Effects of supplementing feedlot steers and heifers with zilpaterol hydrochloride on Warner–Bratzler shear force interrelationships of steer and heifer longissimus lumborum and heifer triceps brachii and gluteus medius muscles aged for 7, 14 and 21 d

Longissimus lumborum (LL) muscles from 117 steers plus LL, gluteus medius (GM), and triceps brachii (TB) muscles from 132 heifers were evaluated for effects of feeding duration of zilpaterol hydrochloride (Zilmax®; ZH; 7.56 g/907 kg on a dry matter basis) and aging time on tenderness. Both genders were blocked by initial weight into six blocks of four pens. Pens were assigned to treatments of control (C), or 20, 30 or 40 days on ZH, with a 3 day withdrawal. Steaks from each subprimal were vacuum aged individually for 7, 14 or 21 days, frozen, thawed, and cooked to 71 °C for Warner–Bratzler shear force (WBSF). All muscles from steers and heifers from ZH30 and ZH40 treatments had higher (P < 0.05) WBSF than those of C. The WBSF of steer LL and heifer TB from the ZH20 treatment was higher (P < 0.05) than C. There was a treatment by aging interaction (P > 0.05) for WBSF of GM steaks from heifers. Percentage of intramuscular fat had little effect on tenderness. Percentages of steer LL and heifer TB steaks with WBSF values below thresholds of either 5.0 or 4.6 kg from the ZH20 treatment were quite high, whereas percentages of heifer LL and GM muscles below 5.0 kg (67%) and 4.6 kg (57%) were low. Feeding ZH ⩾ 20 days generally increased WBSF values, but mean WBSF values for steer LL and heifer TB were below 4.6 kg. Feeding ZH 20 ⩾ days resulted in >40% of GM steaks with WBSF values above 4.6 kg.