Heifers were reared separately on wet fermented (WF) (grass silage based) or dry unfermented (DU) (straw based) diets. Clinical lameness was recorded and physical claw attributes were measured regularly, up to six months after calving at approximately two years of age. Two months before calving, some aspects of animal behaviour and the properties of slurry were studied. The incidence of lameness due to claw horn lesions was significantly higher (P < 0.01) in WF than in DU. WF suffered more serious heel erosion prior to calving (P < 0.05) and had softer claw horn at two (heel and axial sole) out of five sites throughout (P < 0.01 and P < 0.05). The slurry produced by WF had lower total solids content (P < 0.05) and viscosity (P < 0.001). WF spent longer standing and feeding (P < 0.001). It is proposed that standing for longer in less viscous slurry contributed to softer claw horn and more severe heel erosion, predisposing WF to claw horn lesions causing lameness. Reducing contact with low dry matter slurry is recommended for improving claw health.