I would think feeding the deer in the summer would be harder with all the food available out there. But mineral licks should be hit though. you know those rust colored blocks. There may be enough of them out there already being cattle ranches. Don't know.
Now that I think about it----there are tons of salt blocks around for them!!!....and my neighbor has alot of "mineral buckets" for his cattle----would deer possibly eat out of them???? They have calcium, phosphurous, vitamins, etc. Perfect things to make big antlers. I have seen some of my deer lick the cattle's salt blocks.....have to watch the deer and see.
Tell me and I forget. Show me and I remember. Involve me and I understand.
Now that I think about it. If you set a block or just pour that mineral right on the ground, where the deer are more comfortable, in the cover, you may get all the deer to use it and have them abandon the ones in the open. That way its in the soil and harder to get, making them hang around longer. They don't like the open as much as in the cover. May take a little time to get them "trained" but it should work.