Last-minute efforts to have the bill voted on before the holiday break failed.
The vote will also have to return to the House for a vote next year, with new new members taking their sets after the recent mid-term elections.
Supporters are confident about the bill's future, saying support has been stronger because of the amount of public support and publicity surrounding it.
The legislative director for the Society for Animal Protective Legislation, Cathy Liss, said: "Not all is lost, since more people have learned about this brutal trade, and the US House of Representatives has affirmed its strong support through its vote in favor of the legislation this summer.
"There was much business left unfinished by Congress - even funding for most of the US Government was not taken up in the waning days of the session."
She said the Senate's inaction must not impede the overwhelming momentum to end horse slaughter. "The American public, the humane community, and a majority of horse industry organisations and veterinarians support this legislation. We must stay united and strong in our commitment to end horse slaughter. If not, America's horses will continue to suffer. We will not let that happen."