Cadbury is to encourage its cows to burp less to reduce the carbon footprint of its milk chocolate.

The company is working with its 65 dairy farmers in Wiltshire to reduce the emissions of their animals. Production of the famous glass-and-a-half of milk in every bar is responsible for 60% of the chocolate's carbon emissions, according to experts at the Carbon Trust who audited the carbon emissions from the company's products.

Cadbury has sent the farmers a guide for low carbon dairy farming, which includes advice on changing the diet of the cows to cut their eructations.

Farmed ruminant animals are thought to be responsible for up to a quarter of "man-made" methane emissions worldwide though, contrary to common belief, most gas emerges from their front, not rear ends.

Studies show that the production of one litre of milk produces the equivalent of 900g of CO2. More than half of this is down to methane. The average cow emits between 80kg and 120kg of methane a year, equivalent to the annual carbon emissions from an average family car.

Ian Walsh, Cadbury's global head of environment, said the company was looking to reduce the environmental impact of its supply chain. "We are committed to tackling climate change and we rely on scientific research to inform the actions we take." The company worked with the Carbon Trust to work out the life cycle emissions of the chocolate.

Cadbury is not the first to address emissions from cattle and dairy farming. Experts at the Institute of Grassland and Environmental Research in Aberystwyth have also looked at how the diet of farmed animals can be changed to make them produce less methane, a much more potent global warming gas than CO2.

The altered diet changes the way that bacteria in the stomachs of the animals break down plant material into waste gas. The institute is working on a government programme with the universities of Wales and Reading, to investigate how this process could be improved. A similar project in New Zealand suggested that dietary changes could reduce methane emissions from sheep by up to 50%.