| Within the present study, we examined the influence of the temperature-humidity index (THI) as an indicator of heat stress on calves. Associations between the THI and feed and water intake, and behaviour could be detected.
Daily milk intake depended on age, but also increased with increasing THI. This finding shows, how important it is to provide sufficient quantities of milk during stressful situations. There was a clear association between THI and the times of main milk consumptions. With increasing THI calves more and more shifted drinking times to cooler hours. This should be taken into account when calves are fed restricted amounts of milk. Particularly evening feedings often take place during hours with high THIs. As later evening or repeated evening and night feedings are not practicable, ad libitum milk feeding should be implemented. Drinking speed, that decreased with increasing THI.
Interesting findings have been made regarding lying behaviour. We found, that the THI had no influence on so called social lying. Another interesting finding was, that calves increased lying in a nest area compared to an open area, although the average THI was higher in this area.
Another parameter influenced by the THI was water intake. On many farms calves still have no free access to water during the first weeks of live. In combination with the fact, that many of these calves are fed milk restricted, these animals have no chance to balance their hydration status dependent on need.
During the study calves showed a high individuality, that was especially pronounced, as calves were group housed early and fed ad libitum.
In conclusion, as in other studies on cows and calves, it was not possible to define one THI cut-off for heat stress in pre-weaned calves. Reactions on rising THIs are individual, gradual, and complex. Changes were already apparent at relatively low THI values of less than 60. Consequently, often suggested cut-offs for adult cattle at 68 0r 72 should only be used with caution.
These results showed ‚that it is important to monitor calves individually and continuously. This can be achieved using various digital technologies, such as accelerometers, automatic feeding systems, and computer vision supported by artificial intelligence. However, these technologies are currently not well developed and independently evaluated for calves. Given the complexity of heat stress, it is likely that several technologies will need to be connected. |