Well, most animals are naturally suspicious about new food items. Getting them to taste it in the first place is often the biggest hurdle and then they may take a day or so to think about it before they eat it willingly.
In that case mixing garlic with something they like, i.e. yoghurt, can help. Some scientific studies suggest chickens have an instant feedback loop that confirms to them whether what they're tasting is good or bad. Actually all animals and humans have that, but there are margins of error, some large, made pretty obvious by animals eating poisonous things and dying.
After the Fukushima incident my chooks swore off garlic from China, which is what I'd been using mostly. I never told them about Fukishima, they never watch the news, lol --- I don't know why they refused to eat it, but from loving it they went to hating it in a few short weeks and completely refuse to touch it now. So now I use garlic from Spain, Australia, and other countries.
I have some concerns about modern strains of garlic because many modern plants are not nutritionally comparable to the heirloom varieties or the primitive ones, which are what old school natural health care is based on. Comfrey for example is very medicinal --- discounting the dozens of modern cultivars, some of which are useless, and some of which are harmful. Corn for another example used to be a main staple food so nutritious it was capable of keeping animals and humans alive single-handedly for very long periods, but modern corn is just not capable of doing the same. I intend to grow all my own stuff as able because too many plants are near-useless now, they've been bred up to be bloated, water-holding, tasteless versions of their ancestors which travel better, look better on shelves for longer, cope better with massive doses of pesticides etc, and due to all that, they aren't better for us in any way, unfortunately they're worse by far and nutrient-impoverished.
If they won't eat garlic, you could try Rue, some old farmers used to plant it so the birds could eat it free choice and it is supposedly superior to garlic for poultry use. Someone on this forum told me that like some powdery blueish plants it can cause eczema in sensitive people, so best to be aware of that possibility.
Best wishes.
