Response of broiler strains differing in body fat to inadequate methionine: live performance and processing yields

Ross x Arbor Acres (RxAA) and Steggles x Arbor (SxAA) chicks were given all nutrients as advocated by NRC (1984) except for methionine. Corn and soybean meal were the sole CP sources, and both strains were compared when feeds were adequate and deficient in methionine (0 to 3 wk, .65 vs .42% with 24.2% CP and 3.20 kcal AME/g; 3 to 6 wk, .54 vs .46% with 20.7% CP and 3.21 kcal AME; and 6 to 8 wk, .35 vs .30% with 17.8% CP and 3.19 kcal AME, respectively). Cystine exceeded NRC (1984) recommendation with all feeds. Live weights of RxAA broilers were heavier throughout the experiment, but SxAA had the advantage in feed conversion. Adverse effects of low methionine on weight gain were apparent only during the first 6 wk, and SxAA responded to the deficiency more than RxAA from 0 to 3 wk. The SxAA birds had less abdominal fat when processed at both 6 and 8 wk than RxAA. Low methionine increased fat proportions at 6 wk but not at 8 wk. Percentage chilled carcass yield without abdominal fat was similar for each strain and decreased as a result of low methionine at both ages. Low methionine also reduced proportions of skinless boneless breast meat but only at 6 wk and particularly in SxAA birds. The increased ability of RxAA birds to deposit fat enabled additional feed intake, in turn, minimizing repercussion of inadequate methionine.