A simple law of economics: If you outlaw something, the black market quickly fills in the gap where there is demand. That’s what seems to have happened in Miami, where investigators from the SPCA note that illegal slaughterhouses operate, according to the Palm Beach Post.

Horse meat fetches $7 to $20 a pound in the black market. In October, Miami-Dade police agencies seized about 400 horses, in a raid of what they suspected was an unlicensed slaughterhouse.

The black market, plus complaints that horses are traveling in far worse conditions to slaughterhouses in Canada and Mexico, has prompted some states to take action, and two senators to introduce a new slaughter bill in Washington.

The New York Times reports that the Montana Legislature approved a bill allowing the construction of horse-slaughter facilities. Similar bills have passed in North and South Dakota.

On the federal level, Sens. Mary Landrieu (D-La.) and John Ensign (R-Nev.) introduced the so-called “Prevention of Equine Cruelty Act,” the companion bill to the radical anti-horse slaughter act in the U.S. House, H.R. 503. The Humane Association of the United States supports these bills, as they will remove the option of shipping unwanted horses to Canada or Mexico.

Opponents say banning horse slaughter in the U.S. has greatly increased the numbers of unwanted horses. According to the Humane Society, last year the United States sent 98,363 horses to Canada and Mexico for slaughter. As of March 28, 17,758 horses had been sent to those two countries for slaughter.