I often refer to my hens as little artists, and rightfully so: eggs are amazing masterpieces that require extraordinary resources to produce. I find it positively painful to crack some of their beauties, but when I do, it is of some consolation to know that I can feed the clean, dry, crushed shells back to the ladies for use in making new shells. Unfortunately, not all eggshells are of the same structural quality; many factors contribute to shell quality from ambient temperature to stress and diet and I wondered whether the source of calcium in a hen’s diet, eggshells or oyster shell, mattered at all and learned that in fact, it does.
I was surprised to learn that the size of the supplemental calcium source in laying hens matters. Let’s look at some of the facts about eggshell construction and dietary calcium to learn why that is the case.
EGGSHELL & CALCIUM FACTS
- Of the approximately 25 hours needed to create an egg, 18-20 of
them are dedicated to shell formation.
- Laying hens do most of the work of assembling eggshells overnight, which is when calcium is in highest demand. (And we thought they were just sleeping at night!)
- In the 18-20 hours needed to assemble an eggshell, a hen can use and replace the calcium it carries in its bloodstream up to 100 times.
- Eggshells are composed of 97% calcium carbonate.
- The calcium required to make eggshells must be provided in a hen’s diet while carbonate is produced within a hen’s body during the normal course
- Calcium carbonate must be broken down into its components (calcium + carbonate) in the hen’s intestine before the calcium is absorbed into the blood. The calcium is then either stored in the bones or routed directly to the shell gland via the bloodstream.
- Laying hens require three times more dietary calcium than non-laying hens.
- The most common sources of calcium carbonate fed to laying hens are
oyster shell, ground limestone and eggshells.
Supplemental calcium should be provided to laying hens in a dedicated hopper or dish.
I mix oyster shell with crushed, clean eggshells in mine. Find the DIY instructions for this hopper, HERE.
The storage facility for calcium in laying hens is a specialized bone called the medullary bone. Think: sponge-like bone filled with calcium inside a hard, hollow bone. (The hard bone is the cortical bone, which is responsible for strength and stability).
We can see a cortical leg bone in this photo. Medullary bones of a laying hens are inside cortical bones. Hens deprived of adequate amounts of dietary calcium will utilize the calcium stored within their cortical bones to produce eggshells, causing brittle bones that fracture easily and in the most severe cases, the inability to stand. This condition is known as caged-layer fatigue. Birds raised on the ground or on litter floors recycle calcium and phosphorus through consumption of feces, and do not have caged-layer fatigue.
Larger particles of calcium carbonate take longer to digest than smaller particles.
In multiple studies, hens sought out large particles of oyster shell late in the day, prior to the most rapid period of shell formation overnight, in order to supply themselves with a continuous supply of calcium when it is most needed.
The ideal particle size of supplemental calcium ranges between 2mm and 5 mm. A minimum of 25% of supplemental calcium should be provided to laying hens in large particles. The take-home message: laying hens need oyster shell to make strong shells and meet their daily dietary need for calcium.
Eggshells in addition to oyster shell are fine, but not sufficient by themselves. “A hen would have to eat the shells of two or three eggs a day to meet her calcium requirements.”*
More information about feeding chickens at different ages can be found here.
Questions about funky eggs? Thin shell, no shell, double yolk, no yolk, misshapen, egg within an egg? Find out how they happen HERE!
Sources, citations and further reading:
Research on Eggshell Structure and Quality: An Historical Overview
The Avian Skeletal System
Factors Affecting Egg Production in Backyard Chicken Flocks.
How an Eggshell is Made
Bones, Shells & Hen Health
Calcium Requirements of Bovanse Hens
Concepts of Eggshell Quality
Overview of Bone Biology in the Egg-laying hen
*Gail Damerow, Backyard Poultry Magazine Volume 9, Number 3 June/July, 2014