Two experiments were conducted to determine the standardized ileal digestible (SID) lysine (Lys) requirement and the ideal SID sulphur amino acids (SAA) to Lys ratio for 30–50 kg crossbred pigs. In experiment 1, a total of 72 crossbred pigs with an average initial body weight (BW) of 28.9 kg were allotted to one of six dietary treatments in a randomized complete block design. Each diet was assigned to six pens containing two pigs each. Six diets were obtained by supplementing graded levels of L‐Lysine∙HCl to create six dietary levels of SID Lys (0.70%, 0.80%, 0.90%, 1.00%, 1.10% and 1.20%). Responses of weight gain (ADG) and gain:feed (G:F) to increasing the SID Lys content of the diet fitted well with the curvilinear‐plateau model; whereas, for plasma urea nitrogen (PUN) two‐slope linear broken‐line model was well fitted. The optimal SID Lys requirement for the pigs of this period was 1.10%. Experiment 2 was a dose–response study using SID Met+Cys to Lys ratios of 50%, 55%, 60%, 65%, 70% and 64%. A total of 72 crossbred pigs with initial BW of 32.9 kg were randomly allotted to receive one of the six diets. Diets 1–5 were formulated to contain 1.0% SID Lys to be second limiting in Lys and diet 6 contained 1.11% SID Lys to be adequate in Lys. The average optimal SID SAA:Lys ratio for maximal ADG and G:F and minimal PUN was 65.2% using curvilinear‐plateau and linear broken‐line models.

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Requirement of Lys is affected by lean gain capacity of the pigs. Methionine is the 3rd limiting AA in typical swine diets. In Exp. 1, 108 crossbred pigs (initial BW 11.8 kg) were assigned to 6 diets adequate in all AA but with increasing Lys content [0.90, 1.00, 1.10, 1.20, 1.30 and 1.40% standardized ileal digestible (SID) Lys] to determine the SID Lys requirement. In Exp. 2, 108 pigs (initial BW 11.9 kg) were assigned to 6 diets to determine the optimal SID Met+Cys:Lys ratio. Diets 1 to 5 were varied in SID Met+Cys:Lys (50, 55, 60, 65 and 70%) and 1.17% SID Lys (second limiting). Diet 6 was formulated to be adequate in all AA (1.25% SID Lys and 62% SID Met+Cys:Lys). There were 6 pens (2 barrows and 1 gilt) per treatment in both experiments and lasted for 21 d. Data were analyzed by ANOVA using the GLM procedure of SAS. In Exp. 1, increasing SID Lys increased the overall ADG and G:F (linear, P < 0.001) and decreased plasma urea nitrogen (PUN; linear, P < 0.001). Regression analysis estimated the SID Lys to be 1.36, 1.38 and 1.28% for ADG, G:F and PUN, respectively with an average SID Lys requirement of 1.34% for 12-23 kg pigs. In Exp. 2, increasing SID M+C:Lys ratio (diets 1 to 5) increased ADG and G:F (linear and quadratic, P < 0.001) and decreased PUN (quadratic, P < 0.001). Pigs fed a high Lys diet (diet 6) tended to have a lower PUN (P = 0.076) but performance was not affected. The regression models estimated the optimum SID M+C:Lys ratio to be 63.1 and 60.7% for ADG, and 62.5 and 61.1% for G:F, respectively while 62.9% SID M+C:Lys optimized PUN. On average, the optimal SID M+C:Lys ratio for 12-23 kg pigs was 62%.