An article by Dr Michael Mosley, BBC News.

Every year we raise and eat 65 billion animals, that’s nine animals for every person on the globe, and it’s having a major impact on our planet. So what meat should we eat if we want to be eco-friendly carnivores? Is it better to buy beef or chicken, free range or factory farmed? As Dr Michael Mosley discovers for BBC Horizon, the answers are far from obvious.

I like eating meat but I know that my food preferences, and those of a few billion fellow carnivores, comes at a cost.

Nearly a third of the Earth’s ice-free land surface is already devoted to raising the animals we either eat or milk.

Roughly 30% of the crops we grow are fed to animals. The latest UN Food and Agriculture Organisation reports suggest livestock are responsible for 14.5% of man-made greenhouse gas emissions – the same amount produced by all the world’s cars, planes, boats and trains.

If that wasn’t scary enough, meat consumption is predicted to double in the next 40 years as people globally get wealthier. So how will the world cope?

In search of answers I went to the US, one of the world’s largest consumers of meat, and travelled to the wide-open prairies of the Flint Hills in Kansas.

Here cattle are still herded by cowboys and cowgirls, as they have been for 150 years. The cows spend their lives roaming the hills, eating grass until it is time for them to be slaughtered. It seems to be an idyllic form of farming.

Yet there’s a big problem. Armed with a laser methane detector, normally used to spot dangerous gas leaks, I dived into a herd of cows and was soon picking up readings that would have had me sounding the alarms if I had been on an oil rig. These cows are producing huge amounts of methane.

A single cow can belch up to 500 litres of methane every day. Multiply that by the 1.5 billion cattle we have on our planet and that’s a lot of gas. And it has a vast environmental impact because methane is 25 times more potent a greenhouse gas than carbon dioxide.

The problem lies in what the cows eat. Unlike most mammals, cattle can live on a diet of grass, thanks to the trillions of microbes that live in their many stomachs.

These microbes break down the cellulose in grass into smaller, nutritious molecules that the cows digest, but while doing so the microbes also produce huge amounts of explosive methane gas which the cows burp out.

Since grass is what fuels methane production, one way to reduce the belching is to change what the cows eat.

At a feedyard in Texas I saw a very different type of farming, thousands of cattle confined in grass-free, mud enclosures and fed a diet based on a carefully formulated mix of corn, fat, growth hormones and antibiotics.

It looked the opposite of eco-friendly farming. Yet the CEO, Mike Engler, argued that his way of farming is “greener” than raising cows on the prairies – that their greater efficiency leads to less environmental impact.

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