Farm Animal Metabolism and Nutrition

metabolite transfer) must exit the cell into
the fluid environment of the rumen. Exit
is by diffusion and by permeases (Fig.
6.2).
Most of the VFAs, the waste products
of the fermentation, are removed from the
fermentation by absorption through
the rumen epithelium. Not all details of the
absorption process are understood but the
process has been reviewed recently (Gabel,
1995; Rechkemmer et al., 1995; von

Engelhardt, 1995). It had been thought

previously that the major route of absorp-

tion was by passive diffusion of the

undissociated acids through the epithelium.

The absorption rate is influenced by

luminal pH and chain length of the acids in

the direction consistent with passive

diffusion, but the magnitude of influence is

very small relative to differences in absorp-

tion rates observed at different concentra-

tions of VFAs. A schematic diagram

Glucose Availability and Associated Metabolism 139

Fig. 6.5.Summary of putative methanogenic pathway and initial reactions of anabolic pathways in ruminal
methanogens. Composited from Muller et al. (1993), Simpson and Whitman (1993) and Thauer et al. (1993).