Animals, Design, and Treatments

Seventy-five Angus steers were housed in individual feedlot pens (4.9 × 1.8 m) bedded with sawdust in a sheltered barn. The steers were given 4 wk to adapt to the new environment and the high-grain finishing diet (Table 1) before starting the experiment. During the adaptation period, the steers were ear tagged and vaccinated following standard operating procedures of the beef facility. The experiment was a randomized complete design. The steers were initially weighed (448 ± 8.4 kg) on 2 consecutive days, blocked by body weight (BW) and randomly allocated into 1 of 5 treatments. The treatments were: 1) control (no monensin, tylosin, or ADY), 2) antibiotics (ANT; 330 mg monensin + 110 mg tylosin·steer⁻¹d⁻¹), 3) ADY (1.5 g·steer⁻¹d⁻¹), 4) EDY (3 g·steer⁻¹d⁻¹), and 5) a mixture of ADY and EDY (MDY; 1.5 g ADY + 3 g EDY·steer⁻¹d⁻¹). The daily amount of antibiotics or yeast was mixed with 50 g of dry-rolled barley and delivered to each animal once daily by top-dressing onto the feed at feeding time. The control animals received a top-dress of 50 g of dry-rolled barley without antibiotics or yeast.

Ingredients and chemical composition of the experimental diet (DM basis)

ADF, acid detergent fiber; CP, crude protein; NDF, neutral detergent fiber

¹Composition (DM basis): 31.8% DM, 48.2% NDF, 25.9% ADF, 16.4% starch, and 12.1% CP based on 4 samples composited by period.

²Composition (DM basis): 90.2% DM, 97.0% OM, 14.6% NDF, 3.7% ADF, 55.9% starch and 14.5% CP based on 4 samples composited by period.

³Supplied per kilogram of dietary DM: 15 mg Cu, 65 mg Zn, 28 mg Mn, 0.7 mg I, 0.2 mg Co, 0.3 mg Se, 6,000 IU vitamin A, 600 IU vitamin D, and 47 IU vitamin E.

⁴NEm and NEg were estimated from NRC (2000).