Yes I know that we can just not enable collision for the fences, my point is that will look weird, unless we made a gate too, which would require extra collision polys and slightly less performance. In the Age of Empires series the farms have no fences, and it still looks fantastic.
And I do not think we can do the faces material thing, however UE4 features many types of culling, including occlusion culling, which may, if they are using it the way I’ve seen in other engines, mean that they are in fact not rendering any polygons that cannot be seen. Regardless, A few extra polys will be a minor hitch in F/S (Frames/Second) as the overall complexity of all meshes will be quite low.