You also have to remember- there's many different soils in Iowa. Some soils are TOP NOTCH while others are barely good enough to use for pasture. Some farmland the farmers are aggressively fertilizing it and others are not.
Another thing to remember- farmers give cows these minerals in Iowa even if they are grazing on top notch ground. It additionally helps them in many ways from increased reproductive rates, better milking, better disease resistance, etc. It makes a difference!
From my observation- I've put out minerals for about 6-7 years. I believe the racks are growing more MASSIVE and increased size. I know it's causing more TRIPLETS (minerals WILL increase reproduction rates) which some DO NOT WANT- side effect. I notice far less parasites on deer (not sure if that's coincidence or not? Probably MORE the hard winters. I notice younger deer growing better racks. Body weight seems better- might be more food plots there. BASICALLY, some of what I see is probably coincidence, some is food plots BUT I believe the mineral makes a difference and IS supplementing things they do not get OR things they do no get enough of. I believe it WILL make a difference in racks as well. (I put a ingredient list on my website too if you want to look). Look at any "deer breeding farms" that grow trophy whitetails, they use minerals (as well as protein and top genetics (genetics is the big one!!!))
Lastly, you have to remember that deer are starting to grow (or the mineral process is starting) new antlers VERY EARLY. March is the time to put out minerals (or late Feb). Think of all the plants that are not up as well at that time to supply the minerals. The guy who is throwing out his 1st bag in july or august is pretty much completely wasting his money (for the MOST PART) unless it stays around long enough for them to eat it next year.