Effects of feed particle size, calcium concentration and phytase supplementation on InsP6 degradation in broiler chickens fed pelleted diets.

1. The objective of the trial was to study the single and interactive effects of feed particle size in pelleted feed, dietary calcium (Ca) concentration and microbial phytase supplementation in broiler chickens. The studied traits were myo-inositol (1,2,3,4,5,6) hexakis (dihydrogen phosphate) (InsP₆) degradation, pre-caecal digestibility of phosphorus (P), Ca and amino acids (AA) and retention of P, Ca and nitrogen (N).2. Male Ross 308 broiler chickens were housed in metabolism units in groups of 10 and allocated to one of eight diets with seven pen replicates per diet. The 2 × 2 × 2-factorial arrangement included coarse and fine feed particle size (309 or 222 µm), low and high Ca concentration (4.9 and 7.2 g/kg) and without or with phytase supplementation (1,000 FTU/kg).3. Pre-caecal InsP₆ disappearance was higher with coarse than fine feed particle size when no phytase was added (54 vs. 48%) but not when phytase was added (74%; p = 0.046). High dietary Ca feeds decreased pre-caecal InsP₆ disappearance (67 to 59%) and P digestibility (65 to 55%; p < 0.001). Gizzard pH was lower with coarse than fine feed particle size and higher with high Ca than low Ca (p < 0.001). Pre-caecal digestibility of most AA was approximately 3.5%-points lower with high Ca without phytase compared to the other treatments (p ≤ 0.047). Coarse feed particle size caused higher pre-caecal AA digestibility than fine particle size (~2%-points; p ≤ 0.031). InsP₆ disappearance in the crop increased at high Ca concentration when phytase was added (22 vs. 37%; p = 0.011).4. Coarser feed particle size in pellets increased gastrointestinal InsP₆ degradation and nutrient digestibility, likely owing to effects on the gizzard functions. Additional Ca supply exerted antinutritive effects that was not compensated for by using coarser feed particles.