ASHINGTON, Dec. 8 - The government went before the Supreme Court on Wednesday for the third time in recent years to defend an agricultural marketing program that requires producers to pay for advertising that not all of them want, and that some have challenged as compelled speech.

"Beef: It's What's for Dinner," an advertising campaign financed by a $1 assessment on every head of cattle sold, was at issue this time. A dissident group of ranchers who believe their own beef to be superior and who see no benefit in generic advertising won a ruling from a federal appeals court that the assessment, usually referred to as a checkoff, violated their rights under the First Amendment.

Seven years ago, the Supreme Court upheld a marketing program for peaches and plums against a similar challenge. That decision was followed by another three years ago that invalidated a federal assessment on mushroom growers. The law is clearly in a confused state, and Wednesday's argument in Veneman v. Livestock Marketing Association, No. 03-1164, did not necessarily clarify matters.

The government is defending the beef program as "government speech" that presents the government's message - albeit through the mouths of ranchers. Deputy Solicitor General Edwin S. Kneedler told the justices that the Agriculture Department must approve each advertisement and that "Congress itself prescribed the entire message" as part of the Beef Promotion and Research Act, the 1985 law that established the program.

"These are programs of government speech," Mr. Kneedler said. As such, he argued, the First Amendment does not apply; the amendment limits the government's power to interfere with private speech but does not limit what the government itself may say.

But several justices objected that the beef advertisements are presented as the message of the cattle producers, with no indication of the government's role. "Is it government speech even if you don't say this is your government speaking?" Justice Antonin Scalia asked. And Justice David H. Souter said, "The implication is that he who pays is the person whose speech is being broadcast." Later in the argument, Justice Souter observed that the advertising campaign was "really a misrepresentation" and that the government "seems to want to hide the ball."

Justice Ruth Bader Ginsburg pointed out that the government appeared to have contradictory messages on the virtues of eating beef. "What is government speech?" she asked. "If you went to the surgeon general, the message would be 'eat meat moderately.' "

Mr. Kneedler said that in promoting beef, the government was trying to overcome "a structural defect in the market for a fungible commodity," in which few of the many small producers could undertake an advertising campaign.

"Mr. Kneedler, is meat fungible?" Justice John Paul Stevens asked. "I'm sure some steakhouses would disagree with you."

The earlier cases on fruit and mushrooms had not touched on the government-speech issue, so Wednesday's argument was novel despite the general familiarity of the subject. Throughout the argument, various justices wondered aloud whether constitutional doubts about the program could be resolved if, on the one hand, the advertisements acknowledged the government sponsorship or, on the other hand, if it were financed by a tax rather than the mandatory checkoff.

"What's objectionable about a little thing in the corner" that would say "the U.S.D.A. requires this," Justice Stephen G. Breyer asked Gregory Garre, arguing for the Nebraska Cattlemen , a private group that supports the law. "Then the whole problem goes away," Justice Breyer continued.

"That's correct," Mr. Garre replied.

Laurence H. Tribe, representing the dissident ranchers who challenged the program, said that identifying the government as the sponsor would not solve "any problem but deception." He continued: "What is objectionable here is homogenizing all these people. The underlying constitutional principle is that you ought not to be able to force people to engage in speech they don't agree with."

The court has ruled in a series of cases that people cannot be required to subscribe to messages with which they disagree, whether a slogan on a state's license plate or a labor union's political activity. When Mr. Tribe, a professor at Harvard Law School, referred to these precedents, Justice Breyer interrupted him. "Does it make a difference that the fundamental aim of this program has to do with simple commercial advertising?" Justice Breyer asked.

Mr. Tribe agreed that there was at least some difference. "Yes, they're not trying to induce ideological conformity," he said. "The spirit of the program is not bad. But the road to hell is often paved with good intentions."