If the world is to grow enough food for the projected global population in 2050, agricultural productivity will have to rise by at least 60%, and may need to more than double, according to researchers who have studied global crop yields.

They say that productivity is not rising fast enough at present to meet the likely demands on agriculture.

The researchers studied yields of four key staple crops - maize, rice, wheat and soybeans - and found they were increasing by only about 0.9% to 1.6% a year.