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Background:The reduction of crude protein levels in diets for broiler chickens may generate economic,environmental and flock welfare and health benefits;however, performance is usually compromised. Whole grain feeding and phytase may improve the utilization of reduced crude protein diets. Results:The effects of pre-pellet cracked maize (0, 15%and 30%) and phytase (0, 750 and 1500 FTU/kg) in iso-energetic maize-soy diets with three levels of crude protein (22%, 19.5%and 17%) were evaluated via a Box-Behnken response surface design. Each of 13 dietary treatments were offered to 6 replicate cages (6 birds/cage) of male Ross 308 broiler chicks from 7 to 28 d post-hatch. Model prediction and response surface plots were generated from experimental data via polynomial regression in R and only significant coefficients were included and discussed in the predicted models. Weight gain, feed intake and FCR were all influenced by pre-pellet cracked maize, phytase and crude protein level, where crude protein level had the greatest influence. Consequently, the reduction from 22%to 17%dietary crude protein in non-supplemented diets reduced weight gain, feed intake, relative gizzard weight, relative gizzard content and relative pancreas weight but improved FCR. However, the inclusion of 30%cracked maize to 17%crude protein diets restored gizzard weight and 1500 FTU phytase inclusion to 17%crude protein diets increased relative gizzard contents and pancreas weights. Cracked maize and phytase inclusion in tandem to 17%crude protein diets increased weight gain, feed intake and FCR;however, this FCR was still more efficient than broilers offered the non-supplemented 22%crude protein diet. Broilers offered the pre-pellet cracked maize and phytase inclusions reduced AME in 22%crude protein diets but improved AME by 2.92 MJ (14.16 versus 11.24 MJ;P<0.001) in diets containing 17%crude protein. Ileal N digestibility was greater in broilers offered diets with 17%crude protein than those offered the 22%crude protein diet;irrespective of phytase and pre-pellet cracked maize. Conclusion:Pre-pellet cracked maize and phytase inclusions will improve the performance of broilers offered reduced crude protein diets.