A ruminant is a machine for turning inexpensive, renewable, dispersed biomass into high value, high-density products.

But of course there's a price to pay.  It's not the poor animal's fault, but milk, meat and wool aren't the only products of its wonderful fodder-processing powers.  It also returns some of what it takes in to the atmosphere, soil and water in less welcome forms.

Not only can these emissions be bad for the environment, they represent material that could have been captured in food and fibre.  In this sense the ruminant, though the faithful and uncomplaining servant of mankind from prehistory to the present day, is rather inefficient and can be difficult to manage in a sustainable way.

If we look at the nitrogen balance of a grazing dairy cow, for example, we find that as much as 80% of what goes in at the front end comes out of the back and only about 20% is recovered in milk.  Why is this?  For the answer we need to travel to the mysterious world of the rumen.

Here are some facts and figures about the rumen and rumination:

The rumen is a large fermenter containing a complicated mix of microbes that turn forage into the raw materials for the animal's growth and milk production.  Like brewing, this activity takes place without oxygen.

As in any ecosystem (and that's one way to think of the rumen), energy supply is a priority for the organisms that have to live, compete and reproduce there.  The best energy source for rumen microbes is soluble sugar in the feed.

The largest potential source of energy in forage is the sugar bound up in plant cell walls, but this isn't soluble and not very accessible.

The trouble is that forage protein is a more available energy source that cell wall sugar.  Protein breaks down into amino acids.  Ideally these should be absorbed by the animal and built up into its own meat or milk proteins.

But if energy is in short supply in the rumen, the microbes will cheerfully use the amino acids to bridge the gap.  They separate the nitrogen of amino acids from the carbon skeletons.  The carbon-rich parts are fermented.  The nitrogen, in the form of ammonia, is converted by the animal to urea and excreted.

The problem of a mismatch between energy supply and demand in the rumen is basically one of timing.  This diagram summarises the synchronisation issue.

What can we do about it?  It would be helpful if we could reduce the breakdown of protein to amino acids in the rumen.  Another way would be to increase the availability of sugars so that the microbes weren't so tempted to go for proteins as an energy source.

Follow the links to find out more about protein and energy in the rumen.