Vargas-Rodriguez , L. et al. 2015. Effect of Citric Acid, Phytase and Calcium in Diets of Laying Hens on Productive Performance, Digestibility and Mineral Excretion. International Journal of Poultry Science. 14, 4 (Mar. 2015), 222–228. DOI:https://doi.org/10.3923/ijps.2015.222.228.