The Humane Methods of Slaughter Act (7 U.S.C. §§ 1901 et seq).
The Downed Animal Protection Act (proposed legislation in April 2001; not adopted): would have made it unlawful to transfer or market non-ambulatory or "downed" livestock unless humanely euthanized.
On May 13, 2002, President George W. Bush singed into law the "Farm Security and Rural Investment Act of 2002" (Public Law 107-171), which includes a Resolution that the Humane Methods of Slaughter Act of 1958 should be fully enforced to prevent the needless suffering of animals. It also calls upon the Secretary of Agriculture to track violations "and report the results and relevant trends annually to Congress."
for a pdf. version of the 400+ page public law.)