Marijuana is the United States's biggest cash crop with estimates that the annual pot crop is worth $35 billion, according to a study.

The study conducted by public policy analyst and former National Organization for the Reform of Marijuana Laws head Jon Gettman, said marijuana's crop value was ahead of the country's three legal cash crops -- corn ($23 billion), soybeans ($17.3 billion) and hay ($12.2 billion).

Despite intensive eradication efforts domestic marijuana production has increased ten fold over the last 25 years from 1,000 metric tons in 1981 to 10,000 metric tons in 2006, according to US federal government estimates.

Gettman said the study used figures in a 2005 US State Department report estimating US cannabis cultivation at more than 22 million pounds.

He said he gave it a conservative value of about $1,600 a pound compared to the $2,000- to $4,000-a-pound street value often cited by law enforcement agencies after arrests.

The study also found that Marijuana is California's number one cash crop, worth nearly $14 billion a year.

In California, the state's Campaign Against Marijuana Planting (CAMP) seized nearly 1.7 million plants this year, but based on seizure rates over the last three years, Gettman puts California's pot production at 21 million plants, worth about $13.8 billion and responsible for a whopping 38 per cent of total US production.

California contains 13.25 percent of annual marijuana users in the US, according to the National Survey on Drug Use and Health.

Medicinal marijuana advocates say marijuana is California's number one cash crop simply because it's illegal.

"Agriculturally, it's worth the same as alfalfa, which is six cents a pound right now, which would make the total crop, if it wasn't illegal, about $1.3 million as opposed to $35 billion."

The massive expansion of pot production in the face of increased eradication efforts suggests that "marijuana has become a pervasive and ineradicable part of our national economy," that should be put under a system of legal regulation, Gettman said.

While California, Tennessee, Kentucky, Hawaii, and Washington are the top producing states, pot is the top cash crop in 12 states and among the top three in 30 states.

But the increase is also a function of government enforcement efforts, Gettman argued. "In response to the government spraying Mexican marijuana with paraquat in the 1970s, people began to grow in California and Hawaii. Then the government starting flying helicopters and airplanes around looking for marijuana from the sky, so cultivation spread out," he explained.

"America's marijuana crop is worth more than our nation's annual production of corn and wheat combined. And our nation's laws guarantee that 100 per cent of the proceeds from marijuana sales go to unregulated criminals rather than to legitimate businesses that pay taxes to support schools, police and roads," said Rob Kampia, executive director of the Marijuana Policy Project.

Tom Riley of the Office of National Drug Control Policy told the Los Angeles Times that while he would not argue Gettman's numbers, he disagreed with his conclusions.