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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 [BI] and 5 [BT]). Subspecies were concurrently subjected to a 2-period, 2-treatment crossover design. Treatments consisted of either 0 mg·steer·d-1 monensin with no previous monensin feeding (CON) or withdrawal from 200 mg·steer·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 NH-N (RAN), rate of NH production, and CH 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 ( ≥ 0.11). An effect of day after monensin withdrawal was observed ( < 0.01) for total VFA concentration, with an increase following withdrawal followed by a decrease and then stabilization. Monensin × day after monensin withdrawal interactions ( ≤ 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 ( ≤ 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 ( ≤ 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 ( = 0.14) from CON. By d 14, no differences ( ≥ 0.88) remained for acetate, propionate, or the A:P ratio. After monensin withdrawal, monensin reduced ( < 0.01) RAN by 12.3% (2.09 vs. 1.83 m for CON and MON, respectively). Monensin withdrawal and cattle subspecies had no effect ( ≥ 0.23) on rate of NH production or CH production rate. Monensin withdrawal had no effect ( ≥ 0.45) on intake or digestibility parameters. Greater forage OM intake ( = 0.09; 21.2 vs. 19.2 g/kg BW) and OM digestibility ( < 0.01; 72.4 vs. 63.0%) resulted in greater ( < 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.

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 [BI] and 5 [BT]; 398 kg BW). Subspecies were concurrently subjected to a 2-period, 2-treatment crossover design. Treatments were 0 (CON) or 200 mg·steer·d 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 NH-N (RAN) analysis. Additionally, at h 2, ruminal fluid and contents were collected for determination of rate of NH production and CH production rate. No subspecies × monensin interactions were observed ( ≥ 0.12). Monensin had no effect ( ≥ 0.16) on intake or digestibility parameters. No subspecies effect ( ≥ 0.11) was observed for forage OM intake, total OM intake, or OM digestion. Total digestible OM intake tended to be greater ( = 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 ( ≤ 0.01) on pH, total VFA, acetate:propionate ratio, and molar percent acetate and propionate. Total VFA concentration was greater ( = 0.01) in CON steers than in MON steers (66.5 vs. 62.0 m). Monensin decreased molar percent acetate ( = 0.02) from 72.5 to 71.2% and increased molar percent propionate ( < 0.01) from 16.9 to 18.7%, resulting in a reduced ( < 0.01) acetate:propionate ratio (from 4.34 to 3.85). Although not significantly ( = 0.19), monensin numerically reduced the CH production rate by 15.8%. Greater ( = 0.07) CH production rate tended to be observed in BI steers than in BT steers (21.4 vs. 16.6 μmol CH·mL·h, respectively). Monensin had no effect ( ≥ 0.32) on pH, RAN, or rate of NH 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. steers consumed less digestible OM and had a greater CH production rate compared with BT steers, suggesting BT were better able to utilize the available forage resource than BI.

The effect of monensin and protein on digestion and ruminal fermentation parameters in cows consuming low-quality forage (LQF; 4.1% CP chopped bluestem hay) was evaluated. Four ruminally cannulated cows (637 ± 24 kg BW) were used in a 4 × 4 Latin square design (20 d periods). Treatments were arranged as a 2 × 2 factorial: the first factor was monensin (0 or 200 mg·cow-1·d-1) and the second factor was protein (0 or 0.64 kg·cow-1·d-1 CP provided as cottonseed meal). A carrier supplement (0.23 kg·cow-1·d-1) consisting of ground hay, cracked corn, molasses, salt, dicalcium phosphate, and a commercial mineral premix was provided to all animals and allowed for inclusion of monensin. Animals were housed and individually fed at 0600 h daily. To prevent carryover effects from previous monensin feeding, 14 d were required before sampling. However, to obtain the optimal response from monensin, only 10 d were needed for treatment adaptation. Therefore, during d 1 through 4 of each period, animals were fed LQF with no treatment. Day 5 through 14 allowed for treatment adaptation and d 15 to 20 was used for sample collection. Digestion measurements were taken d 15 through 19. Rumen fluid was collected h 0, 2, 4, 8, 12, 16, and 20 on d 20 for VFA analyses. Data were analyzed using the MIXED procedure in SAS 9.3 (SAS Inst. Inc., Cary, NC) with terms in the model including protein, monensin, and their interaction, with animal and period included as random effects. There was no protein × monensin interaction (P = 0.30) or effect of monensin (P = 0.37) on any digestion parameter measured. Protein increased OM digestibility (P = 0.01), total digestible OM intake (P < 0.01), and total digestible NDF intake (P < 0.01) by 10.7, 112, and 71.2%, respectively. Protein had no effect on NDF digestibility (P = 0.13). A protein × monensin interaction (P = 0.17) or effect of monensin (P = 0.15) was not observed for total VFA concentration. Protein increased (P < 0.01) total VFA concentration by 20.0%. A protein × monensin interaction was observed for the acetate:propionate ratio (P = 0.05), with protein reducing the ratio when no monensin was present and increasing the ratio when monensin was present. No protein × monensin interaction (P = 0.51) was observed for acetate or propionate concentration. Protein had no effect on acetate (P = 0.11) or propionate (P = 0.67). Monensin increased (P = 0.03) propionate concentration by 8.6% but had no effect (P = 0.41) on acetate. Results suggest that protein increases digestibility when feeding a LQF but that no added benefit is observed with monensin inclusion. [ABSTRACT FROM AUTHOR]

The effect of including monensin in a protein supplement fed to cattle consuming low-quality forage is not well defined. The objective of this study was to evaluate the effects of monensin and protein supplementation on in situ disappearance kinetics of low-quality forage (LQF; 4.1% CP). Four cannulated cows were used in a 4 × 4 Latin Square design with treatments arranged as a 2 × 2 factorial: the first factor was monensin (0 or 200 mg·hd-1·d-1), and the second factor was protein supplement (0 or 0.64 kg·hd-1·d-1 CP provided as cottonseed meal; CSM). A premix (0.23 kg·hd-1·d-1) consisting of ground hay, cracked corn, molasses, salt, dicalcium phosphate, and a commercial mineral mix, was provided to all animals and allowed for monensin inclusion. Animals were individually housed and fed at 0600 h daily. Prior to sampling, 14 d were required to prevent carryover effects from previous monensin feeding. However, only 10 days of treatment adaptation was required for optimal monensin response. Thus, d 1 to 4 of each period, all animals were provided LQF with no treatment. Day 5 to 14 served as treatment adaptation, and sampling occurred d 15 to 20. Samples of LQF were weighed into 10- by 20-cm polyester bags in replicates of six for each hour. On d 15, bags were placed into the ventral area of rumen and replicates removed after 0, 4, 8, 16, 24, 48, 72, and 96 h of incubation. Upon removal from the rumen, bags were placed in ice water, rinsed, and then frozen until final bag removal. Bags were washed in a commercial washing machine and then dried at 55°C to a constant weight. Dried residues were analyzed for DM. In situ rumen DM degradation data were fitted to the first-order exponential model with discrete lag using the iterative Marquardt method and NLIN procedure of SAS 9.3. There was no interaction (P ≥ 0.16) between protein and monensin for any DM disappearance parameters measured. Protein had a tendency (P = 0.07) to reduce potentially degradable DM (from 58.60 to 50.42%) likely due to the increased (P ≤ 0.01) rate of DM degradation (from 1.87 to 4.76%/h) that it caused. Monensin had no effect (P = 0.12) on potentially degradable DM but increased (P = 0.03) rate of degradation from 3.06 to 3.57%/h. Protein or monensin supplementation had no effect on lag time or residue (P ≥ 0.11). It is likely that the increased rate of degradation caused by protein and monensin will allow increased intake of LQF, thus improving its use in production operations. [ABSTRACT FROM AUTHOR]