From the Document: "Farm bills, like many other pieces of legislation, have become more complicated and politically sensitive, and are taking longer to enact than in previous decades. Legislative delays have caused the past two farm bills to expire for short periods and to be extended. Expiration may become an issue again unless Congress passes a new farm bill by September 30, 2013. Farm bill expiration does not affect all programs equally. For instance, appropriations action such as a continuing resolution can continue some farm bill programs, but not all of them. Discretionary programs can be continued via appropriations action, even if their authorization is expired. But programs with mandatory funding generally cease operations when they expire and are not continued in an appropriation. However, the Supplemental Nutrition Assistance Program (SNAP) and the other programs in the SNAP account receive 'appropriated' mandatory funding, so these mandatory programs may be continued with appropriated funds."
CRS Report for Congress, R42442