Minor league = ligas menores...
If you'd like a different term, there's always "cantera". Literally, it means "breeding ground" or "nursery", as in the place that feeds the majors.
Here's an example in a sentence found at http://edicion.yucatan.com.mx/noticias/noticia.asp?cx=16$4107020000$2829011:
"Rinde frutos la cantera de la Unacar
Varios de sus jugadores, a la Segunda División"

The advantage of "cantera" is that, just like the term "farm" league, it is used to refer to the minors, but not usually to AAA or second division, but the lower levels, where they "groom" players.
You can also say something like "Surgió de la cantera de los Yankees" = "He came up out of the Yankees' farm league"

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Note added at 17 mins (2004-06-17 14:41:18 GMT)
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Another example:
\"Jugó muchos años para la cantera de los Serafines de anaheim\"
\"He played for the Anaheim Angels farm league for many years\"