Effect of monensin withdrawal on intake, digestion, and ruminal fermentation parameters by Bos taurus indicus and Bos taurus taurus steers consuming bermudagrass hay

Effects of monensin withdrawal and cattle subspecies on the utilization of bermudagrass hay (14.3% CP, 72.3% NDF, and 36.9% ADF) were evaluated using ruminally cannulated steers (5 Bos taurus indicus [BI] and 5 Bos taurus taurus [BT]). Subspecies were concurrently subjected to a 2-period, 2-treatment crossover design. Treatments consisted of either 0 mg.steer(-1).d(-1) monensin with no previous monensin feeding (CON) or withdrawal from 200 mg.steer(-1).d(-1) monensin (MON) fed individually in 0.91 kg dried distillers' grains with solubles for 42 d. Withdrawal was evaluated for a 28-d period. Ruminal fluid was collected 2 h after feeding on d 0, 1, 4, 7, 14, and 21 after withdrawal for determination of pH, VFA, ruminal NH3-N (RAN), rate of NH3 production, and CH4 production rate. Hay, ort, and fecal grab samples were collected d 23 through 28 after withdrawal for determination of intake and digestion. No subspecies x monensin, subspecies x day, or subspecies x monensin x day interactions were observed (P >= 0.11). An effect of day after monensin withdrawal was observed (P < 0.01) for total VFA concentration, with an increase following withdrawal followed by a decrease and then stabilization. Monensin x day after monensin withdrawal interactions (P = 0.01) were observed for the acetate: propionate (A: P) ratio and molar percent of acetate and propionate. There was a decrease in molar percent of propionate between d 1 and 4 from 19.1 to 18.0; however, it remained greater (P <= 0.10) for MON than CON through d 7. Withdrawal increased molar percent of acetate from 68.3 to 69.8 between d 0 and 4 for MON steers. The A: P ratio was less (P = 0.01) on d 0 for MON than for CON (3.4 vs. 4.0), but by d 4, it increased to 3.8 and was not different (P = 0.14) from CON. By d 14, no differences (P >= 0.88) remained for acetate, propionate, or the A: P ratio. After monensin withdrawal, monensin reduced (P < 0.01) RAN by 12.3% (2.09 vs. 1.83 mM for CON and MON, respectively). Monensin withdrawal and cattle subspecies had no effect (P >= 0.23) on rate of NH3 production or CH4 production rate. Monensin withdrawal had no effect (P >= 0.45) on intake or digestibility parameters. Greater forage OM intake (P = 0.09; 21.2 vs. 19.2 g/kg BW) and OM digestibility (P < 0.01; 72.4 vs. 63.0%) resulted in greater (P < 0.01) total digestible OM intake (16.8 vs. 13.2 g/kg BW) in BT steers than in BI steers. These results suggest that BT steers are better able to utilize bermudagrass hay than BI steers. Upon monensin withdrawal, steers previously fed monensin continue to have a reduced A: P ratio for at least 7 d.