Texas Prison Systems land holdings constitute 100,000 acres, with some 149,000 employees, er, prisoners (in 1999). Dunno how many Rattlers.
Not all of the farms are on the Brazos.

The song, however, seems to have originated outside the prison system (see posts above); Janie's No. 1 is possible.

The 1833 date mentioned by Barry Finn puts the use of the name back a fair piece.