Coarser Soybean Meal Boosts Phosphorus Bioavailability in Broilers

Grinding soybean meal coarser — to a mean particle size of 1,577 μm — increased relative phosphorus bioavailability in chicks to 30–36%, compared with 20–25% for the finest meal tested at 466 μm, according to a two-experiment study published in the journal's Metabolism and Nutrition section.

The finding runs against the common assumption that finer grinding improves nutrient access. For feed formulators and poultry nutritionists, it signals that soybean meal (SBM) particle size deserves a place next to phytase dose and calcium level when ration phosphorus budgets are built.

How the digestibility trial was run

Experiment 1 used a 4 × 2 factorial arrangement: four mean SBM particle sizes (466, 809, 1,174 and 1,577 μm) crossed with two dietary calcium levels (0.20% and 0.75%). Ross 308 broiler chicks served as the test animals, and researchers measured ileal phosphorus digestibility and total tract phosphorus retention at 20 days of age.

At 0.20% dietary calcium, ileal phosphorus digestibility ranged from 86% to 90% and did not differ across the four particle sizes. The particle-size effect emerged only at the higher calcium level: with 0.75% Ca, the 1,577 μm meal produced the highest ileal phosphorus digestibility, significantly different (P < 0.05) from the other three particle sizes.

Calcium itself proved to be the stronger lever. Chicks fed the 0.20% calcium diets showed higher (P < 0.05) ileal phosphorus digestibility and total tract phosphorus retention than chicks on the 0.75% calcium diets, regardless of particle size. In practical terms, the well-documented calcium–phosphorus antagonism dominated, but coarser grinding partially offset the digestibility penalty imposed by higher calcium.

What the bone ash assay showed

Experiment 2 measured relative phosphorus bioavailability of the four SBM streams against a reference standard, potassium phosphate (KH₂PO₄), using a tibia bone ash assay. Crossbred chicks (New Hampshire × Columbian) received one of 11 experimental diets from 8 to 22 days of age.

Diet 1 was a phosphorus-deficient cornstarch–dextrose–SBM basal containing 0.14% nonphytate phosphorus. The remaining 10 diets supplemented that basal with either 0.05% or 0.10% phosphorus from KH₂PO₄, or with 12.5% or 25% of SBM ground to one of the four mean particle sizes.

The researchers estimated bioavailability with the slope ratio method, regressing tibia ash on supplemental phosphorus intake. The 1,577 μm meal delivered the highest relative bioavailability values, 30–36%, significantly higher (P < 0.05) than the 20–25% recorded for the 466 μm meal. That is roughly a 50% relative improvement in phosphorus value from the coarsest versus finest grind, measured where it matters structurally — in bone.

What changes at the formulation desk

For companion-animal and food-animal practitioners, the study's relevance sits squarely in poultry and swine nutrition, where soybean meal is the dominant protein source and phosphorus excretion carries both cost and environmental weight.

Three practice-level takeaways follow from the data:

• Ileal phosphorus digestibility of dehulled solvent-extracted SBM is high — 86 to 90% — when dietary calcium is held at 0.20%.
• Raising dietary calcium to 0.75% depresses both ileal digestibility and total tract retention of SBM phosphorus; the effect was significant at P < 0.05.
• Increasing SBM particle size from 466 to 1,577 μm raised relative phosphorus bioavailability by 10 to 11 percentage points on the tibia ash slope ratio assay.

The authors conclude that increased particle size may increase digestibility and relative bioavailability of phosphorus in soybean meal, and that dietary calcium level strongly influenced the ileal digestibility values. The study involved no regulatory intervention; it is peer-reviewed nutrition science aimed at feed formulation.

Whether feed mills act on it will depend on how coarser grinds interact with pellet quality and bird performance measures outside this study's scope — a question subsequent trials with grow-out endpoints will need to answer.