Everything on this is an opinion. I’ll give mine.
One of the real problems on this type of stuff is finding studies that actually apply to the way we do things. Somebody has to pay for a study. Normally that is people with money, like the commercial chicken operations. It should not come as a surprise that the parameters of the study apply to their situation, not ours. You can gain a lot of knowledge based on these studies but you have to be really careful how you interpret them. For example, that one that showed bacteria levels drop in the intestines if you give ACV the day before you butcher broilers. That’s a short-term effect. How would that affect them long-term if those broilers had a long-term?
There are a lot of flora and fauna that live in our digestive system. Chickens have a huge amount of flora and fauna in theirs too. Some are good (you might call them probiotics), some are bad, and a lot are neutral as far as scientists know. If ACV kills some of the bad ones, it will also kill some of the good and neutral ones. That’s going to upset the balance in the short term. In my opinion, long-term a new balance will be found if you are consistent in what you give them. If that new balance is better, worse, or indifferent, I don’t know but my gut feel is that it is pretty darn close to irrelevant. That means as long as you don’t get silly about this stuff, it just doesn’t matter.
Another problem with these studies is that they have to be consistent. That one about ACV and butchering the broilers was not. What does that tell you? There may have been differences in the studies that were not accounted for. Something else may have been affecting it, food, water, genetics, age, weight, or who knows what. There may have been a difference in the delivery system. Dosage may have been different. Maybe one set just didn’t have as many bacteria to start with. Or maybe there was a flaw in the way the measurements were taken. So the results really didn’t prove anything except since there were differences it warrants more study. Maybe they can find out what actually caused those differences.
Anecdotal evidence is hard too. If I do something in the garden and it seems to help, I’ll probably try it again. If something seems to hurt, I probably won’t try it again. That’s called experience. But sometimes that is just coincidence. One year I planted butternut squash fairly late to see if that would help with squash bugs. I had very few squash bugs that year and really nice butternut squash most of the winter since they matured enough to store well. But when I was talking to the lady at the Mom n’ Pop garden store I go to about it, they said no one was having problems with squash bugs that year. Planting them late the next year did not work.
That study on 11 humans brings up another point. For these studies to mean anything, you have to have enough of a sample for averages to mean something. 11 people is not much of a sample.
One more before I quit. One of the supposed benefits of using ACV is that the poop doesn’t stink as much. If that is true, is that a good thing or a bad thing? To me it says you are killing more of the good, bad, and indifferent flora and fauna in their digestive tract. Is that helping or hurting them digest their food so it can be absorbed and used by their bodies? I don’t know.
In my opinion, as long as you are pretty consistent in what you do chickens are really good at being adaptable and will be OK. As long as you don’t get silly about it I don’t think you are doing any harm. I also don’t think you are doing a lot of good either. For the true believer it is the best thing since peanut butter and sardines on rye with yellow mustard. For those that want to believe you cannot convince them otherwise. For those that don’t want to believe you can’t persuade them with information currently available and maybe never. It’s just one of those things everyone will have an opinion on which is fine in my book.