Zilpaterol hydrochloride lowers marbling score by dilution of marbling and depression of intramuscular adipocyte volume in M. longissimus dorsi of beef steers.

We hypothesized that zilpaterol hydrochloride (ZH) depresses marbling scores in beef cattle by dilution of the intramuscular (i.m.) adipose tissue in the longissimus thoracis (LT) by increased muscle cross-sectional area. Crossbred steers were treated with ractopamine hydrochloride (RH; 28 d) or ZH (20 d) or no ß-adrenergic agonist (CON). ZH increased LT cross-sectional area (P < .001) and depressed rib fat depth (P < .001), USDA marbling score (P < .05), percent LT lipid (P < .02), total lipid in a 2.54 cm-thick LM steak (P < .05) and most LT lipid fatty acids (P ≤ .05),but had no effect on mean i.m. adipocyte volume (P = .15). ZH increased the relative volume proportion of smaller i.m. adipocytes (250 to 500 pL) and decreased the proportion of larger i.m. adipocytes (2000 and 3000 pL) (P < .05). We conclude that ZH reduces marbling score both by dilution of the marbling and a depression of i.m. adipocyte relative volumes.