lowcarboneconomy.com makes note of an article by Financial Times that highlights the successful experiments to use Mootral, a garlic-based additive, to reduce methane in farm-raised ruminants (those with two stomaches - cows, sheep, camel, etc.). NOTE: Both articles are from March of 2009. Note also the religious importance of ruminants in Wikipedia.
Also worth noting, Methane is 22 times more potent than than CO2. They estimate accounts for 20% of the greenhouse gases. I think this could be a game changer.
Here is another story on changing the diets of cows, 15 farms various diets.
Here is another magic pill, bolus. -Jan, 2009
UK tries fumaric acid in cow's diet. Dec, 2005
Notes from an FAO study published in June of 2009
Lastly, it's good to remember there are generally two (2) type of cows in domestic use, milk cows & meat cows.