Adapted from 'Climate change and risk of thermal stress in dairy cattle' by Roger Jones, CSIRO Centre for Atmospheric and Marine Research.
The temperature-humidity index (THI) is a heat index threshold suited to dairy cattle physiology. The diagram below represents THI ranges and corresponding thermal stress levels.
The example graph shows the THI during the summer period of 1998/9. Through most of this period, at the location in the study, the index exceeded mild stress thresholds and moderate stress thresholds for much of the time. In order to maintain milk production, stress minimisation was required for an unusually long period of time. Extra shade and shelter and ample water supplies reduce the impact of heat waves, although protracted extreme heat can begin to exceed the coping range of the dairy cattle. Additional measures may be more costly and if required in many years rather than a few, can begin to affect the long-term profitability of the enterprise.
Farm management and profitability are based around a historic frequency of exceeding stress thresholds (a coping range). In a stationary climate (one in which there are no long-term trends), the chance of experiencing extreme heat events in any decade does not vary much from decade to decade and the enterprise builds its operations around that expectation.
If a region experiences a warming trend over a decade or longer, there will most likely be more events outside the coping range and hence higher frequency of stress and higher costs.
It can therefore be seen that while at first assessment it may seem that an enterprise may be able to cope with a small increase in average temperature, planners must also take into account the frequency of extremes and how that impacts on their enterprise.
This page is produced with the support of Managing Climate Variability - a consortium of primary industry research and development corporations.