This is a controversial question indeed. Many chicken keepers are worried about their birds consuming any kind of chemical, certainly a valid concern. But, I think everyone should be equally concerned about the health of their birds.
Like anything else, education is the key to better understanding. After researching whether or not I should worm my own flock I stumbled across this article on Worming Chickens, it discusses a product called Flubenvet. I know about this wormer, but considering Flubenvet isn’t available in the United States it’s not much good to me.
Nevertheless, I researched it again and it led me HERE with more information on how to use Flubenvet and where to purchase it online. The Backyard Chicken forum was also helpful with another online source.
About 95% of the egg a hen lays is calcium carbonate by dry weight. Over a year, the calcium she puts into her eggshells could equal 20 times the calcium that’s in her bones. So it only makes sense that a steady supply of calcium will help her stay healthy and produce strong eggshells.
I suggest ground oyster shells, and I recommend putting them in their own container so the hens can take what they need. They like them, they’re readily available in feed stores, and they’re not expensive.
Feed Variances and Calcium
If your hens are fed high-quality layer feed and get nothing else, their calcium intake is probably sufficient. But if they’re in the yard, fed table scraps and scratch feeds along with commercial feed, they are good candidates for a calcium supplement.
Note: Only feed oyster shells to hens already laying eggs.
TBN Ranch Chicken Keeping Resources 