Methionine and cystine requirements of slow- and fast-feathering broiler males from three to six weeks of age.

Two experiments were conducted to first determine Met then Cys needs of broilers from 3 to 6 wk of age and whether differences existed between slow-feathering (Ross x 308) and fast-feathering (Ross x 3F8) males. A corn-soybean meal diet (20.0% CP; 3,150 kcal ME/kg) with graded levels of Met or Cys was offered. The first experiment had dietary Met levels of 0.32, 0.38, 0.44, and 0.50% with surfeit Cys (0.40%). Broilers from both feathering strains responded similarly to supplemental Met. Although body weight was not responsive, F/G improved through to the highest level of dietary Met (linear, P < 0.05). Chilled carcass weight increased with Met (linear, P < 0.05) paralleling F/G; however, no differences were detected in the amount of associated abdominal fat. Breast fillet yield increased with Met to maximize at 0.48% (quadratic, P < or = 0.009). In a satellite study using the same birds in cages and feeds, N retention at d 29 maximized at 0.46% Met (quadratic, P < 0.05). The second experiment had Cys at 0.32, 0.34, 0.38, and 0.46% with Met fixed at a submarginal level of 0.38%. Increasing dietary Cys had no effect on live performance of slow-feathering birds, whereas weight gain of fast-feathering birds achieved maximum at 0.36% Cys (cubic; P < 0.05) with F/G responding similarly. Chilled carcass (cubic, P < 0.002) and breast fillet weights (cubic, P < 0.001) of fast-feathering birds also increased with Cys to maximize at 0.36%, and the amount of abdominal fat was not influenced by feathering or Cys supplementation. Separate measurement of N retention at d 31 failed to detect a difference in protein utilization attributable to feathering, but an optimum was achieved at 0.40% Cys with both broiler sources. Overall results suggest that the Met requirement for broiler males between 3 and 6 wk of age was independent of feathering and approximated 0.46% (95% of the level of maximal response). Cystine requirements once corrected for submarginal Met status indicated a greater demand by fast- than slow-feathering male broilers corresponding to 0.42 and 0.37%, respectively.