Effects of separate calcium feeding on laying hens selected for low (R-) or high (R+) residual feed consumption

The experiment was carried out with a sample of females from the 21st generation of lines R- and R+ selected divergently for residual food intake in the laying period. After a breeding period of 18 weeks the hens of each line had been distributed among climatic rooms into two groups, one fed with a complete commercial feed (control group) and the other one with both a low calcium feed and oyster shells given in two separate troughs (treated group). Egg production was recorded during 77 days, egg and shell traits were obtained during the third and fourth weeks of the experiment, and the voluntary consumption of feed was measured over a period of 28 days. The treatment had a significant effect for both lines on average egg weight (p < 0.05), shell weight (p < 0.01), shell thickness (p < 0.01), and albumen thickness (p < 0.05). However, the line X treatment interaction was significant for yolk weight (p < 0.05) and voluntary calcium consumption (p < 0.001), indicating that the under-consuming line (R-) showed a better response to separate calcium feeding with a 40% decrease of its residual feed consumption, and better egg and shell qualities. The advantage of line R- might be related to the expression of a specific calcium appetite which is masked in line R+ which ingests excess nutrients.