I'm not a fan of infusions and other ways of concentrating anything, or of mixing/hiding things in chicken feed so the chickens can't see it's there and regulate their intake. This includes herbs, seeds, even calcium (that's why I don't like layer feed with its added/hidden calcium that the chickens can't regulate). There's a common misconception that if something is good in small amounts, it must be better in larger amounts, and that is just not true. Dosage is everything, and it's very easy to overdo a good thing and make it dangerous with overdose. Fennel seed can be a muscle relaxant, including relaxing the digestive muscles, which can be dangerous if they get relaxed too much, as digestion relies on peristalsis (the rhythmic contraction of smooth muscles along the digestive tract that move the food along). Additionally, chickens can be very sensitive to nutritional imbalances, and develop problems as a result. So, in short, I would not give my chickens fennel tea, or hide fennel seeds in their feed. If you have a lot of seed to get rid of, offer it straight in a separate container, and if the chickens really want/need them, they'll eat them. And if they don't eat them, then maybe there's a reason why. Or, you could just scatter the whole quantity in the run and have it mix in with the bedding. The chickens may enjoy "finding" hidden seeds while they scratch, so it can be an enrichment activity for them, and whatever doesn't get eaten will just become part of the bedding.
