1. A long-term experiment was made with the Rumen Simulation Technique (Rusitec), in which the fermentation of a mixed ration of hay (1Og/d) and bruised barley (5g/d) was compared with the fermentation of the same diet in the presence of 2, 10 and 50 mg monensin/d.
2. Monensin depressed the production of acetic and butyric acids, markedly increased the production of propionic acid and virtually eliminated the production of isovaleric acid. The production of methane was decreased in the presence of monensin, but this decrease could be accounted for entirely by the changes in the production of volatile fatty acids and redistribution of metabolic hydrogen.
3. The digestibility of dry matter (DM) in the rations declined in the presence of monensin. Determinations of the rates of digestion showed that the digestion of the readily-fermented food in the initial stages was not affected by monensin, but that at 24 h digestion had been inhibited by monensin. The inhibition was due entirely to its effect on the digestion of the fibrous components. Digestion of non-fibrous material was not affected.
4. The efficiency of microbial growth, expressed as g dry weight/mol ATP formed (YATP) and in terms of dm digested, tended to be increased by monensin. This however occurred only at high, non-practical doses.
5. Urease (EC 188.8.131.52) was induced by the addition of urea to the fermentation, but monensin had no effect on urease activity. Although monensin increased the activity of protease in washed suspensions, more food protein apparently escaped degradation. This may have been due to decreased deaminative activity.
6. Monensin altered the mircroscopic appearance of the fermentation fluid, and changed the activity of some enzymes in sonicated extracts, including alkaline phosphatase (EC 184.108.40.206), acetate kinase (EC 220.127.116.11) and succinate dehydrogenase (EC 18.104.22.168). These results are discussed in terms of known sensitivities of rumen microbes to monensin and their contribution to the fermentation as a whole.