The amount of land used for opium cultivation in world's largest producer country Afghanistan dropped by an estimated 21 percent this year because of a major clampdown on poppy farmers, the United Nations' anti-drug chief said yesterday. But good rains after years of drought led to bumper harvests from the poppy crops that were grown, so the overall yield of opium dropped by just 2 percent to 4,519 tons, said Antonio Maria Costa, the director for the UN Office on Drugs and Crime. Last year's yield was 4,630 tons. Costa said Afghanistan is still estimated to produce 87 percent of the world's supply of both opium and its derivative, heroin. He predicted it would take 20 years to eradicate the cultivation of drugs — a mainstay of many of Afghanistan's impoverished farmers, despite government warnings against growing poppies and authorities' destruction of some crops.