Tryptophan is an indispensable amino acid (AA) with an optimal dietary standardized ileal digestible (SID) Trp to Lys ratio (Trp:Lys) ranging from 0.173 at 25 kg of BW to 0.180 at 125 kg of BW in grower pigs. However, estimates of these requirements may underestimate the demands for grower and finisher pigs in commercial conditions because of the relatively greater animal density and environmental challenge found in commercial conditions. Therefore, the aim of the present study was to determine the optimal SID Trp:Lys for grower-finisher pigs of 20–50 (Trial 1), 50–80 (Trial 2), and 80–110 kg BW (Trial 3) in a commercial swine production facility. For that purpose, a total of 4032 Duroc × Yorkshire × Landrace barrows and gilts were used in this study with a randomized complete blocked design. For each trial, 1344 pigs were randomly allotted to 6 dietary treatments with the following SID Trp:Lys levels: (i) 0.150, (ii) 0.165, (iii) 0.180, (iv) 0.195, (v) 0.210, and (vi) 0.225; with 8 replicate pens per treatment (14 barrows and 14 gilts per pen). Synthetic Trp was used to adjust the dietary SID Trp:Lys, while the Lys content remained fixed.
As a result, in Trial 1, the average daily gain (ADG), average daily feed intake (ADFI), gain to feed ratio (G:F), and serum urea N (SUN) were increased quadratically as the SID Trp:Lys increased from 0.150 to 0.225. The optimal SID Trp:Lys for ADG was between 0.171 and 0.188, whereas for G:F, the estimated optimal value was between 0.192 and 0.207. In Trial 2, the same quadratic increase was observed in ADG, ADFI, G:F, and SUN as the SID Trp:Lys increased from 0.150 to 0.225. The optimal SID Trp:Lys for ADG was 0.183 for the broken-line model and 0.200 for the curvilinear-plateau model. As for G:F, the estimated value was between 0.195 and 0.213. Finally, in Trial 3, ADG, G:F, and SUN increased quadratically with increasing SID Trp:Lys, while ADFI increased linearly. The optimal SID Trp:Lys estimated for ADG was 0.184 – 0.201. Likewise for G:F, the best ratios were 0.174 and 0.198 for the linear and curvilinear models, respectively.
In conclusion, this study used the broken-line model to settle the optimal SID Trp:Lys for ADG at 0.171, 0.183, and 0.184, in pigs weighing 20 to 50, 50 to 80, and 80 to 110 kg, respectively. As for, the G:F, the estimated values were 0.192, 0.195, and 0.174, in pigs weighing 20 to 50, 50 to 80, and 80 to 110 kg, respectively.
Liu, J. B., Yan, H. L., Cao, S. C., Liu, J., Li, Z. X., and Zhang, H. F. (2019). The response of performance in grower and finisher pigs to diets formulated to different tryptophan to lysine ratios. Livestock Science, 222, 25-30. https://doi.org/10.1016/j.livsci.2019.01.016