33.668% decrease in horses reported stolen after California banned horse slaughter

California Bureau of Livestock Identification 
Horses Reported Stolen 1994-2003
1994 32
1995 40
1996 41
1997 43
1998 43 39.8 annual average for 1994-1998
1999 26
2000 24
2001 34
2002 24
2003* 24 26.4 annual average for 1999-2003
*(8 as of 4/30/03)
39.8-26.4=13.4 average decrease
13.4/39.8=33.668% decrease in horses reported stolen

Source:  email message from Greg Lawley dated June 27, 2003