Close to 900,000 acres of national forest and grasslands had to be closed last year because "modern-day moonshiners" growing marijuana made them unsafe for visitors and federal employees, officials at a Senate hearing said.

William L. Rice, deputy chief of the Agriculture Department's Forest Service, said Thursday that marijuana production in national forests has become a "billion-dollar" industry since the early 1970s and is increasingly violent.He told a Senate Agriculture subcommittee that 886,000 of the 191 million acres of the government land were closed because of the threat posed by growers. Rice said his law enforcement agents are hampered by the legal prohibition against pursuing marijuana growers for investigative purposes.

A display of guns, grenades and makeshift weapons at the hearing prompted Sen. Tom Harkin to observe that the marijuana farmers are "modern-day moonshiners" but more sophisticated and dangerous.

Harkin, D-Iowa, and other subcommittee members endorsed the Forest Service's request for authority to engage in "hot pursuit" of farmers who flee beyond the boundaries of national forests where they cultivate their illicit crops.

Frank G. Packwood, a Forest Service special agent for the Shasta-Trinity National Forest in northern California, said 70 percent of the state's 872 arrests for marijuana cultivation in 1986 were never prosecuted. He said the lack of a hot pursuit policy was a major reason.

Rep. Ron Wyden, D-Ore., complaining that the $14 billion domestic marijuana industry is "threatening the quality of life all over the West," urged approval of his bill to allow Forest Service agents to conduct investigations, searches, seizures and arrests outside national forests where a drug crime occurs.

His bill also would stiffen penalties for unauthorized use of fertilizers and chemical herbicides by marijuana growers, which he said are ravaging foliage and wildlife in national forests.

Wyden said Oregon has become the third leading state in marijuana cultivation. Packwood said California is the second largest producer of the highly potent "sensemilla" strain of marijuana.

"In this room there is sufficient space to produce $400,000 worth of marijuana," said Harkin, surveying his small hearing room.

He said marijuana is the biggest cash crop on the West Coast, and that much of it is grown on national forests where farms are easily hidden and the law enforcement threat is minimal.

Sen. Wyche Fowler, D-Ga., said hunters, campers and tourists are risking injury or death at the hands of heavily armed marijuana farmers. He said many of the marijuana gardens are protected by booby traps that can "cripple or kill intruders."

Wyden said a sheriff's raid on a drug laboratory in southern Oregon netted "a cache of weapons that could outfit a small army," including an Uzi submachine gun, two .50-caliber Browning machine guns, a semiautomatic rifle, 100,000 rounds of ammunition and 150 pounds of gunpowder.

Forest Service officials said the growers use of a variety of booby traps, makeshift shotguns and snarling guard dogs, some of them trained to receive attack commands by remote radio signals.

"These people mean business," Wyden said.

Harkin said marijuana growers are "modern-day moonshiners, but these are a lot more sophisticated than the old moonshiners."