Poultry Supplements: Grit and Calcium

Most people who keep chickens already know the basics of necessary supplements. For decades, probably even longer, grit and a calcium source have been at the top of the list for poultry keeping necessities, both dependent upon the poultry’s access to natural supplementation.

It is thought by some that providing a good quality layer pellet will meet all calcium needs and additional calcium supplementation is not needed; however, we feel it’s “better safe than sorry” and provide a crushed oyster shell supplement at all times.

egg shell anatomy
From: http://www.pysanky.info/Other/Egg_Bioscience.html

An average sized egg shell contains approximately 2 grams of calcium, and a hen of laying age needs about double that (4 grams) per day in calcium supplement to properly produce eggs.1 To understand the need for calcium supplementation, it is important to fully understand the calcification process – the creation of the egg shell in the shell gland during egg production. Female birds have evolved a special bone feature called the medullary bone that allows them to store extra calcium for egg shell production. Under normal circumstances, birds carry calcium in their blood plasma to use with common biological functions, but that level of calcium is only available for a short period of time and is not a level high enough for calcification. Through special biological & hormonal functions during breeding season, female birds store extra calcium in the medullary bone and are able to quickly withdraw this calcium for the calcification process. This only occurs during breeding season, except in domestic birds that have been bred to lay year-round, in which this process continues the entire laying life of domestic poultry. While the medullary bone has a special regenerative ability, years of continued year-round laying can lead to calcium deficiency in other skeletal bones. “Medullary bone, unlike structural bone, is capable of undergoing rapid absorption and renewal. Unfortunately, resorption of structural bone also occurs causing the symptoms of osteoporosis (Whitehead, 2004). Osteoporosis is caused by a decrease in the amount of fully mineralised structural bone leading to bone fragility and susceptibility to fracture…”.2

Laying poultry may be able to obtain the needed level of calcium from their laying feed, but the form of calcium used in layer feed passes quickly through the digestive system resulting in a limited absorption time and insufficient levels for proper calcification. In a typical egg production cycle, the calcification process occurs in the early morning hours when chickens are roosted and not consuming feed. Limestone is a common calcium supplement, and many people offer egg shells to their flock, which is a good source of calcium. However, studies have shown that, overall, crushed oyster shell is a superior calcium source resulting in the best quality egg shell production. Crushed oyster shell takes longer than other supplements to pass through the crop & gizzard thus providing a longer period of calcium availability, better stabilization of blood calcium levels throughout the night, as well as helping to properly maintain the medullary bone for long-term laying.3,4

At our farm, some of our birds get to free range everyday, but due to breeding restrictions, some have to make do with large runs. While they do have access to large areas of ground, they may not have enough access to “grit”, and they certainly don’t have access to lots of bugs and other goodies that might provide extra calcium. Due to that limited access, our birds are provided free-choice access to our custom-mixed grit/calcium/charcoal at all times.


We typically use Manna Pro Poultry Grit (insoluble crushed granite) to mix with a quality oyster shell. Grit is required for all poultry consuming anything besides layer feeds, such as scratch grains, clippings, or food scraps. Commercial feed breaks down quickly in the digestive tract and typically does not require grit to properly digest. However, because poultry do not have teeth, they must have grit in their gizzard to grind what they have consumed. Without grit there is a high risk of developing impacted or sour crop (not just in the crop) due to things getting backed up and not passing through properly. Even pastured poultry should be provided grit to ensure there is proper sized grit particles for their consumption.5 Additionally, grit helps better digest and get the most nutrients out of what the bird has eaten.

As an added perk to our free choice grit/calcium, we also add a small amount of activated charcoal. For centuries, people have utilized charcoal as a detoxifier to bind toxins & harmful chemicals and prevent them from being absorbed in the body. Activated charcoal works through a process called adsorption where its extremely porous surface binds to toxins, chemicals, heavy metals, and more as it moves through the digestive system and is eliminated naturally. Activated charcoal should be used in moderation because, unfortunately, it can also bind to good chemicals and reduce nutrient absorption.6 We offer this in our free choice mix because we believe that our birds naturally know when and how much to consume.

Disclosure: Mo’s Mini Farm offers this information as a guide to poultry care and as an opinion based on the best reputable information available to us. We attempt to reference factual information from reputable books, websites, and scientific publications. We always encourage others to do their own research which is why we provide footnotes and references for you to utilize.

For overall information regarding all things chicken health related, we recommend The Chicken Health Handbook by Gail Damerow (2015, 2nd edition; Story Publishing). The following sections were referenced for the above information: “Medullary Bone”, pg 70; “The Calcium Connection”, pp 116-117; “Grit for the Gizzard”, pg 40; and “Tale of Two Stomachs”, pg 80.

1 “Nutrient requirements of egg laying chickens”,  2018 Poultry Hub: http://www.poultryhub.org/nutrition/nutrient-requirements/nutrient-requirements-of-egg-laying-chickens/

 2 M. M. Bain, Y. Nys, and I.C. Dunn, “Increasing persistency in lay and stabilising egg quality in longer laying cycles. What are the challenges?”, May 3, 2016; published in British Poultry Science, 57(3): 330–338. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4940894/#S0007title (May 23, 2016)

3 T. G. Taylor, “How an Eggshell Is Made”, March 1970; published in Scientific American, 222:88-95. https://accounts.smccd.edu/case/biol215/docs/eggshell.pdf

4 R. Meyer, R. C. Baker, and M. L. Scott – Department of Poultry Science, Cornell University, Ithaca, N. Y. 14850, “Effects of Hen Egg Shell and Other Calcium Sources Upon Egg Shell Strength and Ultrastructure”, August 28, 1972; published in Poultry Science, 52: 949-955, 1973. http://citeseerx.ist.psu.edu/viewdoc/download?doi=

5 Tiffany Towne, “Grit for Chickens: When in Doubt, Put it Out”, October 18, 2017. https://countrysidenetwork.com/daily/poultry/feed-health/grit-for-chickens-digestive-health-when-in-doubt-put-it-out/

6 Maat van Uitert, “Charcoal For Chickens”, November 7, 2016. https://www.hobbyfarms.com/charcoal-for-chickens/

Additional Resources:


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