BLOOD ELECTROLYTE STATUS OVER THE DAILY LAYING CYCLE AND THE EFFECT OF SALINE DRINKING-WATER ON THE AVAILABILITY OF CALCIUM IN THE BLOOD FOR EGGSHELL FORMATION IN THE LAYING HEN

This study examined whether salt in drinking water reduces the availab ility of calcium in blood and thereby interferes with the process of e gg-shell formation. Blood samples were collected over the course of th e daily lay cycle in laying hens, at 2-h intervals. Blood and plasma s amples were analysed for ionised calcium, total calcium, sodium and po tassium. Birds were divided into four groups, drinking either de-ionis ed water or saline drinking water and laying either good- or poor-qual ity egg shells. Plasma total calcium levels declined over the course o f the lay cycle in all groups, with the highest mean total calcium lev els being found in the birds drinking saline water and laying poor-qua lity egg shells. The plasma concentrations of ionised calcium, sodium and potassium varied cyclically in relation to the lay cycle and there were statistically significant effects of both treatment group and ti me post-oviposition. In general, the plasma levels of these ions were lowest at 14-16 h post-oviposition. In the latter part of the lay cycl e, the ionised calcium levels were highest in the birds drinking salin e water and laying poor-quality egg shells. Therefore, it appeared tha t plasma ionised calcium was not limiting in this group and that the h igher levels of ionised calcium were indicative of reduced utilization of available calcium in these birds. The fluctuations in plasma sodiu m concentrations were very similar to those observed for ionised calci um. For plasma potassium, the lowest concentrations were observed at 1 4-16 h post-oviposition for birds laying poor-quality egg shells and d rinking either de-ionised or saline water. Lowest plasma potassium lev els occurred at 16-18 h and 18-20 h post-oviposition in birds laying g ood-quality egg shells and receiving saline water and de-ionised water , respectively. These results suggest that the reduced egg-shell quali ty observed in birds drinking saline water is not caused by a reduced availability of calcium in the blood.