Why/how? Intelligent straw golems replant as they harvest. Unless you go jumping on the wheat, it's not going to ever go fallow. Golems also handle crops that many other mods don't handle right, like flax and corn.
The straw golems don't hold an inventory. They replant farmed plots only if that block dropped a seed. If it doesn't, that block won't be replanted without some other intervention. As wheat drops 0-3 seeds when broke, eventually you'll end up with an unplanted farm if you let it go long enough. In a 5x8 farm, I have to replant 1-3 blocks every day (on my 24/7 server) manually. When I get into a position where I require full automation, I'll be adding something that goes through and aids replanting on occasion. Or go full computercraft.
Unless of course my straw golems are unique in their ineptitude.