Effects of the antibiotic monensin and an inhibitor of methanogenesis on in vitro continuous rumen fermentations.

1. The effects of a methane inhibitor, ICI 111075, and a propionate enhancer, monensin, were studied using in vitro continuous fermenters. 2. Both compounds increased the yield of substrate energy, carbon and hydrogen in volatile fatty acids (VFA). This was mainly due to an increase in the molar proportion of propionic acid. 3. Improved yields of VFA were accompanied by reductions in methane production and microbial yield. 4. Since published information showed that monensin reduced rumen dilution rate in vivo an analogous in vitro system was proposed in which a high dilution rate control fermenter was compared with a monensin treated fermenter set to run at a low dilution rate. 5. Results showed that the general intrinsic microbial activity of the chemical manipulators was not affected by changes in dilution rate. Changing dilution rate in addition to chemical treatment however resulted in substantial modifications in the net effect on the fermentation. 6. The practical implications of reducing rumen dilution rate as a side effect of chemically manipulating the rumen fermentation could involve changes in food intake, increased importance of secondary fermentations and a reduced effect of nutrients not degraded in the rumen.