Poultry Deciding on calcium level, source, form in layer feeds
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Deciding on calcium level, source, form in layer feeds

Author Ioannis Mavromichalis, publish date Saturday. January 14th, 2017

Deciding on calcium level, source, form in layer feeds

A strong egg demands high levels of feed calcium to be provided in the right form and at the right time each day. | Stephanie Frey, Dreamstime.com

Calcium is a sensitive nutrient in layer feeds requiring careful design during formulation and ingredient selection.

Calcium in layer feed is a popular topic among nutritionists and producers. Whereas for other animals calcium is just another nutrient specification that needs to be monitored during feed formulation, this is not the case for layers. Here, an undersupply or oversupply causes problems that affect productivity, bird health and ultimately the profitability of the operation. Picking the right calcium level to use in any layer feed is just the first step. The right ingredient to supply this nutrient must also be evaluated, and even more, the physical form of this ingredient is a matter of contention among experts. But let’s discuss matters in sequence to understand how complex this issue really is for layers.

Dietary levels

Most layer feeds are formulated with calcium levels between 3 and 4 percent. Although this might seem excessive, it must be noted that an eggshell contains about 2 grams of calcium, a hen consumes about 100 grams of feed daily, and the efficiency of feed calcium utilization is about 50 percent. Younger layers have a better efficiency in extracting calcium from their feed, but as they age, this decreases. Thus, diets for older hens contain slightly higher calcium levels that might reach 4.5 percent in some cases.

Calcium nutrition in layer hens deserves close attention as it affects their overall productivity.


If the hen receives less than her daily requirements in calcium she will draw from her bone reserves. This is enough to sustain egg production for only a couple of days and produces eggs with a thinner eggshell. Then she stops laying eggs until these reserves are replenished. Long-term, her bones become frail, and the layer-fatigue syndrome causes high rates of culling. Keeping the hens well stocked up with calcium reserves during the entire laying cycle is paramount and begins during the pullet grow-out period.


It would be easy if we could just feed layers an excessive amount of calcium just to ensure they never run out of it. This would ensure continuous egg production and maximal bone strength. Lamentably, too much calcium has many negative effects, the most notable being kidney failure (kidneys being the organ through which excess calcium is excreted). Furthermore, excessive amounts of calcium interfere with lipid and mineral digestion and metabolism. As such, it is important to find the minimum requirement that ensures maximum productivity without compromising eggshell integrity.


Crystalline (marble) or amorphous (mined) limestone is the least expensive source of calcium, and it is used extensively in most animal diets worldwide. It also tends to be extremely variable as it contains between 32 and 38 percent calcium. This variation can be critical for layers, because if it is not accounted for during feed formulation it might cause an undersupply of calcium. Limestone in most formulation systems is taken to contain 38 percent calcium, which is the theoretical level of calcium carbonate, the form of calcium in limestone. In addition, limestone contains impurities (other minerals) that can cause further problems in eggshell quality.

Calcium carbonate and oyster shells

Feed-grade calcium carbonate is a pure source of calcium that is guaranteed to contain 38 percent calcium. It is industrially produced and is more expensive than natural limestone. But, for sensitive feeds such as layer feeds, it offers a degree of security that cannot always be found with limestone. Perhaps this variability of calcium in limestone is why oyster shells are often found to be of superior value, but this may have been confounded by the differences in particle size and particle hardness between these two sources.

The daily calcium cycle

The hen receives the majority of her daily ration early in the morning so as not to interfere with egg laying, which happens right afterwards. Calcium in the feed is absorbed within 30 minutes and is used to replenish bone reserves, with any excess being excreted. Nevertheless, the highest calcium requirements are during the dark hours of the day when eggshell calcification intensifies. During these hours, the hen relies on bone reserves and any remaining calcium in her gut. This is why some feed extra limestone during the later hours of the day, or prefer to use a coarser particle size of limestone to reduce its rate of absorption.

Particle size

Finely ground limestone (less than 1mm in diameter) is absorbed immediately after it is consumed. Larger particles remain in the crop and the entire gastrointestinal tract for longer as they are not easily processed. Whereas finely milled limestone is needed to replenish bone calcium reserves, coarse limestone particles are ideal to provide calcium during the long, dark hours during which eggshell calcium is deposited. Most experts would recommend a ratio of 60:40 coarse to fine limestone in layer diets.


For calcium to be absorbed, first it must be dissolved in the gut. Thus, solubility is important, and it depends on gut pH and on the physical form and size of limestone. First, the lower the pH (more acidic), the higher the solubility. This is why some diets for aged hens may contain organic acids. Solubility is also affected by how hard limestone is: marble is harder than amorphous grit limestone. And, of course, the larger the particle size, the less soluble it will be. However, these observations, which are true for in vitro testing, are reversed once limestone is ingested. In the gut, larger and harder particles will be retained longer, causing overall solubility to be higher than in fine and soft particles that pass through the digestive tract faster.

In brief

Calcium nutrition in layer hens deserves close attention as it affects their overall productivity in terms of number of eggs produced, eggshell strength and hen bone health. Deciding on the right level of dietary calcium is as important as finding the right source and form of calcium.

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