Influence of limestone particle size and phytase enzyme on performance, egg quality, and total tract retention of Ca in commercial laying hens

The aim of this study was to evaluate the effect of limestone particle size and phytase enzyme supplementation on production performance, egg quality traits, and coefficient of apparent digestibility of calcium of commercial laying hens. A total of 216 laying hens were randomly distributed in six treatments, and six replicates per each. The design consisted of a factorial arrangement of three particle sizes of limestone from the same source (fine (<0.5 mm), medium (0.5-2 mm) and coarse (2-4 mm)) and two phytase levels (0 and 300 (FTU)/kg) in a completely randomized design. Egg production and egg mass were not influenced by experimental diets. Phytase inclusion numerically increased egg weight in second (P=0.06) and whole trial period (20-26 wk, P=0.1). Fine and coarse particles of limestone increased shell thickness (P < 0.01), compared to medium particles, also phytase supplementation improved shell percent (P < 0.01). Fine and medium sizes of limestone resulted in a higher apparent digestibility coefficient of Ca compared to coarse size ((P < 0.0001),0.43, and 0.45 vs. 0.33). Diets containing medium limestone size showed lighter yolk color than those containing fine and coarse particles (P˂0.05). Medium size of limestone significantly increased digestibility of DM compared to fine and coarse particles. In conclusion, productive performance of laying hens did not respond to limestone particle size, however, fine and medium sizes of limestone through enhancing effect on Ca digestibility, is more suitable than coarse particles for practical applications in young layers.