The National Sustainable Agriculture Coalition (NSAC) has clear and detailed answers to 15 questions about Congress' failure this year to pass a Farm Bill. Here is an example.
What happens if we ... revert to the 1949 Farm Bill?
If a new farm bill is not enacted or the current farm bill is not extended for a period of time, the farm bill commodity programs revert to permanent law contained in the 1938 and 1949 farm bills. Each successive farm bill since that time has suspended permanent law for the period of time provided for the newly enacted farm bill. But the permanent law provision is scheduled to pop back up and become the law of the land again if Congress does not enact a new bill or extend current law.
This peculiar feature normally induces Congress to get its work done on each new farm bill in a timely fashion. Without a 2008 Farm Bill extension or a new farm bill, dairy policy reverts to permanent law on January 1, and grain and other commodities do so once the new 2013 crop is ready for planting.