Broiler breeders females need calcium for the formation of the egg shell. If the shell is formed, the calcium is taken from the blood. If there is not enough calcium available, the bird will use calcium that is stored in the bones. This calcium is stored together with phosphorus in the bones, and if the calcium is released the phosphorus is released as well.

As the formation of the egg shell occurs mainly during the dark period, the calcium supply in the blood should be highest in this period. If there is not enough calcium available, the bird use the calcium from the bones, and will restore this calcium during periods when there is calcium availble. However, with this calcium also phosphorus is needed to form the original calcium/phosphorus structure in the bones. If this phosphorus is not availalbe, the calcium can not be build in, and the bones will get weaker.

As the birds form the egg shell in the night period, this is the time when the calcium is needed. If we feed as late in the day as possible, there is a good change that there is still calcium available for shell formation in the dark period, especially if we use a course calcium source as oyster shells or calcium grit.

If we have to feed early in the morning, most of the calcium will be gone from the blood by the time the shell is formed, especially if we give a fine, highly soluble calcium source. By itself this is not a problem as the bird can use the calcium from the bones and build that calcium back in the bones the next time feed is given.

But as the birds needs phorsphorus to build in that calcium, we have to realise that the phosphorus level in the diet will be more critical when feed a highly soluble calcium source, and especially if we feed early in the morning. If we feed a more course calcium source and/or we feed in the afternoon, less phosphorus is needed in the diet.