Interactive effect of dietary protein level and zilpaterol hydrochloride on feedlot performance and meat quality of steers

Bonsmara type steers were used to determine the effect of dietary zilpaterol hydrochloride (ZH) in combination with different dietary crude protein (CP) levels (100, 120 and 140 g CP/kg) on growth performance and meat quality. Treatment groups (T) consisted of 12 steers each. T1 – 100 g CP/kg + 0.15 mg ZH/kg live weight (LW); T2 – 120 g CP/kg + 0.15 mg ZH/kg LW; T3 – 140 g CP/kg + 0.15 mg ZH/kg LW and T4 – 120 g CP/kg + 0 mg/kg LW. Steers received the diets from an initial LW of 278 kg and fed to a target mass of 390 kg. After a seven day ZH withdrawal period the steers were slaughtered and carcasses electrically stimulated. Samples from m. longissimus thoracis, m. semitendinosus and m. triceps brachii were kept in airtight polyethylene bags and matured for ten days at 4 °C. Zilpaterol treatment increased the average daily gain (ADG) non-significantly (NS) by 9.6%. Zilpaterol treatment improved feed conversion ratio (FCR) by 20% (NS). T2 showed a significant 39.4% improvement in FCR in comparison with T4. Warner-Bratzler shear force (WBSF) of m. semitendinosus and m. biceps brachii samples differed between ZH treated groups. Cooking loss (CL) for m. longissimus thoracis was lower for T2 in comparison with T1, T3 and T4. ZH treatment for 35 days, electrical stimulation and a maturation period of 10 days generally left the WBSF and CL unaffected while the FCR in combination with 120 g CP/kg improved significantly.