Effect of dietary available phosphorus and phytase on production performance of old laying hens and tibia bone quality

Old laying hens are more prone to dietary calcium (Ca) and phosphorus (P) deficiencies as they absorb Ca and P less efficiently than younger hens. In a 2×2 factorial design, the influence of diets with two levels of available phosphorus (AP) and phytase enzyme on the laying performance, egg quality and tibia bone characteristics of laying hens at their late 2nd production phase was studied. The experiment used four treatments of 20 Hy-Line W36 hens of 140 weeks old. Treatments 1 and 2 received a 3.5 g kg-1 AP diet while Treatments 3 and 4 received a 2.5 g kg-1 AP diet for 12 weeks. Treatments 2 and 4 received 250 FTU kg-1 phytase in their diet. Laying performance, egg weight, shell weight and shell thickness were measured. On the last day of the experiment, all birds were weighed and blood samples were collected for serum Ca and P measurements. Ten birds from each treatment were sacrificed and both tibiae were excised, defatted, weighed and ashed for Ca and P contents, cortical thickness and bone breaking strength measurements. Hens fed with 2.5 g kg-1 AP diet without enzyme had the lowest body weight gain, serum P, tibia cortical thickness and tibia breaking strength among the treatments (p<0.05). Phytase enhanced body weight gain, egg shell thickness, serum P, tibia weight/body weight and tibia ash/body weight ratios, tibia cortical thickness and breaking strength, particularly in birds receiving 2.5 g kg-1 AP diet (p<0.05). Phytase may improve bone quality and strength of hens in the late 2nd laying period.