Effect of Monensin and Protein on Digestion and Ruminal Fermentation Parameters in Cattle Consuming Low-Quality Forage

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 4x4 Latin Square design (20 d periods). Treatments were arranged as a 2x2 factorial: first factor was monensin (0 or 200 mg∙hd^-1∙d^-1) and second factor was protein (0 or 0.64 kg∙hd^-1∙d^-1 CP provided as cottonseed meal). A carrier supplement (0.23 kg∙hd^-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 fed individually at 0600 h daily. To prevent carry over effects from previous monensin feeding, 14 days were required before sampling. However, to obtain optimal response from monensin, only 10 days were needed for treatment adaptation. Thus, during d 1-4 of each period animals were fed LQF with no treatment. Day 5-14 allowed for treatment adaptation and day 15-20 for sample collection. Digestion measurements were taken d 15-19. Rumen fluid was collected h 0, 2, 4, 8, 12, 16, and 20 on d 20 for volatile fatty acid (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 no added benefit is observed with monensin inclusion.