United Nations officials in Afghanistan are attempting to create a "flood of drugs" in the country intended to destroy the value of opium and force poppy farmers to switch to legal crops such as wheat.

After the failure to destroy fields of the scarlet flowers in Afghanistan's volatile south, the United Nations Office on Drugs and Crime says the answer is to stop the drugs from leaving the country in the first place.

"Manual eradication is incompetent and inefficient," UNODC chief Antonio Maria Costa said during a visit to the western Afghan province of Herat. "So we want to see more efforts to stop the flow of drugs across Afghanistan's borders and the hitting of high-value targets to create a market disruption.

"We want to create a flood of drugs within Afghanistan. There will be so much opium inside Afghanistan unable to go out that the price will go down."

Officials admit that the plan is a second-best solution to intensive eradication campaigns. Last year the Afghan government succeeded in destroying only 3.5% of Afghanistan's 157,000 hectares of poppy because eradication teams were either attacked or bought off by local drug lords. But the attempt to use brute economics to tackle the country's $4bn (£2.5bn) narcotics industry instead is fraught with problems – not least Afghanistan's thousands of miles of porous borders.

Costa got a first-hand view of that issue this month from the porthole of a UN helicopter chartered to fly along a portion of the 580-mile border that separates Afghanistan from Iran.

The vast swaths of desert are thinly populated with a scattering of mud brick villages, and there is little to stop smugglers crossing the border.

While the Iranians, fed up with the problems created by the country's 1 million heroin addicts, have taken steps to build ditches and walls along the frontier, the Afghans lack even a fraction of those resources.

On the Afghan side of the border, Costa visited one of 24 squalid border checkpoints supported by a sprinkling of EU money, where the commanding officer told the UNODC chief that his men needed heavy weapons to defend themselves against the much better armed smugglers who race through the huge gaps in the border.

The task of beefing up Afghanistan's defences on this vast stretch of border is supported by just two UNODC officials, and they say that while their Afghan colleagues have been ready for months to start joint border patrols with their Iranian counterparts, progress has stalled because of bureaucratic infighting between ministries in Kabul.

Their efforts have been further undermined by a recent decree by President Hamid Karzai to close down small cross-border markets which had been a source of economic activity in an otherwise barren wilderness.

The local UNODC officials say the decision by Karzai, apparently taken to protect customs revenues, is "killing the villages".

The governor of Herat province, Ahmad Yusef Nuristani, said young people in the border areas had no choice but to join the drug smugglers to survive. "They were trading areas that kept people busy with legitimate businesses so they would not be tempted into employment by the drug traffickers," Nuristani said.

Even without attempts to disrupt the flow of drugs out of the country, Afghanistan is doing a good job of destroying the value of its main export. Huge overproduction, which by some estimates twice outstrips world demand, has led to a steady fall in the value of opium. A kilogram is now worth less than one fifth of what it was in 2001. The slump in opium values, combined with last year's soaring worldwide price of wheat, fuelled hopes that farmers would switch crops. However, wheat has fallen by 30% since October and humanitarian handouts of imported wheat last winter also helped to keep prices in Afghanistan low.

Costa said his request that the World Food Programme buy only Afghan wheat had been rejected by "free market ayatollahs who think political stability is less important than free market principles".

The UNODC country chief, Jean-Luc Lemahieu, also warned that the strategy of capitalising on falling opium prices could be torpedoed by Chinese drug dealers looking to Afghanistan to supply China's growing army of heroin addicts. "I think we have a two-year window before the Chinese pick up on the Afghan market. Currently the Chinese dealers source their heroin from the Golden Triangle. The networks have not yet been established."