Target 10 Communicator
January 2011 Edition
USING THE TEMPERATURE HUMIDITY INDEX TO HELP REDUCE THE EFFECTS OF HEAT STRESS IN YOUR HERD.
Heat stress has a number of detrimental effects on dairy cows, most notable are the reduction in milk production, decreased in-calf rates and lower milk protein and fat tests. Higher somatic cell counts, liveweight loss, an increase in clinical mastitis cases and other cow health problems can also add to the losses. Managing heat stress in your herd is important to minimise the adverse effects on milk production and quality, fertility and overall animal health.
Metabolic and Environmental Heat
Cows generate heat internally (metabolic heat) as a result of eating and digesting feed. Like most mammals, the dairy cow needs to maintain its core body temperature within a narrow range (38.6 °C to 39.3°C ). The core temperature fluctuates slightly throughout the day, reaching a peak in the early evening and a low during early morning.
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Cows also take on heat from the environment around them. The cycle of gaining and losing heat absorbed from the environment is on-going and always operates in the context of the metabolic heat a cow is carrying at any given time.
Factors that determine the level of environmental heat a cow gains or loses over time are:
- air temperature and relive humidity;
- amount of solar radiation;
- degree of night cooling that occurs;
- ventilation and air flow; and
- length of the hot conditions.
Cows have evolved a range of physiological strategies to off-load heat to cope in hot environmental conditions but problems occur if temperatures and humidity are high (and remain high) and cows do not have opportunities to get rid of load heat from one day to the next.
Temperature Humidity Index (THI)
Temperature Humidity Index (THI) is a measure that accounts for the combined effects of environmental temperature and relative humidity, and is a useful and easy way to assess the risk of heat stress in your herd.
| THI level | Impacts on herd |
|---|---|
| 72 | Cows are likely to begin experiencing heat stress and their incalf rates will be affected. |
| 78 | Milk production is seriously affected. |
| > 82 | Very significant losses in milk production are likely. Cows show signs of severe stress and many ultimately die. |
THI is a useful and easy way to assess and predict the risk of heat stress; however, it is wise to be conservative. If you have a herd of high-producing Holstein-Friesian, it is better to overestimate the risks of heat stress using a lower THI than get caught out.
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Dairy Australia's Cool Cows program provides a six day THI forecast for Shepparton in The Country News each week. Six day THI forecasts for several other locations across the Murray dairy region are also available at Dairy Australia’s Cool Cows website. Visit www.coolcows.com.au , click on ‘Weather Forecaster’ and select the location nearest you.
The Cool Cows weather forecast enables you to monitor the daily heat stress risk level in your local area and adjust your cooling strategies to suit. You can also subscribe to Cool Cows‘ free Heat Stress Alert service which will automatically e-mail you whenever a high heat load event is forecast for the next few days.
