Limestone particle size in layer diets

English: A specific calcitic limestone source that is widely used in South African poultry diets was evaluated during two concurrent studies. During the first study, the effect of limestone particle size on bone quality, egg production and eggshell quality was determined. Limestone was classified according to particle sizes as small (0-1.0 mm), medium (1.0-2.0 mm) and large (2.0- 3.8 mm). During the second study, the effect of different distribution ratios of small and large particle sizes of limestone on bone quality, egg production and eggshell quality characteristics was determined. Small (0-1.0 mm) and large (2.0-3.8 mm) particles limestone from the first study were mixed to obtain the following five distribution ratios used in the second study namely; 0, 25, 50, 75, 100% small or large particles. The experimental protocol for both studies was the same. Initially one hundred and thirty eight, 17 weeks old Lohmann-Silver pullets, were obtained from a commercial egg producer and randomly allocated to six treatments (n=23/treatment). All birds were kept in individual metabolic cages for the duration of the study. The influence of limestone particle size and distribution ratios of particles on feed intake, body weight and egg production was determined for weeks 33 up to week 70 of age. During weeks 54, 58, 62 and 70 of age, the effect of limestone particle size and distribution ratios of particles on eggshell quality characteristics such as shell weight, percentage eggshell, eggshell calcium egg surface area, shell weight per unit surface area (SWUSA) and shell thickness was determined. At 70 weeks of age, the effect of limestone particle size and distribution ratios of limestone particles on bone dimensions (length, width and weight), bone mechanical properties (breaking strength and stress) and percentage bone ash was determined. The results of the limestone particle size study indicated that different limestone particle sizes did not have a significant influence on feed intake (P=0.6229), body weight (P=0.4189), egg production (P=0.3991), egg output (P=0.0599) and feed conversion ratio (P=0.5025). Accordingly different limestone particle sizes did not show any significant effect on mean eggshell characteristics such as shell weight (P=0.9396), percentage eggshell (P=0.3468), eggshell calcium (P=0.9367), egg surface area (P=0.3223), SWUSA (P=0.6111) and shell thickness (P=0.6663) during the entire experimental period. At 70 weeks of age large particles limestone resulted in a significant higher tibia bone breaking strength (P=0.0107) and stress (P=0.0391). No significant (P>0.05) influence of limestone particle size was found on bone length, width, weight, percentage ash and index at week 70 of age. The results of the second study illustrated that different ratios of small and large limestone particles had no significant effect on feed intake (P=0.4066), body weight (P=0.8908), egg production (P=0.2713), egg output (P=0.1457) and feed conversion ratio (P=0.2635) during the entire experimental period. No statistical differences were detected due to a mixture of different ratios of coarse and fine limestone particles on mean eggshell characteristics such as shell weight (P=0.6902), percentage egg shell (P=0.7297), eggshell calcium (P=0.6903), egg surface area (P=0.4474), SWUSA (P=0.7738) and eggshell thickness (P=0.7167) during the entire experimental period. It was concluded that the different limestone particle sizes and ratios of small and large particles limestone in the diets used during the present study, generally had no significant influence on bone and eggshell quality characteristics of Lohmann-Silver laying hens during the later stages of lay, except on bone mechanical properties.