Marijuana: latest US agribusiness. Home-grown pot grabbing larger US market share. CASH CROP

About 12,000 people flocked to a harvest festival in Madison, Wis., last month to celebrate another year's bumper crop. But the bounty in question wasn't corn or soybeans. It was marijuana.

Estimated value of the harvest: between $1 billion and $33.1 billion.

No one can pin down the exact value of the illegal produce. Any way you cut it, though, the domestic marijuana trade is flourishing agribusiness.

Home-grown leaf now supplies a quarter of the United States marijuana market. What's more, sophisticated horticulture gives native plants the dubious distinction of being the most potent smoke in the world.

Domestically produced marijuana is believed to have constituted 18 percent of the total available in 1986. But the estimated 3,000 to 3,500 metric tons of marijuana harvested in the United States in 1987 constituted 25 percent of the ``pot'' consumed here. These estimates come from the National Narcotics Intelligence Consumers Committee, a cooperative body of federal drug agencies.

NNICC says Department of Justice eradication figures also help quantify the increasing role of the US in the domestic marijuana industry. The number of plants eradicated jumped from 4.7 million in 1986 to 7.4 million in 1987.

``People go into it for the money; there are 18 to 20 million users,'' says J.W. Seward, coordinator of the eradication desk at the Drug Enforcement Agency (DEA). With big consumer demand and only a ``couple thousand'' eradication agents, law enforcement is at a gross disadvantage, he says.

Since the early 1980s, the proportion of US crops reported as sinsemilla, the unpollinated female cannabis plant with its higher content of tetrahydrocannabinol (THC) - the psychoactive chemical in marijuana - has continued to increase.

In 1987, 42 percent of the plants eradicated were sinsemilla; in 1983 the figure was 26 percent. The THC content in these plants ranged from 7.6 percent to 20 percent. Average THC content for imported marijuana is 3 to 6 percent, according to the DEA.

Marijuana greenhouses were seized in 45 states in 1987, compared with 39 states in 1986. Total pot-growing greenhouses seized rose from 1,077 to 1,192.

Under a recent statute, law enforcers can confiscate property of pot farmers. Thus, many growers have uprooted and moved to public lands.

``The national forests have become one of the prime growing areas, and the problem is growing annually,'' says Jay Humphreys, a spokesman at the US Forest Service.

In 1987 the service found 3,034 marijuana-growing areas and eradicated 254,947 plants. ``We think we only got 40 percent of what's being grown,'' Mr. Humphreys says.

The Forest Service has closed 886,000 acres of forests to the public because of the infiltration of pot growers, who run the gambit from ``flower children'' caught in a '60s time warp to dangerous organized criminals.

``We're not a law-enforcement agency, never wanted to be. But circumstances forced us to train as law enforcers and carry weapons. It's not something we're happy about,'' Humphreys says.

Rangers were assaulted by growers 75 times in 1987. They confiscated 101 firearms, and 31 growing sites located contained potentially lethal booby traps.

In a lot of depressed areas, local people consider work on marijuana plantations ``honest work for an honest day's pay,'' Humphreys says.

Mr. Seward of the DEA says lax legal sanctions have nurtured this attitude. He says that in the past, the ``vast majority'' of pot-cultivation arrests never resulted in jail time. He predicts that the mandatory jail sentences included in the omnibus drug legislation, recently passed by Congress, will wake people up to the seriousness of growing and using marijuana.

But some experts are skeptical. ``This highly profitable underground market is not going to be eliminated by brute force,'' says Marvin D. Miller, a lawyer in Alexandria, Va., and a member of the board of the National Organization for the Reform of Marijuana Laws.

NORML believes marijuana is wrongly categorized with dangerous narcotics like heroin and cocaine. It says that in spending some $3.8 million a year on marijuana eradication, the DEA is wasting taxpayer money attempting to enforce an unenforceable prohibition.

The organization asserts that legalization and taxation of marijuana would raise revenue that could be better spent to educate the public about the dangers of more serious substances.

Brian Murphy, a former pot farmer who left the field after being arrested and convicted, says, ``Marijuana is the most benign of all substances. A lot of people use it like a beer.''

Mr. Murphy, now a spokesman for NORML, says that 50 million Americans consume marijuana regularly, even as prices have skyrocketed.

(Federal figures put the cost of domestic marijuana at $500 to $1,500 a pound in 1987, compared with $350 to $650 a pound in 1983.)

There are even people outside the recreational-use community who say that current antipot laws are too severe.

A DEA administrative law judge, Francis L. Young, issued an opinion last month recommending that marijuana be reclassified under the Controlled Substances Act to make it available for medical treatment of cancer patients undergoing chemotherapy.

``The record on marijuana encompasses 5,000 years of human experience,'' Judge Young wrote. ``Despite the long history and extraordinarily high numbers of social smokers, there are simply no credible medical reports that suggest that consuming marijuana has caused a single death. By contrast, aspirin, a commonly used medicine, causes hundreds of deaths each year.''

Lester Grinspoon, a Harvard University associate professor of psychiatry, agrees. He has been part of the effort to get pot reclassified.

``There is no such thing as a harmless drug, but marijuana is much less harmful than alcohol or tobacco,'' he says. ``It is much less toxic than thousands of medicines prescribed by doctors every day.''

Yet Dr. Grinspoon, who has studied the effects of marijuana since 1967, concedes that pot poses a risk of lung damage because of its high tar content - four to five times that of tobacco cigarettes. ``The lungs were only meant to breathe fresh air,'' he says.