Maybe try something louder than a rattling food bucket? If you pair feeding times (or even just a treat) with a louder noise, like a bell or whistle, they would eventually learn to come. Can you start them off in a smaller area at first, a few acres? Then once they get the idea that X sound means yummies and to hurry over, let them have the big area?
Is there any treat that they will bend over backwards for? That they love more than life itself? I knew a horse who went bonkers for sugar cubes, so he'd do just about anything for a taste of one. That would certainly help, if you know what they love!