IGR is not effective in controlling flies on its own in most places. The UNL and OSU data I have seen has both showed that. The only places it has a chance to bring the fly load under the economic impact level (typically 200 flies/HD depending on who you talk to) is when your cattle are the only cattle around for miles. IGR can be used in combination with other fly controls (sprays, mists, rubs, pour-ons, tags ect) to create a good overall program though. The company that makes IGR tests it every year & it has always been very effective at preventing new horn flies from developing. The challenge in the real world is getting the targeted amount of IGR in them every day.
I would check your level of IGR- IGR is approved for a certain range (I forgot what it is lets just pretend its 5-10 grams/100 lbs/Hd/d). Are you feeding the lowest approved amount (then any cow that under-consumes does get a big enough dose)? Are you feeding for a 1200 lb cow when your cows actually weight 1400? Are you not factoring in that the calves will eat some too and therefore an under-dosing the heard?
Some cows just aren't going to eat the mineral, and some cows aren't going to eat enough and therefore you will always have some "home grown" horn flies. But if fed at a level where almost all the cows get an effective dose you will reduce the amount of "home grown" horn flies. Which may or may not do you any good, depending on your neighbors!
