title: Effects of dietary zilpaterol hydrochloride on feedlot performance and carcass characteristics of beef steers fed with and without monensin and tylosin.
abstract: 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 Treatments were arranged as a 2 (no zilpaterol vs. zilpaterol) x 2 (monensin and tylosin withdrawn vs. monensin and tylosin fed during the final 35 d on feed) factorial.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 Zilpaterol was included in the diet for 30 d at the end of the finishing period and withdrawn from the diet for the last 5 or 6 d cattle were on feed.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.
authors.affiliation: Intervet Inc, Millsboro, DE 19966, USA. jayden.montgomery@targacept.com
authors.forename: J L
authors.initials: JL
authors.lastname: Montgomery
authors.forename: C R
authors.initials: CR
authors.lastname: Krehbiel
authors.forename: J J
authors.initials: JJ
authors.lastname: Cranston
authors.forename: D A
authors.initials: DA
authors.lastname: Yates
authors.forename: J P
authors.initials: JP
authors.lastname: Hutcheson
authors.forename: W T
authors.initials: WT
authors.lastname: Nichols
authors.forename: M N
authors.initials: MN
authors.lastname: Streeter
authors.forename: R S
authors.initials: RS
authors.lastname: Swingle
authors.forename: T H
authors.initials: TH
authors.lastname: Montgomery
mesh_terms: D000318:Adrenergic beta-Agonists / Q000008:administration & dosage / Q000009:adverse effects / Q000494:pharmacology*; D000818:Animals; D001823:Body Composition / Q000187:drug effects*; D002417:Cattle / Q000502:physiology*; D019587:Dietary Supplements; D004435:Eating / Q000187:drug effects; D008297:Male; D008460:Meat / Q000592:standards; D008985:Monensin / Q000008:administration & dosage* / Q000494:pharmacology; D011897:Random Allocation; D014297:Trimethylsilyl Compounds / Q000008:administration & dosage / Q000009:adverse effects / Q000494:pharmacology*; D015645:Tylosin / Q000008:administration & dosage* / Q000494:pharmacology
chemical_list: D000318:Adrenergic beta-Agonists; D014297:Trimethylsilyl Compounds; C475840:Zilpaterol; D008985:Monensin; D015645:Tylosin