Degradation of Hexakisphosphate in the Gizzard and Ileum of Broilers Fed Diets with Two Calcium to Phosphorus Ratios and Phytase During Subclinical Necrotic Enteritis

Phytate is a polyanionic molecule with the capacity to chelate Ca, forming mineral-phytate complexes. The presence of these complexes decreases the efficacy of phytase. Ca-phytate formation is dictated by molar ratios of the constituents, gastrointestinal pH and the concentration of dietary Ca relative to P (Sommerfeld et al., 2018). The hypothesis of this study was that a high dietary Ca to P ratio will reduce the ability of phytase to hydrolyze phytate, which will exacerbate necrotic enteritis (NE) in broilers. A total of 768 Ross 308 male broiler chicks were randomly distributed to 8 treatments in a factorial arrangement. Factors were: NE challenge (no or yes), phytase level (500 or 1500 FTU/kg; both using 500 matrix values for Ca (0.16 %), P (0.15 %) and Na (0.035 %; Quantum BlueTM, AB Vista, Malborough, UK) and Ca level (0.6 or 1.0% Starter, 0.5 or 0.9% Grower, 0.4 or 0.8% Finisher) whilst maintaining the same level of avP (0.40 Starter, 0.35 Grower and 0.35 Finisher). There were 48 pens, 16 birds per pen and 6 replications per treatment. Half of the birds were challenged with 5000 oocysts of field strains of E. acervulina and E. brunetti, and 2500 oocytes of E. maxima (Eimeria Pty Ltd) on d 9, and 10⁸ CFU per mL of Clostridiumperfringens Strain EHE-NE18 (known to express NetB toxin, CSIRO) on d 14 and d 15. Gizzard and ileal digesta were collected at d 29 for determination of phytate and its inositol phosphate (inositol x-phosphate, IPx: IP3, IP4, IP5 and IP6) esters, based on the methods of Walk et al. (2018).

The NE challenge as a main effect increased the concentration of inositol (3.840 vs 2.404 μmol/g DM; P < 0.01) in the gizzard. Phytase inclusion at 1500 FTU/kg increased IP3 (0.870 vs 0.388 μmol/g DM; P < 0.001) but decreased IP4 (0.607 vs 2.038 μmol/g DM; P < 0.001) and IP5 (0.133 vs 0.262 μmol/g DM; P < 0.05) in the gizzard. Also, narrower dietary Ca:P increased inositol level (3.547 vs 2.697 μmol/g DM; P < 0.05) in the gizzard. In the ileum, a challenge × Ca:P interaction was detected for IP5 (P < 0.05), with wider Ca:P increasing IP5 both in challenged and unchallenged birds. Additionally, a phytase × Ca:P interaction was observed for ileal IP3 (P < 0.01), IP4 (P < 0.05) and IP6 (P < 0.01). In groups fed 1500 FTU/kg, wider Ca:P increased IP3 and IP4 compared to those fed 500 FTU/kg. The IP6 was increased by wider Ca:P both in birds offered 500 and 1500 FTU/kg phytase diets. The NE challenge as a main effect decreased IP3 (0.688 vs 1.175 μmol/g DM; P < 0.05) and IP6 (6.47 vs 10.49 μmol/g DM; P < 0.05), but it increased inositol (30.33 vs 18.70 μmol/g DM; P < 0.001) in the ileum. The results reconfirm that the majority of IP degradation occurs in the gizzard. Furthermore, these findings support the consensus that superdosing phytase diminishes the anti-nutritive effect of phytate and yields more soluble lower esters and inositol. In conclusion, though superdosing of phytase might increase IP destruction with narrower Ca:P, an accumulation of inositol in the ileum during NE might exacerbate the incidence.

ACKNOWLEDGEMENTS: This study was funded by AB Vista Feed Ingredients (UK).