Not long before his death last year Norman Borlaug, the US agronomist responsible for what became known as the green revolution, was asked what had been the greatest disappointment in a seventy-year career that spanned the globe. He didn't hesitate. ‘Africa,’ he replied.

His answer was not surprising.

Having helped transform India's agriculture through improved seed, better techniques and judicious use of fertiliser, he tried to replicate his success in Africa. His efforts were in vain. The problems have proved intractable.

Some five decades after independence, one in three of the 700 million people living