INFLUENCE OF THE $-AGONIST, ZILPATEROL, ON GROWTH PERFORMANCE AND CARCASS CHARACTERISTICS OF FEEDLOT STEERS

One hundred forty crossbred steers (373 kg) One hundred and forty crossbreed yearlings steers were used in a randomized complete block design experiment (373 kg) were used in a 42-d finishing trial. Steers were (14 pens, 10 steers/pen) to evaluate the influence of blocked by weight and randomly assigned, within weight supplementation of a steam-rolled wheat-based finishing diet groupings, to 14 pens (10 steers/pen). Pens were 510 m with with 6 mg/kg (as-fed basis) zilpaterol during the final 6 weeks 64 m overhead shade, automatic waterers, and 17 m of the finishing period on growth performance and carcass fence-line feed bunks. The trial was initiated July 22, 1997. characteristics. Supplemental zilpaterol did not influence (P > Treatments consisted of a steam-flaked wheat-based finishing .20) DM intake (8.55 vs 8.45 kg/d), but enhanced (P .20) KPH, fat thickness, or marbling score. for 48 h the following measurements were obtained: 1) Adjusting to a constant carcass weight, zilpaterol longissimus muscle area (ribeye area), taken by direct grid supplementation increased gross (boneand trim-in) primal reading of the eye muscle at the twelfth rib; 2) subcutaneous cuts (1.7%, P < .01), boneless closely trimmed primal cuts fat over the eye muscle at the twelfth rib taken at a location (2.9%, P < .05), and boneless closely trimmed retail cuts 3/4 the lateral length from the chine bone end; 3) kidney, (3.2%, P < .10). We conclude that zilpaterol supplementation pelvic and heart fat (KPH) as a percentage of carcass weight can have marked beneficial effect on growth performance and and 4) marbling score (USDA, 1965). Yield of boneless, carcass yield of feedlot steers. Enhanced growth performance closely trimmed retail cuts from the round, loin, flank, rib, accounts for 55% of the net economic value of zilpaterol chuck, brisket and plate, as well as yields of subprimal cuts supplementation (benefit to the feeder), while increased from tenderloin, knukle, inside round, gooseneck, heel, carcass cutability accounts for 45% of the net value (benefit outside and inside skirt, back rib, short plate, triangle, chuck to the meat packer and retailer). roll, chuck tender, flank steak, shank and neck were removed Introduction carcass/pen). In determining steer performance, initial and Zilpaterol chlorhydrate is a catecholamine analog fill. Steer ADG (kg/d) was based on carcass adjusted final belonging to a family of compounds pharmacologically weights (carcass weight/.628, where .628 is the average known as $-2-agonists. The $-agonists enhance growth dressing percentage of all 140 steers/100). Energy gain (EG) efficiency by stimulation of $-adrenergic receptors on cell was calculated by the equation: EG = (.0493BW )ADG , surfaces. In muscle tissue, $-agonists promote protein where EG is the daily energy deposited (Mcal/d), ADG is live synthesis and cell hypertrophy by inhibition of proteolysis weight gain (kg/d) and BW is the mean body weight (kg; (rate of protein synthesis, per se, is not affected). In adipose NRC, 1984). Maintenance energy expended (Mcal/d, EM) was tissue, $-agonists promote lipolysis. This redirecting of calculated by the equation: EM = . 077BW (Lofgreen and cellular energy metabolism in favor of protein synthesis has Garrett, 1968). The NE values of the diets for maintenance and prompted the referencing of some $-agonists as gain were obtained by means of the quadratic formula ( “repartitioning agents” (Reeds and Messermann, 1991). The ), where a = -. 41EM, b = . 877EM + . 41DMI + EG, c objective of this study was to evaluate the influence of = -. 877DMI, and NE = . 877NE . 41. This trial was analyzed zilpaterol supplementation during the last 42 d of the feeding as a randomized complete block design experiment (Hicks, period on growth performance and carcass characteristics of 1973). Pen means were used as experimental units. Treatments steers. effects on yields of trimmed cuts were adjusted to a constant Experimental Procedure covariate in the statistical model. 2 2