A total of 405 23-week-old ISA ® Brown layers were distributed in a completely randomized experimental design in a factorial arrangement with nine treatments consisting of three dietary calcium levels (3.5, 3.75, and 4.5%) and three limestone particle sizes (100% fine limestone (FL), 70% FL + 30% coarse limestone (CL) and 50% (FL) + 50% (CL)), with nine replicates of five birds per cage. The following parameters were evaluated: percentage of lay, defective eggs, egg weight, egg mass, feed intake, feed conversion ratio (per kg eggs and per dozen eggs), and mortality. Dietary Ca levels significantly affected lay, with birds fed diets containing 4.5% calcium producing less eggs as compared to those fed 3.0 and 3.75% Ca. Egg production linearly decreased as dietary Ca levels increased, but blood Ca levels (mg/L) increased in 28-week-old birds. The interaction of dietary Ca levels and limestone particle sizes resulted in a reduction in tibial ash Ca content as dietary Ca levels increased and as fine limestone was replaced by coarse limestone. It is concluded that a dietary Ca level of 3.75% and 100% fine particle limestone are required to maintain adequate egg production and available Ca blood level. INTRODUCTIONCalcium is the mineral with the highest concentration in the body of poultry, consisting of 1.5% of its body weight. It is more than one third of total mineral body content of an adult bird (Klasing, 1998) and represents one third of eggshell components.During lay, structural bone loss (medullary and cortical) and the subsequent development of osteoporosis are associated to the medullary bone. Medullary bones in females are shaped in response to estrogen. The role of these bones in mineral homestasis is more important than their structural skeletal function, and it is associated to bone mineralization and eggshell formation. Studies on layer skeleton using fluorochrome (Cransberg et al., 2001) showed that there is no formation of medullary or cortical bone during eggshell formation; only the shape of medullary bones are changed. However, the high mobilization of minerals for eggshell deposition may result in osteoporosis and consequently, in bone fragility.Calcium has an essential role in nutrition and calcium turnover determines the optimal calcium level for bone formation, including the tíbia, but the relationship between calcium turnover and the development of osteoporosis is still not clear (Cransberg et al., 2001

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