By Michael Wellington
The introduction of Black Angus and Wagyu cattle into extensive northern Australia has been watched with keen interest. As I was required to deliver a project as part of my livestock consulting internship, I spent some time looking at differences in adaptability among breeds due to renewed interest in this space.
I found that most heat stress research has focused on the feedlot and dairy industries. This is logical because these industries have some level of control over the environmental conditions cattle experience. Also, feedlot and dairy cattle are more susceptible to heat stress due to higher metabolic heat production. Conversely, beef cattle in extensive environments have greater scope to regulate their heat load through behaviours such as shade and breeze seeking.
There are several heat stress measures that account the effect of factors beyond just temperature on the heat load of cattle. For example, the Heat Load Index (HLI) is one means of measuring the combined impact of a range of weather conditions such as temperature, humidity, wind speed, and solar radiation on an animal’s heat load.
Differences in heat tolerance between breeds and classes are indicated by the point at which a heat load begins to accumulate. Figure 1 shows the Upper Heat Load Index (HLI) threshold for several breeds. Black Bos taurus breeds begin to accumulate heat load at a HLI figure of 86, while for 100% Bos indicus animals the figure is 96. It follows that the greatest determinant of tolerance is the Bos indicus/ taurus content of the animal. This is because Bos indicus cattle have physiological heat tolerance adaptations such as a dewlap and large ears which dissipate heat away from the body. However, other factors such as coat length and coat colour also have an impact on tolerance.
Figure 1: Upper Heat Load Index threshold (point at which cattle begin to accumulate heat load) for various genotypes, baseline level is 86 (adapted from Gaughan et al. 2008).
Due to its complexity, the HLI is more applicable to intensive rather than extensive environments. Another measure, the Temperature Humidity Index (THI), provides a good indication of the stress experienced by an animal and can be derived from commonly available temperature and humidity measurements, as the name suggests.
I used historical Bureau of Meteorology data to generate average daily maximum and minimum THI figures for 57 locations across Australia. This data can be used to answer questions about the suitability of breeds in certain locations. For example, extensive beef cattle veterinarians have suggested that THI figures greater than 79 for more than two weeks in the month of calving will have considerable impacts on cow and calf mortality. I used the data I generated to show the number of days a location’s maximum THI is above 79. This is mapped in Figure 2.
Figure 2: Map of Australia showing the expected number of days per year when the maximum THI would be greater than 79 for 57 locations.
This map shows that all of northern Australia experiences more than fourteen days of THI greater than 79, although these would not necessarily be consecutive.
While this map illustrates the widespread nature of the heat stress issue, it is arguably the average monthly/daily minimum, rather than maximum, THI maps which are more important in informing the distribution of cattle genotypes. This is due to the effect of accumulated heat load. If thermoneutral conditions are not reached overnight, then cattle accumulate heat load over several days. Research has shown that an extended period of hot nights is when most widespread heat stress fatalities are observed. Analysis of the monthly minimum THI figures indicated that much of northern Australia is suitable for Bos taurus genotypes with the exception of humid, coastal regions such as Cape York, the northern Gulf, Top End, and coastal parts of the Kimberley and Pilbara. In any case, understanding your temperature humidity profile and the tolerance of potential breeds to your circumstances is important if you are looking to introduce new genetics.