At a time when security is being intensified at airports, power plants and stadiums, agriculture officials say farms and the food supply remain among the nation's most exposed targets. 

And agroterrorism — attacks on the food supply or turning agricultural materials into weapons — is nearly impossible to guard against, they warn.

Fertilizers can be used to produce powerful bombs, pesticides can become chemical weapons and just a tiny amount of deadly bacteria can taint the food supply for thousands of people.

``Agriculture is vulnerable,'' said University of Georgia agriculture dean Gale Buchanan, part of an association of academics that has formed a task force on the issue. ``There's no way you can put guards around fields or animals.''

For years, agriculture officials have warned of the dangers of agroterrorism, a threat that intensified as the nation's farms have consolidated into massive agribusinesses.

Today, contagious diseases have the potential to spread rapidly in places where hundreds, even thousands of animals are confined in close quarters, such as Midwest feedlots, North Carolina hog farms and Georgia poultry houses.

And because farming takes place in rural areas, the nation's corn, wheat and peanut crops often have nothing more than scarecrows watching over them.

That's been the case on Frank Lipinski's 360-acre farm near Buckley, Mich., for as long as he can remember. But lately, much of his time has been spent thinking about security, concocting elaborate doomsday scenarios: fertilizers and equipment turned into weapons, crops blighted, milk tanks sabotaged, livestock infected.

``If you think like a terrorist, I guess there's no end to the things you could do,'' Lipinski said. ``It's kind of mind boggling. But who thought someone was going to crash a plane into a high-rise building?''

The chemical industry has urged pesticide dealers to tighten security. Crop dusters, which were grounded by the FBI following the attacks, have new procedures for preventing unauthorized flights.

The Washington-based Fertilizer Institute has asked farmers to secure chemicals that could be used to make bombs. On top of the list: ammonium nitrate, which Timothy McVeigh used in 1995 to blow up the Oklahoma City federal building in what until last month was the deadliest act of terrorism on U.S. soil.

Terrell Hudson, a cotton, corn and peanut grower near Unadilla, Ga., about 150 miles south of Atlanta, said he's most worried about terrorists using microorganisms such as anthrax to infect livestock and crops.

``Anybody with a little technical knowledge or biological knowledge,'' Hudson said, ``has the capability of doing some pretty unimaginable things.''

The American Farm Bureau has asked President Bush to appoint a high-level agroterrorism specialist in the new Office of Homeland Security.

``We just want heightened awareness,'' said Farm Bureau spokesman Christopher Noun in Washington. ``This is not a new issue for the American Farm Bureau. We've been working on it for the last few years, particularly in the area of animal health. If a disease like foot-and-mouth disease were ever let loose here, it could devastate animal production.''

A foot-and-mouth outbreak that began last year in Britain has resulted in the destruction of about 4 million animals and bans on the U.S. import of most overseas meat and livestock.

The foot and mouth scare and occasional threats from animal rights groups were all the warnings Rob Robertson needed to increase security on his 2,000-acre farm in Roca, Neb., where he raises cattle, corn and soybeans.

``We all need to be vigilant and keep a lookout for anything out of the ordinary around livestock facilities, pastures or roadsides,'' Robertson said. ``We're all in this together.''

For dairy farmer Wayne Bancroft, who has 300 head of cattle near Traverse City, Mich., fears of agroterrorism have forever changed carefree life on the farm.

He recently installed locks on doors leading to the 1,500-gallon milk holding tank to prevent tampering. Tours by school groups have been nixed. And anyone he doesn't know has to show identification.

``We won't hardly let them out of their vehicle unless we know who they are. We won't let them chat their way into the barns or buildings,'' Bancroft said.

``It sort of takes away some of the freedoms we've always had when you have to be so cautious, but what else can you do?''