Methane emission from ruminant livestock is currently estimated
to be around 100 million tonnes of methane each year and, after
rice agriculture, represents the biggest man-made methane source.
The loss of methane from ruminant livestock is a problem not
only in the respect of greenhouse gas emissions, but also to farmers
in that food converted into and released as methane is food not
being converted into meat and/or milk.
Methane is produced in the guts of ruminant livestock as a result
of methanogenic microorganisms (belonging to the Archaea). The
composition of the animal feed is a crucial factor in controlling
the amounts of methane produced, but a sheep can produce about
30 litres of methane each day and a dairy cow up to about 200.
As with rice agriculture, methane emission arising from ruminant
livestock is, by definition, entirely due to man. With a continuing
expansion of meat and dairy product consumption around the world,
the demand for ruminant livestock and so the size of this methane
sink has grown rapidly.
Intensive rearing methods, developed to provide large amounts
of meat and dairy products at low prices and to a wide consumer
base, has led to very high densities of ruminant livestock and
strong local methane sources. An additional, but important, source
of methane due to ruminant livestock is that of animal waste and
this aspect is discussed in the 'Methane
Sources - Waste' section.
Potential for control
The best studied and applied methane reduction strategy has been
that of altering the feed composition, either to reduce the percentage
which is converted into methane or to improve the meat and milk
yield. Improvements in the overall quality of animal feed may
allow meat and dairy production to be maintained at the same level
with fewer animals and so less total methane emission.
Relatively recent ruminant methane reduction strategies have
included the introduction of methane inhibitors, both biological
and chemical, with the animal feed, to kill off or at least reduce
the activity of the methanogenic microorganisms in the gut.