I have two of that type of feeder with the rubber flap on top.  My cattle will use it if I put some sweet feed in it but will not use it for salt or dry mineral.  The last bag of dry mineral I put in it was in it for so long untouched that it got rock hard and was a total waste.

I've gone back to a white salt block in a rubbermaid type tray near the waterer for salt and Mineralyx tubs for mineral. Both of those get used. I like the Mineralyx product. I am in a copper and selenium deficient area and they need supplement of those two especially.

As mentioned in a thread below, I think a good mineral program is the key to 100% preg rate in cows and successful unassisted calving. I want mineral to be consumed.

I keep trying to think of a different use for those flap feeders....

Jim at Dawn