Hens of the laying hybrid Shaver Starcross 288 were used in two consequential experiments. The first experiment (for 10 months) was conducted on hens at the age from 18 to 60 weeks (1<sup>st</sup> cycle; <i>n</i> = 50) and moulted hens after 10 months laying from 70 to 110 weeks of age (2<sup>nd</sup> cycle; <i>n</i> = 50). For the second experiment (for 8 months) in the 1<sup>st</sup> cycle 18 weeks old hens were bought (<i>n</i> = 60) and hens from the first experiment after moulting were used in the 2<sup>nd</sup> and 3<sup>rd</sup> cycle (<i>n</i> = 32 and <i>n</i> = 28). During the experiments eggshell qualitative parameters and calcium utilisation (%) were determined. The results indicate that the percentage of eggshell decreased with the increasing number of cycles. The significant correlation of the eggshell proportion and the age of hens was higher in long, advanced cycles (<i>P</i> < 0.05). The shell strength in both experiments decreased slightly in relation to the age of hens and repeated laying cycles. The shell deformation had an opposite trend to the shell strength, i.e. it increased with the increasing laying. The average shell thickness showed a decrease in the repeated laying cycles, more significant in shorter cycles (<i>P</i> < 0.01). The specific shell gravity maintained balanced values both in cycles and by the age of hens. The shell density in all cycles in both experiments showed a decreasing tendency in relation to the age of hens. In dietary calcium utilization for the shell formation a significant decrease was recorded in the second cycles after moulting (<i>P</i> < 0.05).
Effects of various levels of dietary calcium during pregnancy on maternal calcium utilization and fetal growth in rats.