from the above link
“The value of vinegar has long been exploited by professional poultry farmers. Acidifying water alters the gut’s bacteria, slowing the growth of nasty bacteria, and giving a boost to good bacteria. Acid also helps control coccidiosis and Clostridium bacteria, which can cause a fatal disease called necrotic enteritis. Vinegar (acetic acid) is a cheap, accessible source of acid that anyone can find. It is, however, not a great acidifier...other organic acids such a butyric or proprionic acids actually work better...(the reason revolves around the pKa of the acid....high-school chemistry, anyone?) I have never found any study that showed any value to apple cider vinegar specifically, and several studies (the Journal of Applied Poultry Science in 2011, and Asian Australasian Journal of Animal Science), showed that broiler (meat) chickens grew slower when fed 0.5% apple cider vinegar or formic acid vs. pure water.
Acidified water also affects laying hens by making the calcium in her feed a little less digestible (again, based on chemistry....calcium is a positive ion, and dissociates better in a more alkaline environment.....seriously, who ever knew that this stuff might matter?). Professional farmers regularly add baking soda to their feed when heat stress is expected....this maintains egg shell quality when hens' feed consumption drops due to the heat.
Using vinegar in the water also helps keep bacteria from growing in your water system. It also smells good, and there is some evidence that birds will drink a little more, possibly because of taste.
At the end of the day, vinegar (apple cider or not) is an organic antibiotic that has a place in helping to control bacteria levels in your flock and altering bacterial populations in the gut. Just remember that it has some minor negative consequences for the hens, as it makes some nutrients less available to the birds.” The Chicken Vet
