Horn flies will only fly half mile to a host animal. If you have your cattle half mile from any others, you will see a reduction in flies. If they are closer than that, you will see more flies for a couple of weeks, then the numbers will be lower again. I think it takes two weeks for a hatch...
Alabama cattle are probably closer to the neighbors cattle than cattle in Mt. SD, ND, Wyoming, Nebraska, Kansas, Idaho, etc. so we don't see much of that problem.
Remember what I posted a long time ago about blocks? Takes a lot of licks to get one ounce of a block of something, unless it is chewable like a previous post mentioned. (Like 3,000 licks to get 1 oz. of salt)~
Dave Wieland, Independent Nutritionist ( he used to write the "consultating Nutritonist" column for BEEF magazine. I still miss his column in that publication~it was a definite asset), has this to say about fly control:
..."horn files are the most costly parasite; they cost the beef industry more than $800 million each year. It is estimated that 500 horn flies per animal will result in the loss of 1 pint to 1 quart of blood loss per day! This amount of blood loss may lead to lowered milk production, resulting in reduced weight gain in nursing calves, reduced weight gain in other classes of cattle and a lowered immune response. In addition to these losses, flies can also transmit diseases such as pinkeye, anthrax, and tb..."
"With the advent of larvacides, used either as a bolus and or as a feed additive, producers were given an exellent method of of control that acts on a completely different part of the fly's life cycle. Instead of killing the adult, larvacides, as their name implies, acts of the larva. In one group of these compounds, insect growth regulators (IGR) kills developing fly larva in the manure of treated cattle. Another class, methoprene, interrupts the horn fly's development into an adult. A convincing argument for the use of a larvacide is that there is no known resistance to these compounds. As far as effacy, larvacides should play a MAJOR PART pf a fly control program."
"The most important fact that producers need to remember is that regardless of the methods used, we will never be successful in eliminating all flies; breeding sites are too numerous, fly life cycles are short enough that reinfestation is inevitable, not all flies will be killed by any one method of control, not all producers or their neighbors in a given area practice good fly control, and the cost of such a goal would be prohibitive. Our goal should not be to eliminate all flies but should be to attempt to get fly numbers below an economic threshold, considering both the cost of control and the potential loss of production if no contol is practiced."
Hope this helps!