Representives John Holland and Vicki Tobin condemned the growing use of the term "harvest" to describe horse slaughter, calling it an insult and outrage to horse lovers.
"This fraud attempts to reduce the horse - the animal which in partnership with man built this nation - to a commodity such as corn, wheat, barley, or oats," the pair said.
"Although there is no market in the US for the 'crop', proponents of this fraud want to ensure healthy horses are killed so there is a continuous supply of meat on the hoof that must continually be shipped to overseas markets that Americans do not own nor profit. This is referred to as the never-ending cycle of breed and dump."
The term harvest was being used in a bid to mask a cruel process with a word that has pleasant associations.
Pro-slaughter groups should at least be honest in the language they use to describe the slaughter process, they said.
Not only were cattle and agriculture associations promoting horse "harvesting", but organisations such as the American Veterinary Medical Association (AVMA), American Association of Equine Practitioners (AAEP) and the American Quarter Horse Association (AQHA) were often quoted and named in their articles and speeches as supporters.
"These are the very organisations that are entrusted to promote equine welfare and care," said the alliance.
"They are organisations that have seen recent results of a three-year long US Department of Agriculture Freedom of Information Act probe by animal cruelty investigator Julie Caramante which resulted in the release of photos and reports from investigations of the department that clearly depict the cruelty and abuse inherent with the entire horse killing process.
"The three-year cover-up by the USDA has been dubbed by some in the media 'Slaughtergate'. It is hardly a harvest," the alliance said.
"Horses are not food animals in America. They are trusted work, service, sport, therapy and companion animals. It is time for Americans to stand up and end the hold the predatory foreign market has on the American equine industry.
"It is time for Americans to stand up and let their legislators know that horses are not crops, and that it is imperative that The Prevention of Equine Cruelty Act of 2009 be passed swiftly and without hesitation by the Congress and signed into law by President Obama."