Intersucking, i.e. sub-adult or adult cattle sucking at the udder of herd-members, is a problem often reported in dairy herds that leads to udder health problems. The causes of intersucking are virtually unknown, and until now all available countermeasures treat only the symptoms of this behavioural problem. We examined (I) whether intersucking occurs prior to weaning and continues thereafter, and (II) whether its establishment is related to the feeding management during weaning. On 10 dairy farms, we observed the sucking and feeding behaviour of a group of dairy calves (2-5 animals per farm, 38 in total) 1 week before and 1, 5 and 9 weeks after weaning. Calves were weaned off milk at an average age of 21 weeks (S.D. 4.7). We analysed the calves' food rations in order to estimate the adequacy of their energy density. Thirty-five (92%) of the 38 calves performed intersucking already before weaning, with the frequency being highly variable between individual calves (x(med)=5.0 bouts/day, range 0-24.7). Calves with a high intersucking activity before weaning were likely to continue intersucking after weaning (P<0.05). Before and after weaning intersucking could be observed at nearly every time of the day, but was most frequent around feeding. Calves with a long feeding duration (x(med)=260min, range 186-374) showed less frequent intersucking 1 week after weaning (P<0.05). In addition, the frequency of intersucking increased with decreasing adequacy of their rations energy density (r=-0.88, P<0.01), and the highest intersucking activity was recorded on farms with restricted availability of food. The results indicate that intersucking occurs already before weaning, and its establishment may be prevented by a ration and a feeding management that ensures an optimal transition from pre-ruminants to ruminants.