The standardized ileal digestible isoleucine-to-lysine requirement ratio may be less than fifty percent in eleven- to twenty-three-kilogram piglets¹

Most studies concerning the Ile requirement in pigs have been carried out using blood products as a protein source, and these have a relatively low Ile content relative to the other branched-chain AA (BCAA). There are indications that an excess supply of one BCAA can affect the utilization of the other BCAA. Little information is available concerning the Ile requirement in pigs when the supply of the other BCAA is moderate (e.g., in cereal- and soybean meal-based diets). The objective of the present study was to evaluate the response of piglets to Ile supplementation under different nutritional conditions. In all experiments, piglets were housed individually and had ad libitum access to feed during a 3-wk period. The first experiment was carried out to study the response of piglets to an increasing Ile supply by using 2 sources of L-Ile differing in degree of purity. Piglets received either a control diet with 48% standardized ileal digestible (SID) Ile:Lys or 1 of 4 other diets containing graded levels of either source of L-Ile to provide 52 or 56% SID Ile:Lys. All diets were formulated to provide 1.00% SID Lys in the diet. Feed intake and growth were not affected by Ile level or Ile source. Experiment 2 was performed to exclude a possible interaction between Ile and Lys supply. In a 2 x 2 factorial arrangement, 2 levels of Lys (1.00 and 1.15% SID Lys) and 2 levels of Ile (48 and 60% SID Ile:Lys) were used. Growth and G:F were 8 and 7% greater in piglets receiving the diet with the greater Lys content, but the Ile:Lys did not affect performance. No interactions were observed between the Lys and Ile supplies. In Exp. 3, a 2 x 2 factorial arrangement was used to test the effect of protein source (spray-dried blood cells or corn gluten meal) and Ile supply (50 or 65% SID Ile:Lys) on performance in piglets. Both protein sources had an elevated BCAA content but differed in Leu and Val contents. Protein source or Ile supply did not affect feed intake, growth, or G:F in the piglets. Plasma concentrations after an overnight fast reflected the difference in AA concentrations of the diets. In conclusion, the results of these experiments indicate that the SID Ile:Lys requirement may be not greater than 50% in piglets receiving cereal- and soybean meal-based diets with a moderate BCAA content. In contrast to other studies, we could not confirm that the Ile requirement was affected by BCAA content of the diet.