What has four stomachs and could change the world?

The average dairy cow in America produces 30 litres of milk a day; a cow in Africa, only 1.6. This 19-fold difference—call it the dairy divide—has enormous consequences. Closing even some of it would ease poverty, help children grow up better nourished, reduce emissions of greenhouse gases and perhaps even make civil wars less likely. The good news is that cows can become more productive, thanks to the spread of technologies old and new. But unhelpful traditions—and climate change itself—make it harder.