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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 × monensin, subspecies × day, or subspecies × monensin × 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 × 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. [ABSTRACT FROM AUTHOR]

Effects of monensin inclusion and cattle subspecies on utilization of bermudagrass hay (13.7% CP, 77.3% NDF, and 38.8% ADF) were evaluated using ruminally cannulated steers (5 Bos taurus indicus [BI] and 5 Bos taurus taurus [BT]; 398 kg BW). Subspecies were concurrently subjected to a 2-period, 2-treatment crossover design. Treatments were 0 (CON) or 200 mg·steer−1·d−1 monensin (MON) in 0.91 kg dried distillers’ grains with solubles. Periods were 70 d in length: 20 d of adaptation, 22 d of sample collection, and 28 d for withdrawal of treatment. Steers were group housed during adaptation and moved to individual covered pens for sampling. Hay, ort, and fecal grab samples were collected d 21 through 25 for determination of intake and digestion. Ruminal fluid was collected with a suction strainer 0, 2, 4, 8, and 12 h after feeding on d 42 for pH, VFA, and ruminal NH3–N (RAN) analysis. Additionally, at h 2, ruminal fluid and contents were collected for determination of rate of NH3 production and CH4 production rate. No subspecies × monensin interactions were observed (P ≥ 0.12). Monensin had no effect (P ≥ 0.16) on intake or digestibility parameters. No subspecies effect (P ≥ 0.11) was observed for forage OM intake, total OM intake, or OM digestion. Total digestible OM intake tended to be greater (P = 0.06) for BT steers than for BI steers (14.0 vs. 12.2 g/kg BW). There was an effect of hour after feeding (P ≤ 0.01) on pH, total VFA, acetate:propionate ratio, and molar percent acetate and propionate. Total VFA concentration was greater (P = 0.01) in CON steers than in MON steers (66.5 vs. 62.0 mM). Monensin decreased molar percent acetate (P = 0.02) from 72.5 to 71.2% and increased molar percent propionate (P < 0.01) from 16.9 to 18.7%, resulting in a reduced (P < 0.01) acetate:propionate ratio (from 4.34 to 3.85). Although not significantly (P = 0.19), monensin numerically reduced the CH4 production rate by 15.8%. Greater (P = 0.07) CH4 production rate tended to be observed in BI steers than in BT steers (21.4 vs. 16.6 μmol CH4·mL−1·h−1, respectively). Monensin had no effect (P ≥ 0.32) on pH, RAN, or rate of NH3 production. A subspecies × hour after feeding interaction was observed for RAN, with BT having greater RAN at h 0 and 4, whereas BI had greater RAN at h 2, 8, and 12. Overall, monensin decreased the acetate:propionate ratio and total VFA concentration but had no effect on forage utilization. Bos taurus indicus steers consumed less digestible OM and had a greater CH4 production rate compared with BT steers, suggesting BT were better able to utilize the available forage resource than BI. [ABSTRACT FROM AUTHOR]