Determining an optimum lysine:calorie ratio for barrows and gilts in a commercial finishing facility1,2

Our objective was to determine an optimum Lys:calorie ratio (g of total dietary Lys/Mcal of ME) for 35- to 120-kg barrows and gilts (Pig Improvement Company, L337 x C22) in a commercial finishing environment. Seven (3 barrow and 4 gilt) trials were conducted using randomized complete block designs (42 pens per trial, a total of 7,801 pigs). Six treatments with increasing Lys:calorie ratio were used in each study. Diets were corn-soybean meal-based with 6% choice white grease. Lysine:calorie ratios were attained by adjusting the amount of corn and soybean meal. No crystalline Lys was used. In barrow trial 1 (43 to 70 kg), increasing the Lys:calorie ratio (2.21, 2.55, 2.89, 3.23, 3.57, and 3.91) increased (quadratic, P < 0.01) ADG, G:F, income over feed costs (IOMFC), and feed cost per kilogram of gain, and decreased (linear, P < 0.01) backfat. In barrow trial 2 (69 to 93 kg), increasing the Lys:calorie ratio (1.53, 1.78, 2.03, 2.28, 2.53, and 2.78) improved (linear, P < 0.01) ADG, G:F, and IOMFC, and decreased (quadratic, P < 0.01) backfat. In barrow trial 3 (102 to 120 kg), increasing the Lys:calorie ratio (1.40, 1.60, 1.80, 2.00, 2.20, and 2.40) increased (linear, P < 0.03) ADG and G:F, and numerically improved (linear, P = 0.12) IOMFC. In gilt trials 1 (35 to 60 kg), 2 (60 to 85 kg), and 3 (78 to 103 kg), increasing the Lys:calorie ratio (2.55, 2.89, 3.23, 3.57, 3.91, and 4.25; 1.96, 2.24, 2.52, 2.80, 3.08, and 3.36; and 1.53, 1.78, 2.03, 2.28, 2.53, and 2.78, respectively) improved (quadratic, P < 0.04) ADG, G:F, IOMFC, and feed cost per kilogram of gain, and decreased (linear, P < 0.01) backfat. In gilt trial 4 (100 to 120 kg), increasing the Lys:calorie ratio (1.40, 1.60, 1.80, 2.00, 2.20, and 2.40) improved (linear, P < 0.02) ADG, G:F, LM depth, IOMFC, and (quadratic, P < 0.06) feed cost per kilogram of gain. These studies suggest that feed cost per kilogram of gain decreases, and reductions in biological performance and IOMFC are rather modest when feeding marginally Lys-deficient diets early (35 to 70 kg) in the grower-finishing period compared with the more severe penalties in growth and economic performance of feeding marginally deficient diets in the late finishing period (70 kg to slaughter). The equations (Lys:calorie ratio = -0.0133 x BW, kg, + 3.6944 and = -0.0164 x BW, kg, + 4.004, for barrows and gilts, respectively) best describe our interpretation of the Lys:calorie ratio that met biological requirements and optimized IOMFC on these pigs (PIC, L337 x C22; 35 to 120 kg) in this commercial finishing environment.