Letting fields rest -- a good thing?

Hey folks, second new topic of the day: Wondering if letting a field rest for a year, rather than keeping it hayed and fertilized, is a good thing. And if I am gonna leave it fallow for a year, should I be mowing it down? Concerned if I don't mow it down, there'll be a lot of dead material baled up the following year. But if I do mow it down, and it sits there in swaths, how many nutrients will really return to the fields? Might I just as well hay it if I'm gonna mow it?

    Thanks.

Letting fields rest -- a good thing?