This company used to make all the CIH grinders, I think they still do. We switched to a roller mill mixer when we got out of hogs, so haven't kept up since.

Anyway, they still show a hydraulic roll feed option . 20 years ago, it was a mechanically powered option, v-belt I think. You can add this option to most of the existing mills. It fits in the feed throat, just above the hammers and limits the rate that the hay flake is pulled into the hammers.  We used the biggest screen available but don't know the hole size, maybe 2"???

If I wanted to process small squares and didn't want to go with a mixer wagon, that's the route I'd go again.  Made for some dusty feed, but we added either soy oil or molasses to cut down on the dust.

Dave