The influence of limestone particle size in layer diets on bone and eggshell characteristics

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 in vivo and in vitro solubility, bone quality, egg production and eggshell quality was determined. Limestone was classified according to particle size 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 in vivo and in vitro solubility, bone quality, egg production and eggshell quality was determined. Small (0 − 1.0 mm) and large (2.0 − 3.8 mm) particles limestone from the first study was 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. One hundred and ninety eight, 17 weeks old Lohmann-Silver pullets, were obtained from a commercial egg producer and randomly allocated to six treatments (n=33/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 19 to 32 as well as week 70 of age. During weeks 24, 28, 32 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. The in vivo and in vitro limestone solubility of different limestone particles and the different distribution ratios of particles were determined during week 37 of age. At 37 and 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 first study clearly illustrated that an increase in limestone particle size resulted in a significantly (P=0.0095) increased in vivo and decreased (P<0.0001) in vitro limestone solubility. The inverse relation between in vivo and in vitro limestone solubility could be calculated from the following regression equation of: y = -0.8567x + 77.657 (r2=0.54), where x is in vitro limestone solubility. An increased (P<0.0001) excreta limestone content of small limestone particles was characterized by a decreased intestinal limestone content, indicating a lower retention time. Large limestone particles resulted in a significant decreased tibia length (P=0.0317), tibia weight (P=0.0265) and humerus length (P=0.0060) at week 37 of age. A significant positive effect of large limestone particles on tibia breaking strength (P=0.0150) and tibia stress (P=0.0419) was observed at week 70 of age. No clear tendency regarding the effect of particle size on tibia ash could be recorded. Large particle size limestone resulted in a significantly (P=0.0056) higher percentage tibia bone at week 70 of age. Egg output decrease significantly with an increase in limestone particle size at weeks 24 (P=0.0519) and 70 (P=0.0292) of age. Large limestone particles resulted in thinner eggshells (P<0.0001) at the sharp end of the egg during week 32 of age. Results of the second study regarding the in vivo and in vitro limestone solubility are generally in accordance with that of the first study. During the second study, a significant increased in vivo (P=0.0324) and decreased in vitro solubility (P<0.0001) of limestone was observed with an increase in the percentage large limestone particles in the distribution ratios. The inverse relation between in vivo and in vitro limestone solubility in this study could be calculated from the following regression equation of y = -0.9116x + 78.785 (r2=0.39), where x is in vitro limestone solubility. An increase in the percentage large particles limestone resulted in a significant higher tibia breaking strength (P=0.0123) and tibia stress (P=0.0304) at week 70 of age. However, no clearly defined tendency of different distribution ratios of limestone particles on bone mechanical properties was observed. Higher percentages of large particles limestone resulted in a significant (P=0.0007) higher percentage tibia ash at week 37 of age. However, the effect of different percentages of large particles limestone on percentage tibia ash was not clearly defined and no obvious tendency could be recorded. The effect of different distribution ratios of limestone particles on eggshell quality was generally not significant (P>0.05). The general conclusion of these studies were that irrespective of the limestone particle size and/or particle size distribution, adequate calcium for optimum egg production and eggshell quality for Lohmann-Silver laying hens between 24 and 32 weeks of age was supplied.