The relationship between the date of first oestrus of the breeding season and litter size when mating took place at the first, second, third, fourth or fifth oestrus was studied in 160 Polled Dorset Horn ewes which conceived to their planned mating. The ewes were maintained in one flock with vasectomized `teaser' rams at a ratio of less than thirty ewes to one ram. Checks were made twice daily for ewes in oestrus. At first oestrus, ewes were allocated to be mated at their first, second, third, fourth or fifth oestrus. Ewes to be mated were withdrawn from the flock and penned with a ram for 2 days. All livebirths, stillbirths and premature stillbirths were recorded.
The previously reported pattern of low litter size from matings at the first oestrus rising to a peak at the third oestrus was found. There was an indication that ewes exhibiting first oestrus late relative to other ewes in the flock had lower litter sizes, irrespective of the oestrus at which they were mated. Ewes which exhibited first oestrus early showed no seasonal rise in litter size. Ewes with a later onset of oestrus showed an increase in litter size with successive oestrous periods.