We summarized experimental data to quantify the change in final BW due to a particular implant strategy when cattle are adjusted to the same final body composition. The database developed for this study included 13 implant trials involving a total of 13,640 animals (9,052 steers and 4,588 heifers). Fifteen different implant strategies were used among these trials, including no implant (control), single implants, and combinations of implants. Individual carcass data collected at slaughter were used to calculate the adjusted final shrunk BW at 28% empty body fat (AFBW) for each treatment group within a trial, then the implant treatments were grouped into categories according to their effect on weight at 28% empty body fat (four groups for steers and two groups for heifers). All differences in AFBW between categories were significant (P < 0.01), indicating an incremental anabolic implant dose response in AFBW over unimplanted animals. Values for AFBW ranged from 520 kg in unimplanted steers to 564 kg in steers implanted and reimplanted with Revalor-S. For heifers, AFBW ranged from 493 kg in unimplanted heifers to 535 kg in heifers implanted and reimplanted with Revalor-H. After accounting for differences in mean BW and composition of gain, implanted steers and heifers had 4.2 and 3.1% higher apparent diet ME values, respectively. Increasing the anabolic implant dose increases the weight at which animals reach a common body composition. This study indicates that anabolic implant response is due to a combination of a reduced proportion of the DMI required for maintenance, reduced energy content of gain, and efficiency of use of absorbed energy.

An experiment was conducted using 200 beef carcasses to evaluate the effects of feeding zilpaterol hydrochloride with or without monensin and tylosin on carcass cutability and meat sensory variables. The experiment was conducted using a randomized complete block design with treatments arranged as a 2 (no zilpaterol vs. zilpaterol) x 2 (monensin and tylosin withdrawn vs. monensin and tylosin fed) factorial. Cattle (n=3,757) were fed zilpaterol hydrochloride, a beta(2)-adrenergic agonist, for 30 d at the end of the finishing period and withdrawn from zilpaterol hydrochloride for the last 5 d on feed. Five carcasses (weighing between 305 and 421 kg and free of slaughter defects) were selected from each of 40 feedlot treatment pens. Strip loins from the left sides were collected for sensory analysis and Warner-Bratzler shear force (WBSF) testing, and the rib was collected for 9th, 10th, 11th-rib dissections. A subsample of 3 carcass right sides per pen was fabricated into boneless subprimals according to Institutional Meat Purchase Specifications. Carcasses from zilpaterol-fed steers had greater (P or= 0.26). For the main effect of monensin and tylosin, withdrawal of monensin and tylosin decreased (P=0.01) consumer juiciness scores, although other yield and compositional measurements were not affected (P >or= 0.07). Zilpaterol is a strong repartitioning agent that increases meat yield through increased protein and decreased fat deposition.

Experiments were conducted at 3 US locations (CA, ID, and TX) to determine the effects of dietary zilpaterol hydrochloride (Zilmax, Intervet Inc., Millsboro, DE) and duration of zilpaterol feeding on performance and carcass merit of finishing steers and heifers. At each site, 160 steers and 160 heifers were stratified within sex by initial BW (study d -1) and assigned randomly within BW strata to 1 of 4 treatments in a randomized complete block design (4 blocks/treatment for each sex). The 4 treatments were arranged in a 2 (no zilpaterol vs. zilpaterol) x 2 (20 or 40 d duration of zilpaterol feeding) factorial arrangement of treatments. When included in the diet, zilpaterol was supplemented at 8.3 mg/kg of DM. Each pen consisted of 10 animals. Each animal was individually weighed unshrunk on d 1, 21 or 41, and 66 of the experiment. Following d 66, cattle were slaughtered and carcass data collected. Feeding zilpaterol increased (P<0.01) final BW of steers and heifers by 11.6 and 6.7 kg, respectively. In addition, feeding zilpaterol hydrochloride increased (P or= 0.12) and KPH (P >or= 0.70) were not affected by feeding zilpaterol to steers or heifers. Feeding zilpaterol decreased (i.e., improved; P=0.02) calculated yield grade of steer and heifer carcasses. Marbling score (P=0.002) and quality grade (P=0.002) were decreased when zilpaterol hydrochloride was fed to steers, and the decrease in marbling score and quality grade tended to be greater when zilpaterol was fed for 40 compared with 20 d (zilpaterol x duration interaction, P=0.07). For heifers, marbling score tended (P=0.07) to be decreased and quality grade was decreased (P=0.05) when zilpaterol hydrochloride was fed. In general, it appears from these data that zilpaterol hydrochloride fed for 20 to 40 d at the end of the finishing period enhances growth performance and carcass muscle deposition for steers and heifers.

Four trials, each with a randomized complete block design, were conducted with 8,647 beef steers (initial BW = 346 +/- 29.6 kg) in 3 different locations in the United States to evaluate the effects of zilpaterol hydrochloride (ZH) on performance and carcass characteristics of feedlot cattle. Treatments consisted of feeding ZH (8.33 mg/kg of dietary DM) for 0, 20, 30, or 40 d, at the end of the feeding period, followed by a 3-d withdrawal period before slaughter. Cattle were weighed on d 0 and 50 before slaughter (in 3 of the 4 studies), and on the day of slaughter. Data from the 4 trials were pooled for statistical analyses. No differences (P > or = 0.78) were detected among treatments for ADG and G:F from the start of the study until the final 50 d on feed. Final BW was greater for the average of the 3 ZH-treated groups (P < 0.01) than for the 0-d group. Average daily gain was greater for ZH-treated vs. control cattle during the final 50 d on feed (P < 0.01) and for the entire feeding period (P < 0.01). No differences in DMI were noted for any periods of the experiment (P > or = 0.42) for ZH-treated cattle vs. controls. No differences were noted for DMI among the ZH-treated groups for the final 50 d on feed (P = 0.81) or for the overall feeding period (P = 0.31). Feeding ZH for any length of time increased G:F (P < 0.01) for the final 50 d and overall compared with 0-d cattle. In addition, a linear increase with more days of ZH feeding was observed for G:F during the period that ZH was fed (P = 0.01), as well as for the overall feeding period (P = 0.01). The ZH-treated cattle had heavier HCW (P < 0.01), greater dressing percent (P < 0.01), reduced marbling scores (P < 0.01), less 12th-rib fat (P < 0.01), larger LM area (P < 0.01), less KPH (P = 0.01), and a lower USDA yield grade (P < 0.01) than the 0-d cattle, regardless of the duration of ZH feeding. Dressing percent increased linearly (P < 0.01) with increased duration of ZH feeding, whereas 12th-rib fat (P = 0.07), marbling scores (P < 0.01), and USDA calculated yield grade (P = 0.01) decreased linearly with increased duration of ZH feeding. Feeding ZH increased ADG and G:F and decreased overall carcass fatness. In addition, effects of ZH on measures of carcass fatness were enhanced by feeding the product for a greater length of time.

Our objective was to determine the effects of feeding zilpaterol hydrochloride (ZH), a beta-agonist, for the final 30 d of the feeding period, with or without a terminal estrogen + trenbolone acetate (TBA) implant (Revalor-S; 24 mg of estradiol-17beta and 120 mg of TBA; REV) on meat tenderness and carcass cutout yields. Crossbred steers (n = 2,279) were divided into 6 BW blocks and 24 pens. Within each block, pens were assigned randomly to 1 of 4 treatments: 1) no terminal implant (control); 2) a terminal REV given 91 d before slaughter; 3) no terminal implant plus ZH; and 4) a terminal REV implant plus ZH (REV+ZH). All cattle received Component TE-IS (16 mg of estradiol and 80 mg of TBA) on d - 61 of the feeding period [corrected]. Zilpaterol hydrochloride was added to the diets at a concentration of 8.38 mg/kg (DM basis) during the final 30 d of the feeding period, followed by a 3-d period before slaughter in which ZH was withdrawn from the diet. Carcasses (n = 30/treatment) were selected from the 2,279 cattle and fabricated into subprimal cuts as per Institutional Meat Purchase Specifications. Strip loins were collected, cut into 2.54-cm steaks, and aged 7, 14, and 21 d, after which Warner-Bratzler shear force (WBSF), collagen content, desmin degradation, and muscle fiber diameter measurements were determined. Feeding ZH increased (P < 0.05) yield of the #112A ribeye roll, #116B chuck mock tender, #167A peeled knuckle, #169 top inside round, #171B outside round, #171C eye of round, #180 strip loin, #184 top sirloin butt, and #189A full tenderloin for ZH treatment. Longissimus muscle WBSF at 7, 14, and 21 d postmortem was increased (P < 0.001) with ZH supplementation. Desmin degradation at 7, 14, and 21 d postmortem was not affected with REV or ZH supplementation compared with controls. Zilpaterol hydrochloride had an additive effect with REV on increasing LM fiber diameter (P < 0.001). When fed to cattle that received a terminal implant of REV, ZH potentially increased LM WBSF as a result of induced muscle hypertrophy. During the 21-d aging period, WBSF decreased with aging, suggesting that carcasses from cattle supplemented with ZH might require longer aging time to ensure that acceptable levels of tenderness are reached.