Testing by the department's laboratory in Ames, Iowa, and the internationally recognized laboratory in Weybridge, England, came back negative, said John Clifford, the department's chief veterinarian.
"Needless to say, we are very pleased with these results," Clifford said in a statement. "I do want to emphasize that the most important protections for human and animal health are our interlocking food-safety protocols."
The department ordered additional testing after initial results indicated the disease may have been present in the cow. Officials called those results "non-definitive" and said they didn't resemble normal samples in which mad cow disease is present.
The animal had complications while giving birth and died on the farm where it had lived. Officials have not said where the farm was. The cow was burned and buried after a local veterinarian removed brain tissue for testing. The cow died in April, but the veterinarian forgot to send in the sample until last month.
There have been two confirmed cases of mad cow disease in the United States. A Texas cow tested positive in June, and a Canadian-born cow in Washington state tested positive in December 2003. All three animals are dead. The only way to screen for mad cow disease is to kill an animal and remove brain tissue for testing.
The brain-wasting disease is formally known as bovine spongiform encephalopathy, or BSE. The consumption of meat products contaminated with BSE is linked to about 150 human deaths from a fatal disorder called variant Creutzfeldt-Jakob disease. Most of the deaths were in the Britain, where there was an outbreak in the 1980s and 1990s.