Digestible lysine requirement of male and female broiler chicks during the period three to six weeks posthatching.

Experiments were conducted to determine the dietary digestible lysine requirement of male and female broilers (Ross x Ross) during the period 22 to 43 d posthatching. An amino acid-fortified basal diet containing corn, feather meal, and soybean meal as intact protein sources provided .63% total lysine, 20% CP, and 3,200 kcal MEn/kg. The basal diet contained .51% true digestible lysine as determined with a precision-fed cecectomized adult cockerel assay. Growth rate and feed efficiency of birds fed the basal diet fortified with a surfeit level of L-lysine were equal to those of birds fed a methionine-supplemented corn-soybean meal positive control diet (20% CP; 3,200 kcal MEn/kg). Graded levels of synthetic L-lysine (0, .1, .2, .3, .4, .5, .6%) were added to the basal diet to produce growth response curves. Subjecting the growth data to broken-line analysis indicated that the digestible lysine requirement for maximum body weight gain was .85% for males and .78% for females. The requirement for optimum feed efficiency was higher: .89% for males and .85% for female broilers. Breast meat (Pectoralis major) yield increased quadratically in response to lysine addition, and the responses suggested that the lysine requirement for maximal breast yield was not greatly different from that predicted from the feed efficiency data. Abdominal fat (percentage of live body weight) increased from the first to the second increment of lysine, but it then declined as lysine level was increased further.