Synchronisation can help to tighten up the calving pattern of heifers. \ Donal O'Leary As most livestock farmers will be aware, a healthy cow or a post pubertal heifer will have a heat (oestrus) every 21 days or so. I say “or so” because the actual normal range is 18 to 24 days. On the laws of probability, if you have 100 females in a group, between four and five will come into heat every day over the 21-day cycle. The purpose of a synchronisation programme is two-fold. Firstly it condenses the animal’s naturally occurring heats into a shorter period and, secondly, hormone treatment can bring forward a heat and get an animal back cycling sooner.