@LogCabinChicks
I'm so sorry to hear you have had a blackhead outbreak in your turkeys. It is so frustrating that the chickens benefit from the turkeys (often offering protection against the more lethal chicken mareks's with the milder turkey marke's) but the turkeys in turn often suffer with blackhead perpetuated by the cecal worms in the chickens.
Information is scant. I know. I scoured the internet for hours to find some sort of guideline as to how long you feed Hygromycin B. At the time, I had found one source, from a Canadian veterinary symposium powerpoint that suggested 2 to 4 weeks feeding for most worm types, up to 6 weeks to remove cecal. Repeat seasonally. Unfortunately that link is no longer available.
Elanco, the producer of Hygromycin B, views this wormer as a control agent and meant to be continuous feed....as in fed all the time... to control worms vs. eradicating them.
http://www.elanco.us/products-services/poultry/hygromix.aspx
Most backyard owners simply cannot afford that kind of "control." From what I had gleaned at the time, Rooster Booster had internal response memos that most of their small holder customers used it for 2 to 4 weeks seasonally.
In trying to re-create that research, I have found Rooster Booster's official statement now is that it takes several weeks to slowly rid worms, with
capillary worms taking up to 6 weeks.
http://roosterboosterproducts.com/faq.php
I can give the following research presentation for (A. galli) which is continuous feed for 8 weeks for round worms in poultry. One resource study (Shuma, et al, 1958) stated that Hygromycin B was more easily effective on cecal than round worms, so I can see we'd be back to 6 weeks suggestion.
http://slideplayer.com/slide/4060410/ (See slide #4...this does not say anything about Hygromycin B for cecal)
http://www.stalosan.com/media/7110/Helminthologia.pdf (page 9)
As stated in that post, Durvet has a product that is substantially cheaper than the Rooster Booster Multi Wormer. Strike III can be found in feed stores. I have purchased it as cheaply as $13 for the 1lb package. However, that quickly adds up with large flocks.
There are off label choices that I will list below. Off label use has always been a grey area for the small holder, and each owner must weigh whether or not they feel comfortable using off label products. It appears this will only become more difficult as new regulations take effect in 2017. I will link a good FARAD article below to help your decision whether to use off label or stick with FDA approved sources.
FARAD Literature
https://vet.osu.edu/sites/vet.osu.e...ng the treatment of backyard poultry 2015.pdf
If the eggs/meat are for your benefit only, there is no problem. However (and with the number you've mentioned I suspicion you fall into this category), if you sell eggs or meat to the public, even farm direct, technically, according to FARAD, you are legally responsible for any residue that might be found in the eggs or meat should they be inspected. (It would be an FDA violation). However, small holders are not inspected. (Ffewer than 3,000 birds I believe is considered below inspection levels). So each owner must judge whether or not this is a risk worth taking. I can say that if anyone got sick and they found residue in your eggs, you could have a legal violation and challenge. That risk may be very minimal, but obviously the safest route is to remain in FDA approval if you sell meat and eggs.
Here are alternative choices for treatments for cecal worms. (Of course, in the US, only Hygromycin B is FDA approved for layers of eggs for human consumption)
http://www.poultrydvm.com/condition/poultry-cecal-worms
I personally have chosen to use Durvet Strike III for 3 weeks quarterly. Medically, it is best to rotate meds to prevent resistance in worms. However, as stated in the FARAD, off label use may become more complicated in 2017. But as of now, if any off label product is used, just make sure any residue has cleared, and FARAD gives some suggestions for some of the off label wormers.
Good luck with your flock.
LofMc