"That's a pretty substantial impact," said Dale Bauman of Cornell University, whose research appears in the journal Proceedings of the National Academy of Sciences.

While it has been approved for sale by the US Food and Drug Administration since 1993, the hormone has been banned in Japan, Australia, Canada and parts of Europe. Opponents say it can have harmful effects on both the cows and humans who drink their milk.

Many US grocery chains in the United States have switched to milk suppliers that do not use the synthetic hormone, and Wal-Mart Stores Inc said in March its private label brand of milk would be sourced from suppliers that do not use growth hormones.

Cornell University paid for the study, done with the help of Roger Cady of Monsanto Co, maker of the bovine growth hormone Posilac.

The study focused on environmental, not safety, issues.

FEWER COWS, LESS FEED

The research suggests that, if used on a large scale, bovine growth hormones could reduce the number of cows needed to produce milk, cutting demand for corn and soybeans and reducing greenhouse gas emissions by decreasing the amount of manure they produce.

Decomposing manure produces methane, a greenhouse gas. Scientists say controlling methane emissions from animals would help address climate change.

"All food production has an environmental impact, but many people don't realize that," Bauman said by telephone.

Bauman and colleagues used computer models to calculate the impact that greater use of growth hormones might have. They figured using the hormones would let 843,000 cows produce the same amount of milk now produced by 1 million.

That would save 491,000 tonnes of corn, 158,000 tonnes of soybeans, and total feed would be reduced by 2.3 million tonnes. The change would allow farmers to reduce the amount of cropland needed by 540,000 acres (219,000 hectares) and reduce soil erosion by 2.3 million tonnes a year, they said.

And it would reduce carbon dioxide emissions by 1.8 billion pounds (824 million kg), methane emissions by 90 million pounds (41 million kg), and nitrous oxide emissions by 210,000 pounds (96,000 kg).

Michael Hansen, a senior scientist with Consumers Union, said the study is based on a "false notion" that you can produce the same quantity of milk with less feed.

Hansen said Monsanto in the late 1980s sought to make a label claim that the product increased feed efficiency, but the FDA said the company did not provide enough evidence for that claim.

(Editing by Patricia Zengerle)