ENTERING summer is a good time for dairy farmers to assess their heat stress management programs as heat stress can impose a significant financial and welfare cost on a business.
Heat stress has been shown to decrease milk yield by 10-25 per cent, feed intake by 10-20pc, decrease in the six-week and 100-day in-calf rates, drop conception rates, and increase the risk of clinical mastitis in high yielding cows.
The conditions under which a heat stress event occur can be complex. They can combine the climatic factors of temperature, humidity, wind speed, and sunlight with an animal's characteristics of breed, feeding, stage of milk production and health status.
The Temperature Humidity Index (THI) in Table 1 shows how temperature and relative humidity impact cattle heat stress and therefore a cow's performance.
The table shows the simplified THI; for a detailed THI visit www.publish.csiro.au/ebook/chapter/9781486306473_Appendix_01.
Cattle try to maintain a stable core body temperature through increased water intake, panting, sweating and behavioural changes. If these mechanisms cannot reduce body temperature below acceptable levels, this is when heat stress will occur.
Cow behaviour is a dominant indicator of heat stress progress and a cow goes through the following progression when experiencing heat stress:
To manage the impact of heat stress the following actions can be taken when a high heat day is expected:
In the long term, the following actions can be considered to manage heat stress for the benefit of the business and also to ensure the welfare of cattle:
The impact of heat stress also differs between breeds, with Brown Swiss and Jersey breeds less susceptible than Holstein Friesian, as black-coated cows absorb more solar radiation than cows with lighter coloured coats during the day.
Over the last couple of years, the development of a Heat Tolerance Australian Breeding Value has enabled producers to identify animals with higher tolerance to hot and humid conditions. More information about this value is available at datagene.com.au/BreedingValuesBullABVs.
For further information visit Dairy Australia coolcows.com.au.
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