Veterinary Handbook Contents

Environmental conditions that predispose animals to EHL include recent rainfall, high ambient temperature, high relative humidity, absence of cloud cover or shelter with high solar radiation, minimal air movement over several days, and a sudden change to adverse climatic conditions. 

Measurements that are useful in assessing heat load risk include relative humidity, dry bulb temperature, wet bulb temperature and black globe temperature. 

Humidity refers to the amount of water vapour in the air. As temperature rises, the amount of water vapour that can be present within a given volume of air will rise. Relative humidity is the current amount of water vapour in air, expressed as a percentage of the maximal possible amount of water vapour that could be present, at the current temperature. A relative humidity of 100% means that the air is totally saturated with water vapour and cannot hold any more. 

Dry bulb temperature is the ambient air temperature measured with a conventional thermometer that is shielded from direct rays of the sun.

A wet bulb thermometer is a conventional thermometer with its bulb and lower stem wrapped in an absorbent cotton wick that is wet with distilled water for 30 minutes or more before the temperature is read. It provides a measure of temperature adjusted for the cooling effect of evaporation and air movement.

A black globe (or globe) thermometer is a conventional thermometer with the bulb inserted into a large (15 cm) sphere that is painted with a matte black finish. This measures temperature adjusted for the effect of radiant heat and air movement. 

Mechanical ventilation is used below decks on export vessels to ensure air flow. It has been suggested that Pen Air Turnover (PAT) be used as the preferred measure of ventilation rate for closed decks (LIVE.0211, 2002). This is the ratio of actual ventilation flow (typically in m³/hour) to the pen area being ventilated (in m²). 

The result is a measure that has the dimensions of velocity (m/hour). Measurement of PAT on several export vessels reported values ranging between 100 to 300 m/hour (LIVE.0211, 2002). The current AMSA MO43 regulations define ventilation requirements in terms of air changes per hour and stipulate a minimum of 20 air changes per hour for decks with a ceiling height of 2.3 metres or more, and 30 air changes per hour for decks with a ceiling height of 1.8 metres. These requirements are equivalent to a PAT of between 50 and 100 m/hour (SBMR.002, 2001). 

Monitoring of environmental temperatures (dry bulb, wet bulb, black globe), humidity and wind speed all offer useful information for assessment of heat load. Meat & Livestock Australia has developed a heat load index (HLI) for use in land-based feedlots that is based on measurement of relative humidity, wind speed and black globe temperature. 

Wet bulb temperature is considered to have particular application on board export vessels. The lack of involvement of direct solar radiation below decks on livestock vessels means that the black globe temperature measurement is likely to be of less value as a measure of heat load risk. Decks with livestock also typically have a high relative humidity and relatively constant air flow under mechanical ventilation. 

A variety of indices have been developed in an attempt to better represent heat stress potential. These generally involve some combination of different forms of temperature measurement (dry bulb, wet bulb, black globe), humidity, and wind speed. When heat stress conditions are operating, evaporative heat loss is the most important form of heat loss and this in turn is influenced far more by wet bulb temperature than by dry bulb temperature. In the likely range of values occurring on board export vessels, the simplest approach is to use wet bulb temperature alone as a practical measure of heat stress potential on board export vessels (LIVE.0211, 2002).

Table 14.1: Wet bulb temperature risk criteria for heat stress on export vessels

Livestock Line	Safe	Caution	Danger
Bos indicus cattle	< 28°C	28 - 31°C (non-acclimatised)	> 31°C (non-acclimatised)
		30 - 33°C (acclimatised)	> 33°C (acclimatised)
Bos taurus cattle	< 26°C	26 - 30°C	> 30°C
Sheep (100%)	< 26°C	26 - 29°C	> 29°C

Other work done by researchers at Murdoch University suggests that sheep may be slightly more resistant to heat load and that wet bulb temperatures approaching or exceeding 30ºC may be used as thresholds that favour development of heat stress for sheep. Wet bulb temperature can provide an early warning of likely heat stress conditions and allow preparations, increased monitoring of animal behaviour and signs, and implementation of mitigation or treatment strategies to avoid severe disease and deaths from EHL. Information on other indices is provided here because these indices have been used for assessing heat stress conditions for livestock either in land-based feedlots or on export vessels