Temporal distribution of sucking behaviour in dairy calves and influence of energy balance

So far, most authors in the literature have suggested that cross-sucking in dairy calves is elicited by milk ingestion, as sucking motivation persists for an additional 12–15 min after milk intake. However, cross-sucking without temporal association to milk ingestion has also been noted in studies with longer observation times. Furthermore, it is known that energy deficits influence sucking behaviour. This study is based on the hypothesis that cross-sucking is not only elicited by milk ingestion; hunger can also reinforce sucking behaviour. Seventy-five dairy calves (in groups of 24–27 animals) were observed two times for 20 h each during the milk feeding period, 1 week after grouping and 1 week before weaning. We noted all events of cross-sucking and related them to the calves’ temporally closest visits to the milk feeder, with or without milk ingestion. The daily energy balance (consumed energy minus energy demand for growth and maintenance) was determined for each animal, and the influence of energy balance on the occurrence of cross-sucking was calculated. Thereafter, a daily energy balance was assigned to each cross-sucking bout. Of 919 cross-sucking bouts, 28.4% occurred within the first 15 min after receiving milk, and 71.6% of the sucking bouts occurred independently of milk intake. Of that 71.6%, half (36.3%) took place either during the 15 min before a visit to the milk feeder (with or without milk ingestion) or during the 15 min after an unrewarded visit to the milk feeder. The rest of the sucking bouts (35.3%) occurred without close temporal association to a visit to the milk feeder. The probability of exhibiting cross-sucking behaviour decreased with an increased energy balance (P = 0.002). Furthermore, the association between daily energy balance and time to the nearest visit to the milk feeder differed depending on the timing of the sucking bout (before or after the visit to the milk feeder) and if the calf received milk or not during that visit to the milk feeder (P < 0.001). Energy balance influenced sucking behaviour, indicating that animals with positive energy balance were less likely to perform cross-sucking and that the temporal occurrence of sucking bouts depended on energy balance. These findings show that milk-elicited cross-sucking must be differentiated from sucking bouts triggered by other motivational mechanisms, one of them likely being hunger.