The successful re-breeding of suckler and dairy cows is vital to the economics of farming cattle. The gestation length of cows is 280-290 days, so the optimum interval after calving for the subsequent mating is 75-85 days if a 365 day calving interval is to be achieved. Average calving rates to a given service are of the order of 0.5 to 0.6 (Diskin and Sreenan, 1980). Most practical management strategies allow cows to be mated during a 7-10 week period (giving 3 or 4 opportunities to show oestrus). The chance of conception improves with each oestrus cycle following calving, so the earlier a cow begins to show oestrus activity after calving the more likely she is to be successfully mated, because the early, less fertile oestrus cycles, are likely to happen before the planned mating period.