Early bone growth in chickens genetically selected for a high and low growth rate.

Male chickens from lines divergently selected for fast (FGL) and slow (SGL) growth were compared for the growth of cortical bone. Morphology, histomorphometric, compositional and biomechanical properties of the tibiotarsi were analysed in both lines at 1, 8, 15 and 22 d of age. Tibial morphology (length, volume, cross-section and diameters) was similar in FGL and SGL chickens when compared at an equal body weight. Cortical area was also similar in both lines at an equal body weight but cortex porosity was higher in FGL. Tibial mineral density (ash:volume) was higher in FGL than in SGL at hatching and at 8 d of age. Biomechanical properties were lower in SGL than in FGL at 8 d of age, but then became higher in this line when compared at equal body weight. The very slow growth rate in SGL did not modify bone size when chickens were compared at equal body weight. Bone quality was modified in various ways: in SGL bone structure was strengthened by a lower porosity of the cortex while bone tissue was less mineralized up to 22 d of age. In both lines, cortical growth was slower than in commercial cross-breeds and bone quality (structure and composition) was improved compared to broilers.