Opening Brazil's vast Cerrado region to agriculture has improved - not hurt - the environment, this year's World Food Prize laureates say.

The three men - American Colin McClung and Brazilians Edson Lobato and Alysson Paolinelli - were credited with helping turn the 300 million-acre inland region in Brazil into some of the most productive cropland in the world..

Norman Borlaug, the Iowa native who founded the World Food Prize in 1986, has called the opening of the Cerrado one of the great agricultural achievements of the 20th century.

Critics say farming in the Cerrado has harmed the habitat for frogs, birds and other species.

The three laureates said Friday at the final event of the two-day international World Food Prize symposium in Des Moines that the environment has been enhanced by the coming of agriculture to the Cerrado, which means "closed" or "inaccessible" in Portuguese.

"Don't confuse the Cerrado with the Amazon" rain forest, Paolinelli said. "It's completely different."

Environmentalists say the cutting of large areas of the rain forest in the Amazon has contributed to the buildup of greenhouse gases in the Earth's atmosphere by removing trees, which turn carbon dioxide into oxygen.

Paolinelli, who was Brazil's Minister of Agriculture from 1975 to 1979, said the Amazon forest is being cut by people leaving Brazil's urban areas and by illegal logging, not by farmers.

"It is not economical to cut down the Amazon forest and plant corn," Paolinelli said. "It would take many, many years to pay off."

In the Cerrado, he said, trees are not being cut down to make farmland.

"You don't cut one tree," he said. "We're recovering the pastures in the Cerrado."

McClung, who did pioneering soil research in the Cerrado in the 1950s, said cover crops have been planted in the Cerrado to prevent soil erosion.

"There's no evidence of degradation," McClung said.

Lobato, who built on McClung's soil research, said there was some soil erosion when the Cerrado was first farmed extensively but said that has been corrected.

Brazilian farmers are required to keep 20 percent of their land in natural habitat.

Conservation International, a Washington-based group, is working to get farmers to comply with the set-aside law.

"We need agriculture," John Buchanan, the group's director of agriculture and fisheries, has said. "We don't intend to stop it in Brazil, but we need smarter and better agriculture."

Lobato said that if a farmer is profitable, he or she is more apt to pay attention to protecting the environment.

"A farmer's first concern is to pay his bills," Lobato said.

Ed Schuh, a professor at the University of Minnesota and an expert on international trade, cited research showing that the opening of the Cerrado has slowed the cutting of the Amazonian rain forest.

"The flow of people to the Amazon in the north is coming primarily from people living in the highly populated regions in Brazil's south," said Schuh, who is married to a Brazilian and owns a 1,300-acre farm in the Cerrado.

"Development of the Cerrado in the center of the country is cutting off some of that migration, and people are settling in the Cerrado and not going on the Amazon."