Ideal ratios of standardized ileal digestible methionine, threonine, and tryptophan relative to lysine for male broilers at the age of 1 to 10 days

The objective of this study was to develop ideal amino acid (AA) ratios based on the standardized ileal digestible (SID) lysine, methionine (in the presence of 4.3 g SID cysteine/kg), threonine, and tryptophan requirements, for 10-day-old male broiler chickens. Three experiments were conducted using Ross 308 male chicks fed diets based on corn, soybean meal, and additional graded crystalline AA with 228 g/kg of crude protein. Each diet contained the same amount of basal diets including corn, soybean meal, corn starch, and soybean oil, and graded AA were substituted at the expense of L-glutamic acid in order to keep all diets isonitrogenous. A total of 720 birds were allocated to 6 treatment groups, with 15 birds per cage and 8 cages per treatment using a randomized complete block design. Body weight and group feed intake were measured on day 10. The dietary SID AA levels in each experiment ranged from 2.9 to 6.9 g of methionine/kg, with increments of 0.8 g/kg (Exp. 1), 4.5–8.5 g of threonine/kg, with increments of 0.8 g/kg (Exp. 2), and 1.1–2.1 g of tryptophan/kg, with increments of 0.2 g/kg (Exp. 3), respectively. The methionine requirement was conducted in the presence of 4.5 g total cysteine/kg diet which was 4.3 g/kg diet in SID cysteine. Average daily gain and gain to feed ratio showed linear and quadratic relations to increasing levels of SID methionine, threonine, and tryptophan, respectively (P < 0.01). The SID requirements for broiler chicks at the age of 1–10 days were determined using 12.4 g/kg of SID lysine concentrations in the dietary treatments. The requirement for average daily gain and gain to feed ratios were 3.85 and 3.84 g/kg for SID methionine when the diet contained 4.3 g SID cysteine/kg diet (thus, total sulfur AA requirements were 8.15 and 8.14 g/kg, respectively), 7.11 and 7.38 g/kg for SID threonine, and 1.58 and 1.36 g/kg for SID tryptophan, respectively. By using the upper limit of the broken-line requirement, the ideal SID AA to lysine ratios of 10-day-old broiler chickens were 0.31, 0.60, and 0.13 for methionine (in the presence of 4.3 SID cysteine/kg diet), threonine, and tryptophan, respectively.