The optimal isoleucine to lysine ratio in diets for postweaning piglets

Use of low CP diets for pigs contributes to an increased sustainability and reduced environmental impact of pig production. To avoid over- or undersupply of protein and essential amino acids, it is important to have more precise information on the requirement for essential amino acids in pigs in different production phases. Isoleucine (Ile) can be regarded as a potential limiting amino acid in diets for weaned piglets of today's genotype. Therefore, the present study investigated the optimal Ile to lysine (Lys) ratio in diets for postweaning piglets in a dose-response study with 384 postweaning piglets with six dietary treatments (T1-T6) with eight replicates (pens) per treatment. Treatments included provision of diets with incremental levels of standardised ileal digestible (SID) Ile by increasing the inclusion of free L-Ile to reach a ratio of SID Ile to Lys of 37, 41, 46, 50, 55 and 60% (range 4.40 to 7.15 g SID Ile per kg of diet). The basal diet mainly consisted of corn, wheat, barley, potato protein, haemoglobin powder and soybean meal (CP: 150 g/kg; Net energy: 10.46 MJ/kg; SID Lys: 12.0 g/kg) and was supposed to be deficient in Ile (4.40 g SID Ile/kg of the diet). Daily feed intake (dFI), body weight gain (BWG) and feed conversion ratio (FCR) were measured over a period of 4 weeks, starting 6 days postweaning. Regression analysis with a broken line model and a quadratic model was applied to derive the optimal ratio of SID Ile relative to SID Lys in low-protein diets for postweaning piglets. The piglets fed the basal diet had significantly lower BWG, dFI and higher FCR (P < 0.05) than animals fed with Ile-supplemented diets. Both dFI and BWG showed a dose-dependent response towards increasing the dietary level of Ile. The optimal ratio of SID Ile to Lys, based on regression analysis with the quadratic model, was estimated at 52.6 and 52.5% for dFI and BWG, respectively. It is concluded that the optimal ratio of SID Ile relative to Lys to optimise dFI and BWG in piglets fed a low protein diet in the postweaning period is 52%.