So it sounds like you are saying that the foundations of the silo are in danger of being undermined, is this correct? If so, any good local general contractor (not a "fix-it" guy) would do just fine, probably.
Where my parents live, there are certain "old-timer" contractors who have done lots of work on various farms in the area over the years. Some of them are even farmers themselves. Someone like this would be great to know for you anyway, so you might ask around. You might also think about rebuilding the roof for the silo, too. That might help, depending upon the causes of the erosion.