Chicken keepers have been giving raw garlic to their hens for decades, possibly longer, to help them treat infection and respiratory problems but also to improve their appetite and the size and quality of the eggs they lay. After a few weeks of use, the sulphur from chicken's droppings is also reduced which can make your chicken coop and run smell better.
When freshly crushed, garlic releases allicin and allicetoins that have antibacterial properties. Not only is garlic a super food for us, it is commonly used for treating ailments in many animals.
I often add garlic powder to my chickens' daily feed. Every couple of weeks I mince up a whole bulb, mixed with some chopped fresh mint, and feed it to the chickens free-choice and they seem to love it. Garlic, especially in conjunction with mint and wormwood (artemesia) is a wonderful year-round natural wormer.
Lots of people comment that adding garlic to your chickens feed makes the eggs taste almost "garlicky" but i have not noticed in the slightest.
I grow garlic every year just for my hens. It is easy to grow and requires little attention. Plant June/July with a generous amount of dolomite mixed into the soil and plant your bulbs just below the surface and your garlic will be ready to harvest in January/February. Plant the nice purple type and you will be rewarded with large and juicy bulbs come harvest time.
Do not use they white garlic sold in hanks - this is a Chinese garlic that is blanched in a form of arsnic to kill all bacteria (along with killing most nutrients im pressuming). I have also found that it is rare to get this garlic to sprout due to what it has been treated with.
For those that dont have the space to grow your own, granulated garlic or powdered garlic from your supermarket store will suffice. Just add this to your chickens feed.
I also feed my chickens garlic to boost their immune system. I offer them minced garlic or I put a few cloves of garlic into their waterer to keep them healthier and free from disease. They will eat the green sprouts from the garlic as well.