The gemara relates (daf 15a) that Chagai hanavi said Amon and Moav must take maasar ani on shviis, so the poor can be supported.
I don't understand. If everything is hefker on shviis, why can't the poor come and take from everyone's hefker fields just like anyone else? Why do they need a special takana to support them? My digest of meforshim didn't say anything about this, but it seems an obvious question.

Maybe, since nothing is planted or cultervated (plus being in compotition for everything that grows among the rich and animals) and only until the time of Biur, it's very possible getting their tenth from properly maintained feilds were vastly more then the Hefker of Shviis.