Thin Eggshells Are More Likely to Let Bacteria In

Calcium Supplements for Chickens Help Keep Eggshells Strong and Safe


Sunny side up, over easy or hard boiled? Think about the last time you made eggs. Did the shell break in a crisp, perfect line or was the crack more of a crumble and shatter? If it was the first, your eggshells are strong and protective. If it was the second, you are battling thin eggshells and your backyard hens could use a calcium boost.

Patrick Biggs, Ph.D., a flock nutritionist for Purina Animal Nutrition says strong shells are an important part of egg safety.

“When we raise backyard chickens, we are ultimately producing a consumable food product: farm fresh eggs,” he says. “As flock raisers, it’s our responsibility to produce eggs as safely as we can. One way to help keep eggs safe is by feeding hens for shell strength. Strong shells help keep bacteria out and eggs protected.”

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Strong Shells Keep Bacteria Out

Shell strength is determined by two primary factors: thickness and pore size.

Let’s start by looking at the shell under a microscope. A strong eggshell is about 0.3 millimeters thick and has between 7,000 and 17,000 tiny pores. These pores work to allow oxygen, carbon dioxide, and moisture to pass through, but to keep bacteria out.

“Eggshells with larger pores or thin eggshells have less protective power,” Biggs says. “A strong shell can help deflect bad bacteria, while bacteria can fit through the larger pores of a weak shell.”

The shell is then covered by a thin coating called the bloom, or cuticle, for added protection. Just inside the shell, the inner and outer membranes provide yet another layer of defense.

“These protective shields work together to keep the contents of an egg safe and healthy,” explains Biggs. “However, none of these barriers are effective unless you start with a strong eggshell; it’s the egg’s first line of defense.”


How to Strengthen Thin Eggshells

Realizing the importance of shell strength, many researchers have considered the connection between chicken layer feed and shell formation. The major player in the equation is calcium. Once laid, an eggshell includes 2 grams of calcium. To get this level and still maintain strong bones, a hen requires 4 grams of calcium – all of which must come from her layer feed.

The team at the Purina Animal Nutrition Center recently looked at two different calcium variables during on-farm trials:

  • Calcium in conjunction with other minerals
  • How quickly a hen digests calcium in her digestive system


The first finding showed that adding trace minerals, such as manganese, into layer feed can contribute to more than double the shell strength of diets without trace minerals.

Next, the team looked at calcium digestion and slow-release versus fast-release calcium. It takes 20 hours for a hen to make an eggshell, with calcium needed the entire time. Therefore, both forms of calcium are required. Providing only a fast-release calcium source, like eggshells or foraged foods can leave a calcium deficiency when a hen is sleeping.

“Layer feeds that include Oyster Strong® System take both learnings into account,” says Biggs. “The added calcium in Purina® Layena® and Purina® Layena® Plus Omega-3 have larger particle size to release calcium slowly and added trace minerals for extra shell strength. This provides essential calcium during the entire 20-hour egg formation process so you can collect safe, strong-shelled eggs each morning.”

To try a layer feed containing Oyster Strong® System and learn more about strong shells, sign-up for Purina’s new Feed Greatness™ Challenge at To learn more about raising backyard chickens, go to or connect with Purina Poultry on Facebook or Pinterest.

Purina Animal Nutrition LLC ( is a national organization serving producers, animal owners, and their families through more than 4,700 local cooperatives, independent dealers and other large retailers throughout the United States. Driven to unlock the greatest potential in every animal, the company is an industry-leading innovator offering a valued portfolio of complete feeds, supplements, premixes, ingredients, and specialty technologies for the livestock and lifestyle animal markets. Purina Animal Nutrition LLC is headquartered in Shoreview, Minn. and a wholly owned subsidiary of Land O’Lakes, Inc.


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Thin Eggshells Are More Likely to Let Bacteria In