Heat Stress in Cattle

Cattle are unable to dissipate their heat load very effectively

Compared to other animals cattle cannot dissipate their heat load very effectively. Cattle do not sweat effectively and rely on respiration to cool themselves. A compounding factor on top of climatic conditions is the fermentation process within the rumen generates additional heat that cattle need to dissipate. Since cattle do not dissipate heat effectively they accumulate a heat load during the day and dissipate heat at night when it is cooler.

Figure 1 Cooling of cattle

Heavy cattle cannot handle heat stress compared to lighter weight cattle. Increased fat deposition prevents cattle from regulating their heat effectively. Solar radiation is a critical component that can lead to death loss from heat stress. Typically, proportionality more black hided cattle die during heat waves then other hide colours. Since cattle rely on respiration as a method to manage heat respiratory function is important (Figure 1). Cattle that had severe respiratory disease early in the feeding period will have decreased ability to regulate their heat load.

Managing Heat Stress

Cattle water normal requirements (Table 1).
The water requirements of cattle increases during heat stress. Cattle lose water from increased respiration and perspiration (see figure 1). Additionally, consumption of water is the quickest method for cattle to reduce their core body temperature. Therefore, water consumption will be greater than typical metabolic requirements.

Shade can be critical in determining whether cattle die during extreme heat events, especially for black cattle.

Flies cause cattle to bunch up which decreases cooling. Minimizing breeding areas for flies and applying insecticides to decrease fly populations prior to heat stress times is worthwhile.

During times of increased heat stress cattle should be observed closely to identify if additional strategies need to be implemented.

As heat stress increases cattle will begin to salivate and respiration rates will increase. Eventually, cattle will begin to group together. In severe heat stress cattle will be open mouth breathing with a laboured effort. 

Table 1.  Approximate total daily water intake of cattle at 4^o, 14^o and 21^oC

                                ¹Estimated        Source: National Research Council 1996

• Coat colour and type - cattle with lighter coat colour tend to be more tolerant of heat.
• Body condition - heavier cattle tend to be more susceptible.
• Adaptation - cattle will adapt to heat provided the temperature change is gradual.
• Health - cattle with a prevailing health condition are less able to cope with changes in temperature.

Grazing management 
Another option is to rotate cattle in the evening rather than the morning. The assumption is that the grass will be consumed in the evening and the ‘heat of fermentation’ or digestion is mostly dissipated by mid-morning, thereby reducing the heat load produced by the animal. Another possible option is to graze paddocks that allow access to temporary shade or trees during the heat of the day.

Effects of heat stress on welfare of dairy cattle

Signs of heat stress:

• Reduced feed intake (natural response to reducing metabolic heat)
• Change of feeding patterns – more grazing at cooler times of the day
• Cattle stand rather than lie down
• Bunching or congregating in shade if it is available
• Rapid shallow breathing; open mouth breathing with panting
• Respiration rates increasing with temperatures (14 to 34 °C) - 100 breathes/minute action is needed to reduce stress
• Increased water intake: 100-130 l/day

Heat stress in the welfare of beef cattle

The ability of beef cattle to tolerate heat load varies depending on factors such as: breed, health statue, coat colour, degree of fattening and pen conditions

Managing animals during hot and humid weather:

• Provision of a plentiful supply of clean, cool drinking water
• Suitable shade or shelter: - Trees with large canopies - Natural paddocks and gullies - Shelterbelts - Forestry blocks
• Windy areas
• Handling
• Transport

Animals at high risk of heat stress include: Young animals - Dark coloured animals - Heavy cattle - Pregnant cattle - Sick animals

The temperature humidity index 
Temperature is the most important environmental factor affecting the physiological functions of cattle. Cattle have a thermal neutral zone (optimal experience of comfort in relation to environmental temperature), between the lower critical temperature (LCT) and the upper critical temperature (UCT) (Table 2).  When cattle are within this thermo-neutral zone, they only have to expend minimal energy in order to maintain their body temperature and therefore, maintain performance. Once the ambient temperature goes above or below the thermo-neutral zone, the animal will be required to expend metabolic energy on heat production or heat dissipation therefore reducing the amount of energy available for other bodily functions. The LCT will vary depending on the condition of the animal’s coat, as can be seen in Table 2. The UCT varies depending on humidity levels.

Table 2.  Effect of coat condition on lower critical temperature (LCT) of cattle.

Coat Condition                                 LCT (°C)

Wet or Summer Coat                     15

Dry, Autumn Coat                            7

Dry, Winter Coat                              0

Dry, Heavy Winter Coat                 -8

Cattle also alter their behaviour in order to reduce stress when exposed to extreme temperatures. Once the ambient temperature goes above or below the thermo-neutral zone the animal will be required to expend their energy producing or dissipating heat e.g. increasing or reducing feed intake, lying stretched out or huddled together with other animals.

The most effective way of reducing the LCT and therefore preventing cold stress is through the provision of adequate housing as it prevents the animal’s coat from getting wet which would decrease insulation value and increase evaporative heat loss.

The temperature humidity index (THI; Thom, 1959), shown in table 3, has been widely used as an indicator of thermal stress in livestock. However the THI has limitations as it does not account for wind speed or solar radiation.

Table 3: Thom’s Temperature Humidity Index (extracted from: http;//www.eurometeo.com)