Sometimes we see some leg problems caused by too low calcification already in day old chicks, and when we do a break out we sometimes already see it on the unhatched embryos. Not only in the legs (bending in stead of breaking) but also on the beaks (soft, "rubbery" beaks), sometimes even leading to lower hatches as the embryo cant break the shell with that soft beak.

When we think about it, a possible cause might be the vitamine D3 level that the breeder passes on to the eggs. But when we check the D3 levels in the feed they often seem to be sufficient, and when it would not be sufficient one would expect problems with shell quality and perhaps even leg quality in the breeders. 

But perhaps it is not the level of D3 but the source. Nowadays we often use 25-hydroxyvitamin D3 (often referred to as Hydroxy D3 or HyD) in poultry diets. This product has advantages from a nutritional point of view as it is a more "active" metabolite than the traditional D3 sources.

However, one of the characteristics of this product is that it is more water soluble than the original D3 (which is fat soluble). This could mean that the product is not passed through the egg yolk to the embryo and day old chick as effectively as the traditional D3, and in that way causing problems in the period that the bird is not receiving vitamine D3 from its own feed intake.

As far as I know there is no scientific literature or evidence for this suggestion. But we have seen in the field that by changing the vitamine D3 source in the breeder diet from Hydroxy D3 back to more traditional D3 sources, some problems with leg and beak calcification and with it leg problems seem to disappear.