Limestone Powder in Poultry Feed: Particle Size Matters

Limestone powder is the most common calcium source in poultry feed worldwide. It’s cheap, consistent, and readily available. But not all limestone is created equal. The particle size of limestone has a massive impact on how well birds utilize the calcium.

The Particle Size Problem

When I talk to feed mill managers about limestone, the question I always ask is: what is your particle size distribution? Most cannot answer. They’re buying what the supplier delivers, usually a fine powder (less than 0.5 mm) because it mixes well and doesn’t segregate in the feed.

But fine limestone dissolves too quickly. It spikes blood calcium immediately after feeding, but the blood levels drop off during the night. For laying hens that are forming eggshells overnight, this is a problem.

The ideal limestone particle size for layers is 2-4 mm, with no more than 10% passing through a 0.5 mm sieve and no more than 10% retained on a 4 mm sieve.

For broilers, the ideal is slightly different. Because broilers grow fast and have high calcium requirements for bone development, a mix of particle sizes works best. I recommend:

Broiler starter: 50% fine (<0.5 mm) + 50% medium (0.5-2 mm)
Broiler grower/finisher: 30% fine + 70% medium
Layers: 80-90% coarse (2-4 mm) + 10-20% fine
Breeders: 70% coarse + 30% fine

Measuring Particle Size

The best way to measure limestone particle size is with a sieve stack. The standard method uses sieves of 4 mm, 2 mm, 1 mm, 0.5 mm, and 0.25 mm. Shake 100 grams of limestone through the stack for 10 minutes, then weigh the fractions.

A good coarse limestone should have:

Less than 5% on the 4 mm sieve
30-50% on the 2 mm sieve
20-30% on the 1 mm sieve
10-20% on the 0.5 mm sieve
Less than 10% through the 0.5 mm sieve

I do this test annually with every limestone supplier. The results are sometimes surprising. One supplier labeled their product as “coarse” but 40% passed through a 0.5 mm sieve.

Calcium Content Variability

Limestone calcium content varies by source. High-calcium limestone contains 38-40% calcium. Dolomitic limestone contains 22-24% calcium plus 10-12% magnesium. Dolomitic limestone should not be used in poultry feed because the high magnesium interferes with calcium absorption.

I require a certificate of analysis with every shipment showing:

Calcium content (minimum 37%, target 38-40%)
Magnesium content (maximum 0.5%)
Moisture (maximum 0.5%)
Particle size distribution

Any supplier that cannot provide this should be replaced.

Feed Mill Issues

Coarse limestone does create handling challenges:

Segregation. Coarse particles tend to separate from finer ingredients during mixing and transport. This can cause batch-to-batch variation in calcium levels. Proper mixing time (3-5 minutes in a ribbon mixer) and minimizing vibration in finished feed handling reduces segregation.

Bridging. Coarse limestone can bridge in bins and hoppers. Larger bin openings and vibrators help. The angle of repose for coarse limestone is about 35-40 degrees, compared to 45-50 degrees for fine powder.

Pellet quality. Coarse limestone acts as a filler in pellets and may reduce pellet quality slightly. The effect is small but measurable at high inclusion rates.

Despite these issues, the nutritional benefit of using coarse limestone for layers far outweighs the handling inconvenience. Many feed mills have successfully adjusted their handling systems to accommodate coarse limestone.

Economic Impact

Switching from fine to coarse limestone costs nothing extra for the raw material. The price is the same. The only costs are potential handling system modifications and quality control testing.

The payoff comes from improved eggshell quality. A farm producing 100,000 eggs daily with 5% cracked eggs could reduce breakage to 3% by switching to coarse limestone. At $0.15 per egg, that’s $300 per day in savings.

Summary

Limestone particle size is a zero-cost adjustment that can have substantial benefits. For layer operations especially, switching from fine to coarse limestone is one of the easiest ways to improve eggshell quality. Test your limestone today. You might be surprised at what you find.