The aim of the study was to examine the effects of different methods of supplementation of zilpaterol hydrochloride (ZH) (every day, 1 d on and 1 d off or 2 d on and 2 d off) for the last 28 and 42 days of feeding period (length of feeding) on fat tail adipocyte and muscle fiber size and meat quality. Fifty six Lori-Bakhtiari feedlot lambs (initial BW = 44 ± 4.7 kg) were stratified according to the BW and randomly assigned to individual pens. Lambs were fed a diet with 14% CP and 2.36 Mcal/kg of ME supplemented with 0.2 mg/kg of live weight d −1 ZH. Different methods of supplementation and/or feeding period of ZH had no effect (P > 0.05) on ultimate pH, cooking loss, drip loss and cholesterol content of meat. Method of supplementation had a significant effect on shear force (P < 0.013) with the higher value in the daily supplementation compared to intermittent days of supplementation. Lambs fed ZH daily for the last 42 days had higher shear force compared to the control group (P < 0.05). There was no significant difference in fat-tail adipocyte area between the methods of ZH supplementation (P > 0.05). Lambs that received ZH daily for 28 days and intermittently (1 on and 1 off for 42 d) had greater fiber area (P < 0.01) compared to the control treatment fed a basal diet for 42 d without ZH. Intermittent supplementation of ZH had higher tenderness than daily supplementation (P = 0.025) but, method of supplementation and length of feeding did not affect other sensory characteristics of meat. Overall, feeding ZH daily for 42 d decreased tenderness of meat, while the other meat quality characteristics were not affected in Lori-Bakhtiari fat-tail lambs.
