Effect of bone strength on the frequency of broken bones in hens.

Bird weight, breaking strength of humerus and tibiotarsus and the number of bones broken during culling were recorded for four breeds of end-of-lay hens housed in battery cages. The probability of a bone being broken increased with bird weight and decreased with increasing bone strength. Bone strength increased with bird weight within each breed but the increase in strength was not great enough to prevent the extra damage suffered by heavier birds. There were differences in tibiotarsal strength between the four breeds of bird but overall no breed was more likely to suffer from broken bones than another. The rate of increase of bone strength with weight was similar between breeds and between humerus and tibiotarsus. The results show that differences in bone strength due to the type of housing system in which birds are kept are great enough to affect the ease with which bones are broken during bird handling during removal from cages at the end of lay.