LASHKAR GAH, Afghanistan (Reuters) - Afghan farmer Abdul Ghani looks over his field carpeted in small, green plants and knows this crop will feed his family.

Ghani explains his simple logic that makes him part of an illicit industry that the government says is funding terrorism and threatens to destroy the country.

Ghani said he grew wheat and vegetables such as tomatoes, but got a pittance from those crops compared with the opium he sold to traffickers, who appeared on motorbikes and in trucks at harvest time.

For hundreds of years networks of canals brought water to fields and orchards, producing rich crops, but the irrigation system has collapsed over years of conflict.

Taliban insurgents encourage opium growing and roam the mountains that rise from the valley in the north, and across vast tracts of lawless desert that stretch south to the Pakistani border, officials say.

Ensuring security so anti-drug efforts can go ahead in the province that produces a quarter of Afghan opium will be a main task of 3,300 British troops who will soon be based here.

"The Taliban have promised the farmers to protect their poppy fields," provincial governor Mohammad Daoud told a small group of reporters this week. "They have assured the farmers they will not allow the government forces for eradication."

The drug gangs have taken advantage of insecurity, weak or non-existent policing, rampant corruption and a reluctance to go after powerful figures involved in drugs but supporting the U.S-led war against the Taliban.

With international pressure mounting to tackle drugs, efforts to eradicate the $2.8 billion a year industry have begun early this season to allow farmers time to replant a legal crop.

The United States and Britain, which fund and oversee drug efforts with the government, also stress getting tough with traffickers and providing farmers with alternatives.

Farmers planted more than a fifth less opium last year, largely because of his efforts to shame them and appeals that they stop, coupled with the threat and fear of eradication.

U.S. ambassador Ronald Neumann recalls his visit to Helmand in the 1960s - when his father was the envoy - and the United States helped rehabilitate the irrigation system that turned the valley green.

"There is more linkage here in Helmand between the drug trade and the Taliban and terrorism than there is anywhere else in Afghanistan," Neumann told reporters during a visit to the provincial capital, Lashkar Gah, this week.

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