The effect of zilpaterol hydrochloride on energy and protein metabolism and empty body composition of beef steers



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Two trials (indirect calorimetry and comparative slaughter) were conducted to examine energy and protein metabolism and empty body composition of cattle supplemented zilpaterol hydrochloride (Z). The first trial utilized beef steers (n=20; 463 ± 14 kg) blocked (n=5) by body weight (BW) and source and adapted to maintenance intake for 21 d prior to Z (90 mg/hd/d) or control (C) treatment for 20 d (455 ± 14 kg at start of treatment). No differences in DMI, apparent nutrient digestibility, O2 consumption or CH4 production (P ≥ 0.12) were detected between treatments but Z cattle had greater CO2 production during the maintenance period (P = 0.04; 2,325 vs. 2,185 L/steer; 23.6 vs. 22.4 L/kg BW0.75). Cattle treated with Z tended to have increased heat production (P = 0.09; 12.44 vs. 11.69 Mcal, respectively) but not on a BW0.75 basis (P = 0.12; 0.126 vs. 0.120, respectively) with no treatment difference in fasting heat production (P ≥ 0.32). Control cattle excreted more (P = 0.05) nitrogen in urine (39.8 vs. 32.4 g/d, respectively) whereas cattle fed Z tended to have increased nitrogen retention (P = 0.07; 22.14 vs.14.12 g/d). Supplementation of Z did not improve HCW (P = 0.12) but did increase dressed carcass yield (P = 0.02; 62.12 vs. 60.65%, respectively) and LM area (P = 0.02; 77.81 vs. 70.90 cm2) while tending to lower USDA calculated yield grade (P = 0.06; 1.8 vs. 2.2). The second trial utilized single-sired, beef steers (n=56; initial BW = 590 ± 36 kg) blocked (n=2) by BW and terminal implant and further sorted into pairs (n=14 per block) by BW. Pairs were assigned to 0, 28 or 56 d of feeding, within 28 and 56 d to maintenance (M) or ad libitum (A) intake, and within 56d of feeding, steers within a pair were randomly assigned to either 20d of Z supplementation (90 mg/hd/d) with a 4d withdrawal prior to harvest or no Z supplementation (C). Carcass sides were fabricated 48h post-harvest; samples of 9-10-11 rib primals were dissected into lean, fat, and bone with lean and fat ground and sampled for proximate analysis; all long bones were sliced horizontally and sampled for analysis. Treatment impacted (P ≤ 0.05) live ADG; contrasts indicated A (1.33) was greater than M (0.14 kg/d), and Z (1.12) was greater than C (0.82 kg/d). Similarly, carcass ADG differences (P < 0.01) indicated A (1.04) was greater than M (0.36 kg/d), and Z (1.35) was greater than C (0.71 kg/d). Intake altered BW and empty body weight (EBW); M cattle had reduced BW and EBW (P < 0.01, 585 and 540 kg) than A cattle (647 and 597 kg). Cattle fed at M had less carcass and internal cavity mass (P < 0.01, 359 and 79.4 kg) than A cattle (394 and 93.5 kg). Moreover, mass of total splanchnic tissue was less (P < 0.01) for M cattle than A cattle (59.8 vs. 72.5 kg). Dressed carcass yield was greater (P < 0.01) for Z than C cattle (63.5 vs. 61.6 %). Cattle fed at M exhibited less 12th rib subcutaneous fat and lower U.S. yield grades than A cattle (P < 0.01; M-1.71 cm and 3.3, A-2.46 cm and 4.3) and lower Canadian yield grade (P < 0.01; 53.9 vs. 51.9 for M vs. A). Fat content of the EB (EBF) and carcass (CF) was impacted by treatment with 56d AC having greater EBF and CF (P < 0.05; 34.1 and 30.2%, respectively) than all other treatments except 28d A. In addition, M steers had less (P < 0.05) EBF (30.44 v. 32.30%) and CF (30.29 v. 32.23%) vs. A steers. Moisture of the EB (EBM) and carcass (CM) tended to be impacted by diet (P < 0.10) in an inverse fashion to EBM and CF; whereas Z increased (P < 0.05) EBM and CM. Protein of the empty body and carcass tended to be affected by treatment on an absolute basis with Z steers exhibiting increased protein yield vs. C. Additionally, EB moisture, fat and protein daily gains were different (P < 0.05) with M decreasing gains of all components vs. A (P < 0.01) and Z tending to increase moisture vs. C (P = 0.06). Heat production of AZ was greater than AC steers (P < 0.001; 0.211 vs. 0.184 Mcal/EBW0.75) whereas MZ did not differ from MC (P > 0.05). As a result, efficiency of metabolizable energy intake was 0.499 and 0.293 for C and Z steers respectively. Results from the first trial indicate that Z treatment tended (P ≥ 0.07) to increase nitrogen retention and modify heat production during maintenance by increasing CO2 production. Results from the second trial indicated that days on feed, energy level intake and Z supplementation affected live performance, kill yields, carcass grading factors, empty body and carcass composition and efficiency of gain.



beef, energy metabolism, empty body composition, zilpaterol hyrdrochloride,