Date of Graduation


Document Type


Degree Name

Doctor of Philosophy in Animal Science (PhD)

Degree Level



Animal Science


Paul A. Beck

Committee Member

Jason K. Apple

Second Committee Member

Jiangchao Zhao

Third Committee Member

Jason Kelley


cattle, feed additives, feed efficiency, ionophore, monensin, weight gain


Three monensin levels during a stocker phase (0, 800, 1600 g/ton in a free choice mineral) and two levels during finishing (0 [U] or 37.5 mg/kg diet DM [M]) were used to determine the effects of monensin supplementation during a stocker and subsequent finishing phase on beef calves. Steers (n = 605, BW = 278 ± 27 kg) were fed pearl millet hay with soybean hull and corn gluten feed supplement (0.5% BW daily [AF basis] Block 1) or grazed fall wheat (Block 2), spring wheat (Block 3), bermudagrass (Blocks 4 & 5) or wheat with mixed-grass baleage (Block 6). A subset of calves were transported 1,068 km to Canyon, TX (blocks 1, 2, and 4) or 636 km to Stillwater, OK (block 6) for finishing. Rumen fluid was collected on a subset of cattle (n = 30) throughout the experiment to characterize rumen microbiota by next generation sequencing. There were no treatment × diet interactions (P ≥ 0.96) during the stocker phase or location × treatment interactions (P ≥ 0.19) during finishing, therefore data were pooled for statistical analysis. During the stocker phase, cattle consuming monensin had greater (P = 0.01) final BW compared with the 0 treatment. Steers on 800 and 1600 had greater ADG (P = 0.02) and total gain (P = 0.02) than 0. Mineral intake decreased (P < 0.01) as monensin level increased. Stocker × feedlot treatment interactions were not observed for feedlot performance (P ≥ 0.50). Feedlot cattle consuming monensin had decreased (P < 0.01) DMI and increased (P < 0.01) G:F than those that did not. Cattle consuming monensin during finishing had increased (P = 0.03) fat thickness. During the stocker phase, monensin treatments had decreased alpha community (P ≤ 0.04) compared to the 0 treatment. Cattle previously on the 0 and 1600 treatments during the stocker phase and were fed monensin at the feedlot had decreased alpha diversity (P = 0.04) on feedlot d 14 compared to those that did not.