Stuart Reid, of Lower Hutt, asks :-

Is more methane released into the atmosphere when grass passes through the gut of livestock compared to when the grass just dies and decomposes naturally?

David Pacheco, an animal nutritionist and physiologist with AgResearch, responded.

The production of methane requires anaerobic conditions, that is, when no oxygen is present. So, if grass just dies, the first phase of decomposition will be in the presence of oxygen and very little methane will be released to the atmosphere. However, over time, as oxygen is consumed, anaerobic conditions are established and then methane is produced from intermediate compounds such as acetate (the acid in vinegar) and carbon dioxide.

I think the question needs to be addressed from the point of view of the conditions in which the grass is decomposed, as it will affect how readily oxygen is consumed. Grass clippings stacked on a pile will create a greater opportunity to create anaerobic environment leading to methane formation compared to grass clippings scattered in a lawn. Also, the longer a piece of grass stays under anaerobic conditions, the more the opportunity for methane-producing processes to occur.

In extreme cases, such as landfills, methane can be up to 50% of the gas produced, with carbon dioxide being the other half and some traces of gases such as sulphides. As comparison, in the stomach of a cow the amount of methane would be around 10% the volume of the carbon dioxide produced.