President Theodore Roosevelt’s rigorous trustbusting proved to be the only force that challenged the beef trust’s collusive tendencies.
In 1902, after antitrust lawsuits broke the beef trust’s pools, Swift, Armour, and Morris merged into the National Packing Company to continue fixing prices, subduing competition, and controlling trade while avoiding antitrust charges. 
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[Charles Edward Russel. "Cartoon Showing Decreasing Prices Paid for Cattle, Sirloin, and Chickens by the Beef Trust to the Producers." The Greatest Trust in the World. 1905.]
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[Charles Lewis Bartholomew. The beef trust - don't shoot I'll come down. 1906.]

Meatpackers

“If a packer ever buys cattle to better advantage than his competitor, it is because his cattle-buyer is a better judge of cattle than his competitor's buyer, and that brings in the personal equation—​one man's brains and judgement against another man's—​and that cannot be governed by a monopoly.”

[J. Odgen Armour. The Packers: The Private Car Lines, and the People.]

Investigators

“The supreme court of the United States today decided ... the beef trust case, charging conspiracy among packers to fix prices on fresh meats.​”

[Beef Trust Loses in Supreme Court. The Morning Press. 31 Jan. 1905.]
In January 1905, Roosevelt used provisions under the Sherman Antitrust Act to sue them in Swift & Co. v. United States.
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[Beef Trust Men Face Penalties. Chicago Tribune. 31 Jan. 1905.]
This halted the beef trust’s price-fixing practices, but little else. To further prosecute the trusts, the trustbusters needed more legislative regulation.
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[New York Times. 31 Jan. 1905.]
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Before. The Meat Trust (to the small farmer): My friend, why don't you raise a few cattle each year? The price of beef is high. You will make good money.
After. The Meat Trust (to same small farmer): The price I offer for your cattle is low, is it? Well, you may take it or leave it, my friend. There is nobody else for you to sell to.
[Udo J. Keppler. Tweedledee and Tweedledum. 1913.]