Among their recommendations are limits on the population density of animals and mandatory extensive environmental reviews for new feedlots. They also recommended a ban on the use of antibiotics to promote animal growth, and that the drugs be available to farmers only through prescriptions.
In a new area of concern, the scientists said they were worried about the danger of a flu pandemic spread by feedlots with both hogs and poultry, and recommended new regulations to set minimum distances between the two.
Farm industry representatives said they were not familiar with the new reports and could not address specific findings or recommendations. But they said that many environmental improvements had already been made, and that some experts at universities had said the health risks were minor.
"The livestock industry has been under very intense scrutiny over the past 10 years, and as a result, has gone to great lengths and very high expense to try to improve their environmental record, across the board," said Don Parrish, the American Farm Bureau Federation's senior director of regulatory relations.
"We've definitely improved our game over the past 10 years," Parrish said, and most livestock owners "are being very sensitive to their neighbors and doing the best job they can."
Many of the risks come from the sheer volume of manure. Livestock excrete 13 times more waste than humans -- 133 million tons per year in the United States -- and some individual feedlots produce as much waste as entire cities.
The American Farm Bureau Federation maintains that almost every state regulates the amount of manure applied to the land to protect water supplies.
But the new reports criticized the current techniques.
"Generally accepted livestock waste management practices do not adequately or effectively protect water resources from contamination with excessive nutrients, microbial pathogens and pharmaceuticals present in the waste," the scientists reported.
The number of large livestock operations has surged in the last two decades, and farms with more than 500 hogs now account for three-quarters of the U.S. inventory. In Iowa, the average number of hogs per farm increased from 250 to 1,430 between 1980 and 2000.
California has more than 2,000 dairies, mostly in Tulare and Merced counties, and many have thousands of cows each. But the health risks to the dairy workers and their neighbors have gone unstudied, said Frank Mitloehner, director of the UC Davis Agricultural Air Emissions Center, who was not involved in the new reports.
