Rearing entire male calves for beef is becoming more common, but there is concern that sexual behaviour among bulls may be economically damaging and that there may be aggression between bulls and against stockmen. Behaviour has only been studied in detail in mature bulls or under disturbed conditions.

To compare the development of behaviour in stable groups of Friesian-Holstein bulls and steers prior to sale at one year old, and to assess implications for management.

Twelve bulls and 12 steers were held in an open-sided house at 5 m2/calf, with2 ad libitum hay, concentrates and water. From 5 to 10 months old, each calf was observed for one hour each month and all sexual and aggressive interactions between calves were recorded. At 6½ months, the pen was divided into halves, separating bulls and steers. This reduced the frequency of all interactions. At 10 months, reaction to humans was assessed on a scale from 1 (approach strongly) to 5 (avoid strongly). Bulls and steers were compared on all measures by the Mann-Whitney non-parametric statistical test (* = P < 0.05, ** - P <0.01).