Effect of Monensin and Protein Supplementation on in Situ Degradability of Low-Quality Forage Fed to Cattle

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 4x4 Latin Square design with treatments arranged as a 2x2 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 carry over effects from previous monensin feeding. However, only 10 days of treatment adaptation were required for optimal monensin response. Thus, d 1-4 of each period, all animals were provided LQF with no treatment. Day 5-14 served as treatment adaptation and sampling occurred d 15-20. Samples of LQF were weighed into 10×20 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, then frozen until final bag removal. Bags were washed in a commercial washing machine 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.