Evaluation of High Standardized Ileal Digestible Tryptophan:Lysine Ratios with Ractopamine HCl on Growth and Carcass Performance of Pigs from 110 to 135 Kg

Tryptophan is considered the second or third limiting amino acid in corn soybean-meal-based diets fed to growing and finishing swine. The NRC (2012) SID Trp:Lys ratio requirement for pigs above 165 lb is 17.7% of lysine. However, recent research has reported that increasing standardized ileal digestible (SID) Trp:Lys ratio above 20% in finishing pigs fed Ractopamine HCl (RAC) resulted in improved growth and carcass performance. To further evaluate this response, the objective of this experiment was to confirm the effects of feeding high SID Trp:Lys ratios in diets containing RAC on growth and carcass performance of finishing pigs from 110 to 135 kg. A total of 935 pigs (PIC 1050 × 337, initially 107.6 kg BW) were used in a 22-d trial. Pens of 23 or 24 pigs were allotted by BW and randomly assigned to 1 of 5 dietary treatments in a RCBD with 8 replications per treatment. The dietary treatments included 5 SID Trp:Lys ratios (20, 22, 24, 26, and 28% of Lys). Corn-soybean meal based diets were formulated to 0.90% SID Lys and contained 10 ppm ractopamine. Analyzed nutrients and total amino acids contents of experimental diets were consistent with formulated estimates. At d 22, pigs were transported to a packing plant for processing and carcass data collection. For overall growth performance, increasing SID Trp:Lys increased (linear, P = 0.007) ADFI (2.86, 2.91, 2.89, 2.96, and 3.00 kg) and SID Trp g/kg gain (5.2, 5.6, 6.4, 6.7, and 7.4) linear (P < 0.001). However, there was no evidence for treatment differences for ADG (1.10, 1.14, 1.10, 1.15, and 1.13 kg) or G:F (0.384, 0.390, 0.378, 0.390 and 0.376). Similarly, for carcass characteristics, there was no evidence for treatment differences for HCW, carcass yield, backfat loin depth, lean, carcass ADG or carcass feed efficiency. In summary, increasing SID Trp:Lys increased ADFI and SID Trp g/kg gain, however, there was no evidence for treatment differences for other growth or carcass parameters measured. Further research is necessary to determine why inconsistencies are observed when feeding high SID Trp:Lys ratios to finishing pigs.