I've been wondering whether Rick's gang shouldn't have tried to drive/lead/decoy the herd right on past the farm. Anyone not able to fight could just hide in the house until they're gone. Those who did the decoying could get a decent lead on the herd, get out of their line of sight, and then circle back. It's not like the zombies are automatically going to besiege the house just because it's there. As long as you don't draw their attention to it.
I suppose by the time they were aware of the threat, it was a little late to come up with a plan like that.