Sulfur amino acid requirements and interactions in broilers during two growth periods.

Total sulfur amino acid (TSAA) requirements and methionine and cystine combinations were investigated during two growth phases (0 to 21 days and 21 to 42 days) of broilers using practical-type diets. For the first growth phase (0 to 21 days), the basal diet contained .62% TSAA to which increasing increments of DL-methionine were added. Both overall growth and feed efficiency improved as methionine levels increased to a level of .82% TSAA. When methionine-cystine combinations were used with the same basal diet, it was found that cystine could supply 54% of the TSAA content. When expressed on a millimolar basis, the methionine requirement was 41%, while cystine could account for 59% of the TSAA's. During the second growth phase (21 to 42 days) a basal diet consisting of .52% TSAA was used along with DL-methionine supplements to determine sulfur amino acid requirements. The trend was towards a slight decrease in requirement from .82% during the first growth period, to between .70% and .76% TSAA in the diet, represented mainly by methionine. When sulfur amino acid combinations were used during this period it appeared the replacement value of cystine for methionine decreased to between 38 and 43% of the TSAA level. Results also suggested that a more efficient way to express the sulfur amino acid requirement may be on a mole per unit of feed basis.