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Calcium and Oyster Shell 

Q: Why feed oyster shell separately?

A: If many people are adding calcium as a free offering, then why can’t it not just be added to the feed? In other words, if the layer needs 4% calcium, and the feed has 3%, why not just put oyster shell into the feed and skip a step?

Here’s the short version of what was a fascinating lecture by Dr. Gita Cherian, one of my very favorite professors at Oregon State.

If all the calcium that she needs (about 4%) is in the feed, the hen’s unique physiology causes her to have too much calcium when she doesn’t need it much, during the day, and not enough when she does need it, building shells at night. That is stressful to her and can result in poor quality eggshells because when she needs it, she has already pooped it. 4% calcium in the feed results in a lot of pooped calcium that should have ended up in eggshells. You’ll see a lot of white in her droppings.

And here’s the long version:

When a hen consumes calcium, it does not go directly from her digestive tract to the egg-building construction zone. The calcium goes into the gizzard before going to the bird’s glandular stomach. If it is in small particles as it is in a pelleted feed, it stays there very briefly, then passes into the rest of the digestive system.  As calcium particles are reduced in size enough and dissolved by the digestive acid and the gizzard’s grinding action, they move on to the body’s needs in general.  Calcium regulates heartbeat, muscle contraction, blood pressure. It is arguably the most important mineral in the body. It is the material from which bones are made – calcium carbonate.

When a hen lays an egg, she builds the contents of the egg over several days but finishes up the calcium carbonate shell the night before it is laid. She’s not eating at night, so she pulls the calcium from her blood. An egg uses nearly all of what is circulating in her bloodstream. If she didn’t have a place to keep some spare calcium, she’d be out of calcium, which would be a disaster, but fortunately, she does have a stash. Her leg bones (medullary bones) are a reservoir in which the calcium is kept until it is needed. It’s like a calcium bank into which she deposits dietary calcium and withdraws calcium for body functions and for eggs. She has to take it out of the feed, put it into her leg bones, take it out again and put it into the shell.

When she eats a large flake of oyster shell (not oyster shell flour which has been ground up), the large size of the piece keeps it in her gizzard awhile. Then as it slowly dissolves its calcium goes into the bloodstream and from there to the leg bones for storage or into the egg shell for the egg that is being constructed that night. It’s like a constant supply of calcium without interruption.

Hens like oyster shell and bits of calcium-bearing stone. Not all hens have access to oyster shell, and other calcium sources, limestone chips and bone chips that have large particle size work, too. But oyster shell is common and cheap, and they like it, so it is our most common source of supplemental calcium. By providing it separately we give her what she needs to build an egg with the least amount of stress on her body and the least wear and tear on her metabolism.

Calcium carbonate is a cheap feed ingredient, and many chicken feeds do contain up to 4% calcium in a fine powder form. Some is good, but not the whole 4%. It’s more than she can use at once, and it won’t be there when she needs it. It won’t kill her, but it is not the best way to meet her calcium needs. Long-term egg production and a hen’s healthy bones pay us back for providing her the calcium she needs in the best form for her use.