Farms have the lowest CpS of any fully upgraded Building when a reasonably large number of Buildings have been purchased, meaning that their greatest use later on is improving the CpS of fully upgraded Cursors. The 150th Farm costs 533 billion cookies but only increases the output of Farms by 20 CpS (before bonus multipliers) with all upgrades; it however contributes 26,520 CpS (plus multiplier) to the Cursor rate and per mouse click, if you have 200 cursors and the Quadrillion Fingers upgrade - a 1326x difference(!). (Please update numbers)
Farms were not included in the Classic version of Cookie Clicker.
Farms are currently the worst buildings in game. It is not recommended to buy farms for CPS mid-late game, as they are only really useful for Synergy effects.
Statistically, Farms are the most efficient building to buy first!
The name given to the achievement 'I hate manure' may be a reference to Back to the Future, as in the films this line is often quoted by a member of the Tannen family, when they end up in a pile of manure.