A feedlot experiment was conducted under commercial conditions in the Texas Panhandle using 3,757 feedlot steers (average of 94 steers/pen) to evaluate the effects of feeding
zilpaterol hydrochloride with or without
monensin and
tylosin on feedlot performance and carcass characteristics. The experiment was conducted using a randomized complete block design. Treatments were arranged
as a 2 (no
zilpaterol vs.
zilpaterol) x 2 (
monensin and
tylosin withdrawn vs.
monensin and
tylosin fed during the final 35 d on feed) factorial. Steers were fed for a total of 161 to 167 d, and treatments were administered during the final 35 d that cattle were on feed. When included in the diet,
zilpaterol,
monensin, and
tylosin were supplemented at 8.3, 33.1, and 12.2 mg/kg (DM basis), respectively.
Zilpaterol was included in the diet for 30
d at the end of the finishing period and withdrawn from the diet for the last 5 or 6 d cattle were on feed. Cattle were harvested and carcass data collected. There were no
zilpaterol x
monensin/
tylosin interactions (P >or= 0.12) for ADG or G:F. Feeding
zilpaterol increased ADG (P < 0.001) by 0.20 kg and G:F (P < 0.001) by 0.029 kg/kg during the last 35 d on feed. Likewise, when feedlot variables were measured throughout the entire 161- to 167-d feeding trial, ADG (3.4%; P < 0.001) and G:F (3.9%; P < 0.001) were increased. Feeding
zilpaterol increased (P < 0.001) dressing percent and HCW and decreased (P < 0.001) total
liver abscess rate compared with controls. In addition,
zilpaterol increased (P < 0.001) LM area by an average of 8.0 cm(2). There was a
zilpaterol x
monensin/
tylosin interaction (P = 0.03) for marbling score.
Zilpaterol decreased (P < 0.001) marbling score regardless of
monensin and
tylosin treatment, although withdrawal of
monensin and
tylosin for 35 d decreased marbling to a greater extent (31 vs. 17 degrees).
Zilpaterol decreased (i.e., improved; P < 0.001) calculated yield grade regardless of
monensin and
tylosin treatment, but feeding
zilpaterol in combination with the withdrawal of
monensin and
tylosin for 35 d decreased calculated yield grade to a greater extent (0.49 vs. 0.29) compared with the
zilpaterol,
monensin, and
tylosin combination treatment (
zilpaterol x
monensin/
tylosin interaction, P = 0.03). Results suggest that
monensin and
tylosin can be withdrawn from the diet during the
zilpaterol feeding period (final 35 d on feed) with minimal effect on animal performance, although feeding
zilpaterol in combination with
monensin and
tylosin seemed to moderate effects on carcass quality.