Records of calving, excluding first calving, on 2862 cows, the daughters of 63 bulls used in AI were analysed to estimate the incidence and variation of twinning between bull progeny groups. The mean incidence of twinning was 2·8% and though the incidence increased with later parities the effect of parity was not significant. The incidence for bull progeny groups ranged from 0 to 8·8%. The intraclass correlation between parities for cows within bulls was 0·064±0·018 and a sire half sib group estimate of heritability for twinning, based on a single calving, was 0·43±0·012.

A study of 1567 lactations of 336 Friesian cows in 10 herds has been undertaken to establish to what extent the shape of the lactation curve varies between parities, cows and herds, and to assess the importance of variations in seasonality from year to year, with particular reference to herd production forecasts.

In general, cows calving the same parity at the samé time of year showed similar curves, modified only by total yield and abnormal seasonality of production. Production forecasts based on month/parity subclasses are therefore adequate, provided due account is taken of extréme variations in weather conditions or forage growth.

Data on calves sired by Charolais bulls and their contemporaries in 22 commercial herds in Northern Ireland indicate that the Charolais calves were significantly heavier than their contemporaries at weaning as well as at birth. Although the weight of dam was also significantly greater for the Charolais calves, all the evidence indicated that this was not associated with calf weight and did not, therefore, invalidate the conclusion on calf weights.

The present results suggest that although interaction between breed of sire and sex is not statistically significant for calf weights neverthe-less the superiority of Charolais sires was much greater for males than for females. The interaction was significant for daily gain to weaning. Separate analyses for the two sexes indicated that the Charolais-cross males gained significantly better than their contemporaries whereas no significant difference in gain could be detected in the females.