I'd suspect badgers, especially if the feed has any molasses content - they've a sweet tooth. I used to leave lick buckets open all the time until I found one clawed into pieces by a badger. Now I put the lids on at night and lick bucket consumption has reduced by 40%. Don't forget badgers visiting your land may well be urinating and defaecating bTB, which can be contracted by ovines, as they go, as well as leaving saliva (also infectious) on the feeder.