17.20 Natural digestive fermentations in herbivores

We have already discussed the natural digestive fermentations in herbivores:

  • the crucial contribution made by fungi to the herbivore’s ability to digest its food (see The Chytrids in Chapter 3; CLICK HERE) and the metabolism of anaerobic chytrids (CLICK HERE to view the Resources Box on chytrid metabolism).
  • Ruminant biology and the microbial community of the ruminant gastrointestinal tract, which was considered in the section Anaerobic fungi and the rise of the ruminants in Chapter 15; CLICK HERE to view the page.

What we want to do here, briefly, is add a quantitative view of rumen fermentation; essentially the ‘engineering’ aspects of ruminant animals which will give you some numbers to excite and interest your friends.

The foregut of ruminants consists of four chambers: rumen, reticulum, omasum and abomasum. The rumen is the largest and, combined with the reticulum, forms a fermentation vessel that has a volume of 100 to 150-litre in cattle and about 10-litre in sheep. This fermenter vessel is loaded with:

  • solids in the form of the plant biomass on which the animal feeds and
  • fermenter fluid in the form of copious quantities of saliva produced by ruminants: about 150 l day-1 in cattle and 11 l day-1 in sheep (that is, about one fermenter volume of saliva day-1).

Retention times for liquids and small particles, including microorganisms, in the rumen are in the range 10-24 h, but for large particles, like plant fragments it ranges from 2 to 3 days. The rumen is a continuous culture system but it is not well mixed and because of the heterogeneous nature of the substrate plant material it may become stratified.

The ruminant animal relies on microorganisms to convert the plant biomass into nutrients accessible to the animal. The concentrations of these populations in rumen fluid are in the range 109 to 1010 ml-1 bacteria; 105 to 106 ml-1 protozoa and about 109 ml-1 thallus-forming units of anaerobic fungi (Trinci et al., 1994).

So an average cow contains about 1.5 × 1014 anaerobic fungi and an average sheep contains about 1011 anaerobic fungi.

The total number of dairy cows in the world is 2.4 × 108, but in addition there are about 1.3 × 109 beef cattle worldwide (source: FAO at http://www.fao.org/ag/aga/glipha/index.jsp). In total, therefore, there are approximately 1.6 × 109 cattle worldwide, which amounts to a grand total of 2.4 × 1011 litres of rumen fluid walking around containing 2.4 × 1023 anaerobic fungi in the world’s cattle herd. Just the cattle herd.

There are about 109 sheep in the world, each one a walking 10-litre fermentation culture. And then there are all those antelopes, bison, buffalo, camels, deer, goats, oxen, wildebeest ... and we’ve not even mentioned the hind-gut fermenting horses, zebra, rhinoceros, elephants, tapirs, sloths, pigs, peccaries, guinea pigs, chinchillas…and rabbits!

There are a lot of chytrids doing a lot of fermentation in the world; that’s why we call it the biggest and most widely distributed submerged liquid fermentation process. You do the maths; and you might like to consider the climatic impact of the greenhouse gas methane released as a by-product of fermentation in herbivores!

Updated December 17, 2016