Computer simulation of the dairy herd production cycle has permitted an assessment of the effects of oestrus detection rate on herd performance. Calving to first service interval is inversely related to the level of oestrus detection and in turn both these parameters govern the subsequent calving to conception interval and the herd culling rate for failure to conceive. Where the breeding season is nine to 10 months long, a poor oestrus detection rate is reflected in an increased culling rate for failure to conceive and a high mean calving index. This results in a reduction in annual milk yield and margin over purchased feed per cow. Where the breeding season is restricted to four months or less, a low level of oestrus detection is reflected almost solely in a high rate of culling for failure to conceive.