Three assays were carried out to determine methionine + cystine requirement and the digestible methionine + cystine: digestible lysine ratio of males broilers, Avian Farms, from 1 to 20, 24 to 38 and 44 to 56 days of age. Each experiment was a completely randomized design with five levels of methionine + cystine, six replications and 20 birds per pen from 1 to 20 and 24 to 38 and 16 birds per pen from 44 to 56 day of age, respectively. The digestible methionine + cystine levels were 0.731-0.916% (in increments of 0.046); 0.712-0.881% (in increments of 0.042) and 0.631-0.781% (in increments of 0.037) for 1 to 20, 24 to 38 and 44 to 56 days of age, respectively. Feed intake, weight gain, feed conversion and carcass quality were evaluated. There were linear positive and quadratic effects of the methionine + cystine levels on body weight gain and feed conversion, respectively, in the 1 to 20 days phase. Using the broken line model, the requirement of digestible methionine + cystine (total) and the ratio of digestible methionine + cystine:digestible lysine for the 1 to 20 days phase was 0.808 (0.907% total) and 70%, respectively, according to the results of feed conversion. For the phase 24 to 38 days, weight gain, feed conversion and absolute leg quarter weight responded quadratically to increasing levels of digestible methionine + cystine. Using the broken line model, the requirement of digestible methionine + cystine (total) and the ratio of digestible methionine + cystine:digestible lysine for the 24-38 days phase were 0.767 (0.844% total) and 72%, respectively, according to the results of body weight gain. The absence of a significant response for body weight gain and feed conversion in the third phase (44-56 days) demonstrates that the lowest level of methionine + cystine met the birds requirement. Nevertheless, according to the best biological response the digestible (total) methionine + cystine were 0.668% (0.738% total) and 71%, respectively. Methionine + cystine quadratically affected breast yield and breast fillet yield in the 44 to 56 days phase.
amino acids; nutrition; poultry; protein
