Cows, sheep and other ruminants are thought to be responsible for around one-fifth of global methane production but the precise amount has proved difficult to quantify. Methane production from animals is often measured using respiration chambers, which can be laborious and are unsuitable for grazing animals. Archaeol is thought to come from organisms called archaea, which are symbiotic or 'friendly' microbes that live in the foregut of ruminant animals. These microbes produce methane as a by-product of their metabolism and this is then released by the animal as burping and flatulence. Two groups of cows were fed on different diets and then their methane production and fecal archaeol concentration were measured. The animals that were allowed to graze on as much silage as they wanted emitted significantly more methane and produced feces with higher concentrations of archaeol than those given a fixed amount of silage, supplemented by concentrate feed.