Did Shane upset the Eco-system? The Farm was special in it's lack of Zombies, like there was a "Sonic Rodent Fence". Could it be rounding up the occasional Zombie and putting it into the "Stink Box" Barn has served as that "Sonic Fence". Zombies weren't attracted by the fresh meat, because the stench of all those trapped Zombies put off a smell that covered the Fresh Meat human smell, so Zombies weren't drawn there. Now, when the Zombie corpses are cleaned up, you no longer have that stench to cover the Fresh Human meat and maybe hordes will be drawn to such a large cache of fresh Human meat without competition.
Possible, but I don't see it as likely. Atlanta, the school they went to for medical supplies, those areas were crawling with zombies, but none wandered off in favor of better hunting grounds.
Easiest explanation is just that it's a rural area. Their sense of smell has to be limited, and when you've got miles of nothing around, nothing to draw you through all the woods and fields to that farm. Every now and then one wanders by, but just not in great frequency because it's so isolated. Even with the "sonic fence", they were still attracting attention occasionally, so it can't have been doing much to discourage them...