MADISON, WIS. -- The dairy farmers of America are standing on the threshold of technology's new frontier. Many do not like what they see.

Bovine Growth Hormone, or BGH, a new wonder drug capable of increasing a cow's milk output by as much as 20 percent, has provoked a bitter nationwide controversy that ultimately could affect the way Americans think about science and "progress."

The four companies that produce BGH herald the drug as an enormous technological breakthrough -- the first major success for genetic engineering and the burgeoning biotechnology industry.

Opponents of BGH charge that the drug will lead to massive milk surpluses, driving down prices and threatening the livelihood of small dairymen. Consumer advocates question BGH's safety, even though scientists say the drug is harmless, virtually duplicating a hormone produced naturally by cows.

Nowhere has the debate over BGH raged more fiercely than in Wisconsin, the country's top dairy state. It is here that the chemical companies have chosen to fight for acceptance of the drug, and it is here that small farmers and enemies of biotechnology have chosen to oppose them.

The issue became such a political hot potato early this year that on April 22 Gov. Tommy G. Thompson (R) approved a one-year ban on sale or use of BGH. A similar "moratorium" immediately went into effect in neighboring Minnesota.

Also responding to the outcry, the U.S. Food and Drug Administration has delayed commercial marketing of BGH and asked the National Institutes of Health to make a fresh appraisal of possible dangers to humans who drink milk or eat meat from animals treated with the drug. NIH officials said their analysis is expected to take months.

"There was so much controversy over BGH that we thought it would have an adverse impact on the consumption of milk," Thompson said in an interview. "We can't play with something like that in Wisconsin."

But Democratic state Sen. Russell Feingold said Thompson, who is running for reelection in November, banned BGH because "when push came to shove, he could read the polls. It was a very skillful move, because now he goes down in history as the Republican governor who acted against the drug."

According to Feingold, who has led the anti-BGH fight in the Wisconsin legislature for four years, "a natural coalition" among processors, dairy farmers and consumer groups was responsible for winning the ban.

"It was clear that BGH would drive up production and end up driving dairy farmers off the farm," he said. "At the same time consumers were tired of being forced to accept products they don't want."

There are signs that consumer attitudes may have just as much to do with BGH's fate as science or economics. Robin Douthitt, a University of Wisconsin consumer economics professor, found in a May survey that 77 percent of Wisconsin consumers would prefer to drink milk from untreated herds, and that 67 percent were willing to pay as much as 22 cents more per half-gallon for it.

The study also found that 71 percent of consumers were concerned that BGH could be a health hazard. Among the farmers polled, this figure rose to 78 percent.

Even dairymen who favor the drug understand the message of such survey findings.

"You can't stop technology," said Fred Machado, who has a 1,200-cow dairy spread in the San Joaquin Valley near Fresno, Calif. "My only concern is what consumers think. If people won't drink it, I can't use it."

BGH, also known as bovine somatotropin or BST, was developed as a synthetic growth hormone by four chemical and drug companies: Monsanto, American Cyanamid, Upjohn and Eli Lilly. The drug is almost identical to a hormone produced naturally by cows, and in tests has enabled cows to produce as much as 20 percent more milk.

Research has shown that BGH will show up in milk from cows that are injected with it, but it breaks down harmlessly into its component chemical parts in the human digestive tract. Advocates of BGH also note that milk from untreated cows has natural BGH in it.

The drug does, however, affect cows. Fertility is hindered, and opponents of the drug have charged that it promotes mastitis, an udder disease, and metabolic changes in a cow's digestive system.

University of Wisconsin researcher Terry Smith said his work has found no changes in cows' metabolic systems, no excessive incidence of mastitis and a "very predictable" effect on fertility. "Since the cows are producing more milk for a longer time it's only natural that their pregnancy would be delayed," Smith said.

Nonetheless, consumer groups continue to oppose BGH, charging that the Food and Drug Administration approved it for experimental trials without ascertaining its possible effects on human health. The NIH review is intended to answer this objection.

Meanwhile, the Douthitt poll showed that the scientific debate has done the chemical producers a lot of damage. Monsanto spokesman Larry O'Neill sharply rebuked opponents of BGH for "working full time, cynically trying to alarm consumers, then pointing to that concern as a reason why people won't use it."

Highest on the companies' enemies list is the Foundation on Economic Trends, a Washington-based consumer advocacy and anti-BGH group led by Jeremy Rifkin.

"The favorite phrase of Monsanto to describe us is 'food terrorists,' which we find a delightful Freudian slip, since Monsanto is one of the largest toxic polluters in the country," said John Stauber, director of the foundation's BGH Education Campaign.

"Any time you go after companies like these and cost them hundreds of millions of dollars by keeping the products off the market, you'll be branded as an extremist," Stauber said.

O'Neill acknowledged that Monsanto, like the three other companies, has invested "tens of millions of dollars" to develop and promote BGH, but emphasized that the companies intend to fight the battle for acceptance until it is won.

The struggle is doubly important because BGH is the first biotechnological product of its type to be ready for commercial distribution. Douthitt called it "in some ways an unfortunate choice" for that role, because its chief selling point is that it produces more of a substance that is in great abundance.

Up to now, political opposition to BGH has been carried along largely by small farmers concerned that more production in an industry historically plagued by enormous surpluses could undermine prices.

"We had this feeling that we had no power over this product, that we were going to have to accept it and it was going to put another one-third of family farmers out of business," said John Kinsman, a dairyman who milks 80 cows on a farm near Lime Ridge, Wis., about 60 miles northwest of Madison.

But many economists have challenged this assertion, saying that BGH will punish the inefficient, not the small, farmer. The drug, they say, is "size neutral."

This means that costs per cow to use BGH and monitor its effectiveness will be the same no matter how many cows the farmer has. What will change is the record-keeping load and the amount of attention each cow will need.

Researcher Smith said tests have shown that the most productive non-BGH milkers respond best to the drug, while cows that give less milk under ordinary circumstances may not be suited to BGH. Computer technology is the easiest way to track performance.

"It is going to be quite demanding on management skills," said Bruce Marion, a University of Wisconsin agricultural economist who has written extensively on BGH. "This is the factor, not herd size, that is going to make the difference. If you do a good job you could get 15 percent more milk. If you do a poor job you could get nothing."

For Kinsman, however, better management means "another hired person to keep the records" or longer days in a business where "people are already worn out. Any project that takes more management skills -- forget it."

Feingold, Kinsman and other opponents of BGH do not question that the drug does what supporters claim. Thus, it should enable farmers to produce more milk with fewer cows at lower cost, as advertised. BGH is a major innovation.

Unfortunately for the chemical companies, Douthitt said, "There seems to be nothing in it for consumers. Milk is already the cheapest beverage you can buy, milk produced from BGH-treated cows is not qualitatively different and there are health questions."

So why bother with it?

"It's different and it's scary because we don't understand it very well," the University of Wisconsin's Marion said. "But in the main we have allowed new technology to come in; we have not used economic injury as a criterion for regulating it."

Banning BGH, Marion said, "sets an uncomfortable precedent regardless of how you feel about the family farmer."

"The health of people or animals is one thing, but once you get into social control of new technology it becomes immensely difficult to deal with. What's the next product we're going to ban?"