Comparison of changes in the bone mineral content and egg shell weight and egg shell ratio in brown and white egg layers during the first egg laying period

In this experiment computer tomography (CT) was used to follow the changes in the bone mineral content of laying hens and to examine its correlation with egg shell quality in brown and white egg layers during the first egg laying period. The experiment was carried out with 45 TETRA SL (brown egg layer) and 45 TETRA BLANCA (white egg layer) hens, which were scanned four-weekly by a Siemens Somatom Emotion 6 multislice CT scanner between 20 and 72 weeks of age. The CT measurements consisted of overlapping 10 mm thick slices covering the whole body of the hens. The estimation of the bone mineral content (BMC) was perfomed by scanning phantoms with hydroxyapatite densities equal to 0 and 200 mg/cm3 and Hounsfield units of the bones (101-550) were linearly converted into hydroxyapatite densities. Based on the results it was established that changes in the bone mineral content of the laying hens showed an increasing tendency during the experimental period in both examined genotypes. Changes were parallel in both of the examined genotypes, but the measured values were mostly higher in the TETRA BLANCA hens than in the TETRA SL layers. Similar tendency was observed also in the changes of the egg shell weight in both genotypes. However, despite of the similar changes in bone mineral content and egg shell weight, only very low correlation was found between these traits in both examined genotypes (r=0.118 in the TETRA SL and r=0.173 in the TETRA BLANCA hens, respectively). Based on the results it was concluded that computer tomography seems to be useful tool for the in vivo examination of changes in the bone mineral content of laying hens. However, the low correlation coefficients between the bone mineral content and egg shell weight in this study indicate that instead of the whole bone structure of the hens only the examination of those bones seems to be needed, which are in close connection with the egg shell formation