1. Analyses are presented of 1628 calvings from 397 pedigree Hereford cows kept in three herds whose records were made available by their owners. In addition a further 541 calvings were examined from a sample of 100 cows taken at random from the herdbook entries.
2. Mean age at first paternity for 71 homebred bulls was 25·6 mo. Mean age at first calving varied between the three herds from 29·5to 32·8 mo, with 39·7 mo for the random cows.
3. Random herdbook cows produced an average of 3·3 calves per cow compared with 4·2 from those in the three herds; these latter cows had an average calving interval of 13·1 mo.
4. The pattern of cow and calf disposal varied partly according to the differences in herd age and culling policy. While one herd retained more than 80% of heifers and subsequently sold half of them after two or three calvings for breeding, another retained only one-third but kept these for their full working life. Similarly the percentage of bull calves sold for breeding varied between 43 and 83%.
5. The net reproductive rate, or number of calves weaned per 100 cow-herd-years was 74, 88 and 67 for the three herds, and 66 for the random herdbook cows. The mean generation intervals were 4·1 yr for sires and 5·8 yr for dams whose breeding life in the herds had been completed.
6. The biases in reproductive statistics estimated from herdbook entries were examined, and it was concluded that such data can be of use where better herd records are unavailable.