In small-scale farming practice, the term grit refers to insoluble mineral material that poultry ingest to mechanically process feed in their digestive tract. It is therefore a physically acting component that supports digestion. Typically, it consists of small stones (often siliceous gravel). On the other hand, there is limestone (calcium carbonate), which also fulfills a nutritional function because it is soluble.

Appearance and identification in feed

Both grit and coarse limestone consist of small, sharp-edged particles. Their structure is firm, mineral-based, and odorless. Particle size (so-called fraction) varies depending on the animal category—from very fine grains to fragments several millimeters in size.

Color depends on origin:

• Light shades (white, gray) for limestone
• Darker or mixed shades for siliceous stones

In mash (loose) feed mixtures, they are clearly visible. Grit and coarse limestone appear as heavier, shinier, or sharper particles among other components, most often cereals. Fine limestone appears as dust. In pelleted feeds, grit and limestone are more difficult for the farmer to distinguish, as they are incorporated into the pellet structure.

Which animals is it used for and why?

The use of grit is typical primarily for poultry due to the specific structure of their digestive tract. Poultry have no teeth, so mechanical breakdown of feed occurs in the gizzard (poultry have two stomachs: the glandular stomach and the muscular stomach). Here, grit or coarse limestone act as “millstones,” helping to grind grains and fiber, thereby increasing nutrient digestibility. Without grit, digestion may be impaired, feed utilization reduced, and in some cases, gizzard dysfunction may occur.

Limestone in feed mixtures

In feed mixtures, calcium components—primarily in the form of calcium carbonate (limestone)—are very common.

Limestone is a fundamental source of calcium, which is essential for:

• Eggshell formation
• Bone mineralization
• Proper function of the nervous and muscular systems

In laying hens, calcium is one of the most important components of the feed ration.

In practice, limestone is used in two basic fractions:

• Fine fraction – Finely ground limestone dissolves quickly in the upper part of the digestive tract. Calcium thus becomes rapidly available for metabolic processes. This type is particularly suitable for meeting immediate calcium needs during the day.

• Coarse fraction – Larger particles remain in the gizzard for a longer period. In this respect, they partly act like grit. Calcium is released/dissolved gradually, which is particularly important for laying hens during eggshell formation, which takes place mainly at night. This “slow-release” calcium source ensures a stable level in the body and reduces the risk of poor-quality shells.

How much grit and limestone do poultry need?

Requirements vary depending on species and productivity. In laying hens, calcium content in the feed mixture typically ranges between approximately 3.5–4.5% Ca, corresponding to about 7–10% limestone in the complete feed mixture.

In free-range systems, poultry usually consume insoluble grit as needed, or it should be provided in an accessible location so the birds can ingest it freely.

Conclusion

Grit represents an important, though often overlooked, component of poultry nutrition. While the insoluble form supports mechanical digestion, limestone also plays a crucial role as a calcium source. The correct combination of fine and coarse fractions has a direct impact on health and productivity, especially in laying hens.

Sources used in this article are available from the author, Ing. Lucie Humeni.

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Frequently Asked Questions

1. What is the main difference between grit and limestone?

Grit is an insoluble mineral material (e.g., siliceous gravel) that helps mechanically grind feed in the poultry gizzard. Limestone (calcium carbonate), in contrast, is soluble and primarily serves as a source of calcium for the body.

2. Why do poultry need grit?

Poultry have no teeth, so feed breakdown occurs in the gizzard. Grit acts as “millstones” that help crush grains and fiber, improving nutrient digestibility.

3. What role does limestone play in laying hens?

Limestone is a key source of calcium necessary for strong eggshell formation, healthy bones, and proper muscle and nervous system function. In laying hens, calcium is one of the most important components of the feed ration.

4. What is the difference between fine and coarse limestone fractions?

The fine fraction dissolves quickly and meets immediate calcium needs. The coarse fraction remains longer in the gizzard and releases calcium gradually, which is especially important during nighttime eggshell formation.

5. How much calcium do laying hens need?

Calcium content in feed for laying hens typically ranges between 3.5–4.5% Ca, corresponding to approximately 7–10% limestone in complete feed mixtures. Insoluble grit is usually consumed according to need, especially when birds have access to outdoor runs.