Lysine needs of summer-reared male broilers from six to eight weeks of age

Experimentation was conducted to estimate dietary lysine needed to optimize production of summer-reared broilers between 42 to 56 d of age. Male Ross x Ross 308 chicks were placed in floor pens of an open-sided house and provided common feeds from placement to 42 d of age. During the subsequent 42 to 56 d, birds received a corn-soybean meal basal diet (18% CP and 3,250 kcal/kg ME) established to provide limiting essential amino acids favorably balanced near requirement levels with the exception of lysine. Four 0.10% increments of L-lysine isonitrogenously displaced L-glutamic acid from the basal diet to provide analyzed values progressing from 0.85 to 1.25%. Body weight gain and mortality were not altered as dietary lysine increased; however, feed conversion linearly improved. Chilled carcass yield, amount of abdominal fat, and recovery of skinless boneless breast meats were not affected. Measurements on nitrogen balance and plasma levels of total protein, albumin, glucose, and uric acid taken at 49 d from a concurrent study using sample birds in raised-wire cages and identical feeds also failed to define a requirement. However, plasma aspartic transferase increased to a maximum approximating 1.05% lysine, whereas free lysine concentration linearly increased to the highest level. Overall data supported a lysine requirement no less than 0.95% that was greater than the previous minimum of 0.85% obtained under similar terms without heat stress. Suppression of growth from heat stress appears to reduce the absolute need for lysine; however, increased dietary concentration appears necessary to accommodate depressed feed intake and improve its effectiveness.