This study was conducted at the poultry farm to animal science department, Faculty of Agriculture, Ferdowsi University of Mashhad, for the period from 12/8/2018 to 26/9/2018. The objective of the study was to evaluate the influence of different levels of calcium (Ca) and available phosphorus (AP) with or without probiotic (Pro) on overall growth performance and criteria of the bones of broiler chickens. Four diets were formulated and fed to broilers from days 1 to 10 with different mineral status;1) standard diet (0.96% Ca and 0.48% AP) as recommended by the Ross 308 strain guidelines; 2) Low1 (0.864 % Ca and 0.432% AP); 3) Low2 (0.768 % Ca and 0.384% AP); and 4) Low3 (0.672 % Ca and 0.336% AP). On the eleventh day, each treatment of low-Ca and AP treatments except the standard treatment (control group) was divided into six groups (low1, low2, and low3 with Probiotic (or (low1, low2 and low3 without probiotic). The bacteria concentration was (2 x 109 CFU/g). While the restriction was continued until the end of trial. Results showed that lowering the Ca and AP levels did not influence on weight gain (WG) and feed intake (FI), but the feed conversion ratio (FCR) was influenced by reduced Ca and AP levels (P=0.036). Birds fed diet containing low-Ca and AP with probiotic or without it, had a better feed conversion ratio compared with control group (P=0.01 and P=0.005) respectively. The percentages of ash, calcium, and phosphorus of tibia were decreased in birds fed the low-Ca and AP diets with or without probiotics compared with the control group. Broiler chickens fed with the low-calcium and phosphorus+ probiotics diets showed a higher percentage of calcium and phosphorus in the tibia ash compared to those fed on the same diets without probiotics. The results also showed that the length, width and relative weight of femur were not influenced by Ca and AP-deficiency or by probiotic supplementation, but the breaking strength was influenced by calcium and available phosphorus deficiency. Whereas the breaking strength has reduced in birds that suffer Ca and AP deficiency with or without probiotic compared with the control group. In conclusion, it is possible to decrease dietary Ca and AP levels by 10% or 20% during the grower and finisher phases without affecting growth performance and bone criteria.

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