Have you asked round? probably a mistake or as said someone trying to be helpful. If it is a tractor load , there are only a couple of folk with tractors so ask them .Or if it is a small amount probably one of the many Sunday drivers we have had around today that has decided to give your ponies a feed from whoevers silage it is outside your gate.( I found a bunch of kids feeding my turkeys chicken sandwiches and buns).It would have been put there for a reason not dumped, the farmers have dump areas for bad silage and no one around here would even think of dumping bad silage on someone elses land. I dont think the ponies would have touched it if it was blue mouldy, but good job you found it.
I will ask for you if you like.