I have lots of customers that buy mineral with IGR. Some (only a few) buy the mineral and then hand mix the IGR in with it. But I recommend buying it already mixed, as the equipment at the plants can mix it in so much better.
Yep, you will see added weight from using IGR. The secret is to get it out early enough (after about 3 days of 60-65 degrees) and leave it out long enough, say the first of Oct. or until the first good frost. That keeps the flies from wintering over and you won't have as big a hatch of flies next spring.
Also, know when you buy this product that you will NEVER get rid of all the flies. IGR controls HORN FLIES. So you will see other flies on the cattle. But horn fllies are pretty much what causes the cattle to bunch up during the day. When cattle are bunched, they aren't eating. When they aren't eating, they aren't gaining...
IGR keeps the Horn Flies down to a very manageable level. However, if you have cattle across the fence that aren't on fly control, you won't see as much benefit from the IGR, as horn flies will travel a ways to a host animal. We have had customers move their cattle from being close to neighboring cattle, and in 2 weeks (life cycle of the horn fly) they see a big difference in fly load.
Horn flies are the flies that are on a cows back, that point downward...
Hope this helps!