The rush across the developing world to stop food leaving the region is a perfect example of the old adage: be careful what you wish for.

For years, governments of poor countries, and their champions in the rich world’s development campaigns such as Oxfam, have been complaining bitterly that farm-gate prices have been driven down by overproduction and dumping by US and European farmers. Now, food prices are rising. But the governments involved, rather than celebrating, are scrambling to stop their farmers benefiting too much at the expense of their urban consumers.