A simple method is presented to estimate lamb's paternity errors when natural mating (NM) complements artificial insemination (AI). This error depends on the number of days between insemination and natural mating (N), the rate of oestrus synchronisation failure (F) (or insemination failure rate, if sinchronisation data are not available), and, the number of days in which the new born lambs are considered to be sired by AI rams (IC). The necessary number of days to guarantee AI paternity is also supplied. Results are provided in easily handled tables.
Having in mind the simplicity of the model, the paternity errors provided here should be considered as mere indicators aiming at helping in deciding how many days after insemination natural mating sires can be used in order to avoid paternity error above a predetermined maximum value.