There is no scientific evidence but enough anecdotal evidence over the years to keep this idea going. I myself have noticed increased female offspring in meat rabbits, chickens and sheep with the use of vinegar as opposed to when it has not been used. The pH of the animal most certainly influences what happens in the reproductive system as well as all the other body systems. It affects it to such a degree that the eggs have a completely different taste and odor than eggs produced from hens that have not had ACV on a continual basis.
It happens with enough frequency that people note it and pass it along as a possibility, even if it has not been proven by science. A placebo affect is something that takes place in someone's mind that has an effect on their perceptions and can even effect the body...increased female offspring after initiating ACV in the livestock management is not happening in the mind, it's actually happening in real life. I know it's happened enough in my own animal husbandry that I do not discount it as being only in my mind. It definitely has plenty of anecdotal evidence for many a long year from all points of the compass and with all types of animals. For the record, I noted the phenomenon years ago before I ever read anywhere that it was a possibility...I just happened to be using it for my rabbits to clear up some pink eye in kits and decided to just keep it in the water from there on out in my rabbitry. Later, having read about this idea of female offspring, I looked back on records kept on our breedings and litters and started seeing some correlation with our own records after the ACV was initiated. Coincidence? Maybe. Later using it with chickens I noted the same thing. I'm not much a believer in coincidence.