KABUL (Reuters) - Long the world’s largest producer of opium, the raw ingredient of heroin, Afghanistan has now become the top supplier of cannabis, with large-scale cultivation in half of its provinces, the United Nations said on Wednesday.

Between 10,000 and 24,000 hectares of cannabis are grown every year in Afghanistan, with major cultivation in 17 out 34 provinces, the U.N. drug agency (UNODC) said in its first report on cannabis production in Afghanistan.

While some countries grow cannabis on more land, Afghanistan’s robust crop yields -- 145 kg of resin per hectare compared to around 40 kg per hectare in Morocco -- make it the world’s largest producer, estimated at 1,500-3,500 tons a year.

“This report shows that Afghanistan’s drug problem is even more complex than just the opium trade,” said Antonio Maria Costa, head of UNODC in the report.

“Reducing Afghanistan’s cannabis supply should be dealt with more seriously, as part of the national drug control strategy.”

For years Afghanistan has been the world’s largest producer of opium, a paste extracted from poppies and processed into heroin. While land cultivated with poppies fell by 22 percent last year, record yields meant production fell only 10 percent.

FUNDING INSURGENTS

The illegal opium trade is said to fuel the insurgency in Afghanistan with the Taliban siphoning off millions of dollars from the trade by imposing taxes on farmers and smugglers in return for ensuring safe passage of the drug.

“Like opium, cannabis cultivation, production and trafficking are taxed by those who control the territory, providing an additional source of revenue for insurgents,” the report said.

As with opium, most cannabis cultivation takes place in the south of the country where the insurgency is strongest, UNODC said, with more than two-thirds (67 percent) of cannabis farmers also growing opium.

One of the main reasons cannabis is so widely grown, UNODC said, is because of its low labor costs and high returns. Three times cheaper to cultivate than opium, the net income from a hectare of cannabis is $3,341 compared to $2,005 for opium.

“The entire process is a non-expensive, fast industrial process, which is indeed somewhat worrying,” Jean-Luc Lemahieu, head of UNODC in Afghanistan, told reporters in Kabul.

“We have already enough problems with the opium so we don’t want to see the cannabis taking over.”

Afghanistan still grows far more opium than cannabis, however, and Lemahieu said it was unlikely to overtake the poppy crop as it required a lot of water to grow -- in short supply in Afghanistan -- and had a very short shelf life.

“You can walk around with opium for 10 to 15 years and, perhaps, like the wine it gets better with the time. For cannabis ... you need to process it really immediately,” said Lemahieu.

While cannabis production in 2009 was valued at an estimated $39-94 million, this is only about 10-20 percent of the total farm-gate value of Afghanistan’s opium production, because so much more opium is grown.

While some of the cannabis is consumed within Afghanistan, most of the drug is smuggled abroad following the same routes as opium, UNODC said. In 2008, 245,000 kg of cannabis was seized in southern Kandahar near to the border with Pakistan.

“As with opium, the bottom line is to improve security and development in drug-producing regions in order to wean farmers off illicit crops and into sustainable, licit livelihoods, and to deny insurgents another source of illicit income,” Costa said.