The Optimal Standardized Ileal Digestible Lysine and Total Sulfur Amino Acid Requirement for Finishing Pigs Fed Ractopamine Hydrochloride1

A total of 1,887 pigs (97 kg initial BW) were used in a 28-d study to simultaneously examine standardized ileal digestible (SID) Lys and SID TSAA requirements of pigs fed diets containing 5 ppm ractopamine HCl. Four SID Lys (0.66, 0.79, 0.92, and 1.05%) and 4 SID TSAA (0.47, 0.52, 0.57, and 0.63%) concentrations were evaluated. Methionine hydroxy analog was used as the Met source. The diet with the greatest concentration of both Lys (1.05%) and TSAA (0.63%) was used in each of the Lys and TSAA titrations; hence, there were 7 diets but 8 treatments. The Lys diets were formulated with a minimum SID TSAA-to-Lys ratio of 69%, and the TSAA diets were formulated with 1.05% SID Lys. From d 0 to 28, increasing SID Lys increased ADG (linear, P < 0.01), reaching an apparent plateau at 0.92% SID Lys. However, ADG was not affected by SID TSAA concentration (P = 0.76). Increasing dietary SID Lys improved G:F (linear, P < 0.01), and increasing SID TSAA tended to improve G:F (linear, P = 0.09). Although the response was linear, the greatest improvement in G:F was observed as the SID TSAA increased from 0.47 to 0.52%, resulting in an optimal SID TSAA-to-Lys ratio of 57% (at 0.92% SID Lys). Regression analysis indicated that the maximum G:F was obtained with a SID TSAA-to-Lys ratio of 58%. In summary, a SID TSAA-to-Lys ratio of no more than 58% optimizes growth performance of finishing pigs fed 5 ppm ractopamine HCl.