The Influence of Two Dietary Calcium and Phytase Levels on the Performance of Broiler Chickens Challenged with Necrotic Enteritis

Calcium is an important cation in chicken diets, being the most abundant element in the body of the chicken. Its functions include: mineralization of bones, blood clotting, enzyme activation, neuromuscular function, muscle contraction, and intracellular signaling. However, high dietary Ca is a limiting factor for phytase efficacy and the formation of insoluble Ca-phytate complexes decreases Ca and P availability. Furthermore, Williams (2005) has shown high dietary Ca concentration in the gastrointestinal tract to be associated with pathogenesis of necrotic enteritis (NE). This study investigated the hypothesis that high dietary calcium would decrease phytase efficacy and decrease performance in chickens either challenged or unchallenged with NE. Ross 308 male broiler breeder (n=768) were randomly distributed to 8 treatments in a factorial arrangement. Factors were: calcium level (0.6 or 1.0%), phytase level (500 or 1500 FTU/kg) (Quantum BlueTM, AB Vista, Malborough, UK) and NE challenge (no or yes). There were 48 pens, 16 birds per pen and 6 replicates per treatment. Performance was measured weekly with the exception of feed intake (FI) and feed conversion ratio (FCR) which were not measured on d 21. The FCR was corrected for mortality (FCRc). Half of the birds (384) were challenged with 5000 unattenuated sporulated oocysts each of E. acervulina, E. brunetti and E. maxima (Bioproperties Pty Ltd) on d 9, and 108 CFU per mL of C. perfringens strain EHE-NE18 (known to express NetB toxin, CSIRO) on d 14 and again on d 15.

Body weight was reduced as a result of challenge for every time point measured post challenge (P < 0.05), being 10.7% lower on d 42. Challenge also increased FCRc on d 28 (P < 0.001) and d 35 (P < 0.01). There was no Ca effect on body weight or gain at any time point (P > 0.05). Low and high Ca FCRc was on d 14, 1.205 vs 1.176 (P < 0.05), d 28, 1.556 vs 1.507 (P < 0.05) and d 35, 1.478 vs 1.399 (P < 0.05). Low and high Ca FI was: d 28, 2259 vs 2161 g (P < 0.05) and d 35, 3205 vs 3052 g (P < 0.05). Body weight (BW) was higher in birds fed the high phytase level compared to the low phytase level: d 14, by 23g (P < 0.05), d 21, by 41g (P < 0.05), d 28, by 66g (P < 0.05), d 35, by 100g (P < 0.05), d42, by 69g (P < 0.05). Birds fed the high phytase level had greater livability than those fed low phytase on d 0-21, 89.9 vs 92.3%, (P < 0.05). Birds fed the diet with high phytase and high Ca had the lowest FCRc, d 7, 1.079 (P < 0.05) prior to NE challenge. Birds not challenged with NE and fed high phytase and high Ca had the highest weight gain on d 42. The lowest weight gain was observed in NE challenged birds fed diets with low phytase and low Ca on d 42.

The results reject the hypothesis that high calcium decreases phytase efficacy and performance of birds challenged or unchallenged with NE. The result evaluated as FCR (FI to live bird wt) resulted in the same conclusion.

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