There is a connection between daily peaks in activity and spikes in temperature- if we focus in on the period of October, there is an obvious and close link between changes in daily group activity and changes in average temperature.
Potential reasons for this are:
- Warmer temperatures normally indicates more sunshine, which encourages more activity from the calves
- Lower temperatures at this time of year often indicates rainfall, which discourages activity
- Warm weather reduces the energy requirement to maintain body temperature, allowing more energy to be used on other activities such as play or socializing
The lower thermal neutral zone for calves of this age (12-40 days) is shown in the table below, as it regularly decreases as the calf gets older. The higher limit is around 26°C (Spain and Spiers, 1996). If the air temperature moves outside of this thermal neutral zone (TNZ) then the calf will react to help maintain its core body temperature. We will look at heat stress in more detail later on.
It appears that the colder temperature have limited impact on the calves in this project, at least in terms of activity. This is probably helped by fact several factors:
- They are housed under a roof, reducing exposure to rain and snow
- Smaller hutches in their housing provide further shelter from wind and rain
- The surrounding buildings help to prevent excess wind and draughts
We examined in a previous blog post (https://wellcalf.com/blog/impact-of-heat-stress-on-calves-part-2/) how the temperature within the shed and particularly in hutches can be much higher than the outside air temperature. This also showed that the temperature for the calves went above 26 degrees for several hours on multiple days. Now we want to look at the calf behaviour on those days and contrast it with behaviour on those days where the calf was definitely not heat-stressed.
The activity peaks in August are only slightly higher than October, but the main differences are that the peaks are more sustained (over many hours, rather than one short period) and that the night-time minimum is much higher during the August heatwave. Our tag picks up head movement as well, as some of these increased activity could indicate irritation and consequent head/ear movement.
One factor we will look into is whether the lying times were decreased during this heat stressed period, as we would expect- “Behavioral modifications observed during heat stress include seeking shade, changing posture (standing vs. lying), body alignment away from the sun, reducing locomotion during hot times of the day, and bunching to seek shade from other animals” (Roland, 2016). It seems that there could be reduced lying time even at night, when the temperature dropped below 20°C. This suggests that the discomfort of the calf could be going on for longer than just the hottest hours of the day. Our results seem to disagree with the suggestion of reduced locomotion at the hottest times, though this would require further analysis.
There is a close link between activity and air temperature for pre-weaning calves when it is not too cold or hot, though the changes are not extreme in these cases. When the calf is heat stressed, it appears to become much more active, especially at night.
During periods of cold stress, the calf seems to manage these with much less impact on its activity. In this set-up, it suggests that the calves are able to keep warm enough to stay relatively comfortable even when outside temperatures would suggest it is stressed.
We will look further next time at the impact of high temperatures on intakes and whether heat stress had a significant and prolonged impact on calf growth.
Our project has followed groups of calves from the age of 12 days through to weaning. Animal movement was measured by our Smartbell tags.
For each day, we calculated the individual calf activity and then looked at the group average. Once calves reached weaning (from 40 days) then their data was removed, so that all calves were at a comparable stage.