Experiment 1: A total of 5,059 (PIC 800 × Camborough and DNA 600 × 241; initially 24.3 ± 1.99 lb) were used in an 18-d trial to evaluate SID Lys:CP ratios for 25 to 45 lb pigs in diets with and without DDGS resulting in diets that contain high or low levels of SBM on growth performance. Pigs were weaned at approximately 21 d of age and housed in mixed-sex pens with approximately 35 pigs per pen. A pelleted early nursery diet was fed to all pigs from weaning until the beginning of the experiment. When pigs reached approximately 24 lb, pens were assigned to 1 of 12 treatments in a completely randomized design with pen serving as the experimental unit. A total of 143 pens were used, resulting in 11 or 12 replications per dietary treatment. Experimental treatments were arranged in a 2 × 6 factorial with main effects of DDGS (0 or 15%) and SID Lys:CP ratio (6.01, 6.22, 6.45, 6.70, 6.97, or 7.26). Pigs were weighed at the beginning and at the end of the study on d 18. Overall (d 0 to 18), a SID Lys:CP × DDGS interaction was observed (linear, P < 0.001) for feed efficiency where increasing SID Lys:CP ratio in diets without DDGS improved (P < 0.001) F/G quadratically, with the poorest F/G on the lowest and highest SID Lys:CP ratios; whereas in the diets with DDGS, F/G worsened (quadratic, P < 0.002) as the SID Lys:CP ratio increased above 6.45. For the main effects, ADG increased quadratically (P = 0.021) as the SID Lys:CP ratio increased, with the greatest ADG with ratios of 6.45 to 6.97. Conversely, ADFI increased linearly (P = 0.018) as SID Lys:CP increased up to the highest level tested. A tendency was observed for pigs fed no DDGS increased (P = 0.071) ADG and improved (P = 0.010) F/G. In conclusion, formulating diets to a SID Lys:CP ratio of 6.70 without DDGS and 6.45 in diets with DDGS improves feed efficiency, and further increasing the ratio leads to a reduction in feed efficiency, possibly due to nitrogen becoming the limiting factor as crude protein is decreased. Additionally, feeding diets without DDGS can lead to an improvement in ADG and F/G.
Experiment 2: A total of 4,167 pigs (initially 28.7 ± 0.59 lb) were used in a 21-d growth study to evaluate SID Lys:CP ratios for 25 to 55 lb pigs. Pigs were weaned at approximately 21 d of age and pens of pigs were randomly allotted to 1 of 10 dietary treatments in a randomized complete block design. Pens of pigs were blocked based on sow farm origin, date of entry into the facility, and average pen weight. A total of 160 pens were used with 80 double-sided 5-hole stainless steel fence line feeders, with feeder serving as the experimental unit. For each feeder, 1 pen contained approximately 26 gilts and 1 pen contained approximately 26 barrows. There were 8 replicates per dietary treatment. A pelleted phase 1 diet was fed to all pigs with 1.25 lb/pig budgeted, followed by a phase 2 diet fed in meal form until beginning of the experiment on d-21 post-weaning. Experimental treatments were arranged in a 2 × 5 factorial with main effects of SID Lys (1.15 or 1.30%) and SID Lys:CP ratio (6.00, 6.22, 6.46, 6.72, and 7.00). From d 0 to 21 (overall), a SID Lys:CP × SID Lys interaction was observed (linear, P = 0.012) where increasing the SID Lys:CP ratio reduced (P < 0.0001) feed efficiency at both SID Lys levels, but the magnitude was greater in diets formulated to 1.15% SID Lys compared to 1.30% SID Lys. Additionally, ADFI increased (P = 0.013) as the SID Lys:CP ratio increased. Increasing SID Lys increased (P < 0.001) ADG in phases 1, 2, and overall, but not from d 14 to 21. In conclusion, increasing SID Lys:CP ratio worsened feed efficiency, especially when the ratio increased beyond 6.46. Data suggests that diets for 25 to 50 lb pigs should be formulated to a SID Lys:CP ratio of 6.46 or lower to maintain feed efficiency.