Growth rates vary from horse to horse but most feet grow at a rate of about five to eight millimetres in a month (more in the summer or with increased exercise). The length of time required to grow enough new wall horn to push out the old nail holes is often outstripped by the need to re-shoe the horse to maintain mechanical efficiency. A regular shoeing interval of five to seven weeks is fairly common in North America. In Europe, the shoeing interval is often a little longer, around eight to ten weeks. The shorter interval presents the option of restoring mechanical efficiency sooner between each shoeing while the longer interval presents fewer nail holes in the wall.