We use the Lewis Cattle Oiler and we sell mineral with IGR. We have lots of satisfied customers that use and swear by the IGR. Here on this place we can't get cattle to eat enough mineral in the summertime. So we did buy a Lewis cattle oiler and I guess we aren't totally sold on it.
With the IGR the cattle only need consume 2 oz. for the IGR to do its work. We have a pasture across the road and the cattle do eat it there. Once I came home from town and the neighbors cattle were all bunched in a fence corner (no fly control) and ours were 5 miles down the road in thier pasture and were all out grazing. That's how you tell if the IGR works. Cattle quit bunching up and continue to graze. You will never get rid of ALL flies so don't expect that. What you will see with IGR is less of a fly load, so the cattle are able to keep eating, which means they keep gaining weight. You need to get it out early enough in the spring and keep it out late enough in the fall.
The cons of the Lewis Cattle Oilers are if you have cattle in more than one pasture you need more oilers, meaning one for each pasture. Ours plugged or something so the oil didn't get on some of the rubs. In the winter you have to move the oiler to the cattle and everyone I know that has one, the oiler is still in the last pasture and wasn't with the cattle in the winter. Guess it is management just like anything else.