50, 2.75, and 3.0 in phase 2; 1.64, 1.84, 2.04, and 2.24 in phase 3; and 1.12, 1.32, 1.52, and 1.72 in phase 4. In Exp. 2, Lys:calorie ratios were 2.41, 2.71, 3.01, and 3.31 in phase 1; 1.75, 2.0, 2.25, and 2.5 in phase 2; 1.38, 1.58, 1.78, and 1.98 in phase 3; and 1.02, 1.22, 1.42, and 1.62 in phase 4. For the overall study, added dietary fat decreased (P < 0.01) ADFI and increased ADG (P < 0.04) and G:F (P < 0.01). Carcass yield, backfat depth, and percentage lean were not affected (P > 0.14) in Exp. 1, and a tendency for increased backfat depth (P < 0.09) and decreased percentage lean (P < 0.08) was observed in Exp. 2. Increasing Lys:calorie ratio increased ADG and G:F in Exp. 1 (quadratic, P < 0.04). In Exp. 2, increasing Lys:calorie ratio increased (linear, P < 0.01) ADG and G:F. In both experiments, increasing Lys:calorie ratio decreased (linear, P < 0.01) backfat depth and increased (linear, P < 0.01) percentage lean, but carcass yield was not affected (P > 0.57). These data indicate that 6.0% added dietary fat decreases ADFI and increases ADG and G:F. Also, the third (3.56, 2.75, 2.04, and 1.52 g Lys/Mcal ME) and fourth (at least 3.31, 2.5, 1.98, and 1.62 g Lys/Mcal ME) Lys:calorie ratio regimens appeared to maximize growth performance and carcass leanness in gilts and barrows, respectively.