Heat stress

Excessive heat load (EHL) or heat stress, describes the situation where lotfed livestock (primarily cattle) are not able to dissipate body heat effectively and their body temperature rises above normal. 

EHL initially leads to reduced feed intake and production losses but in extreme cases, EHL can lead to tissue organ damage and death. 

The factors that contribute to body heat load in cattle are complex and include environmental conditions and animal characteristics. 

Environmental conditions 

A combination of two or more of the following conditions can lead to excessive heat load in Australian lotfed cattle: 

• recent rainfall
• a high ongoing minimum and maximum ambient temperature 
• a high ongoing relative humidity
• an absence of cloud cover with a high solar radiation level 
• minimal air movement over an extended period (4–5 days)
• a sudden change to adverse climatic conditions.  

Animal characteristics 

Some cattle are more susceptible to excessive heat load (EHL) than other cattle based on: 

• Breed: Bos indicus cattle are more heat tolerant than Bos Taurus, genetic variations also exist within breeds. 
• 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 to EHL. 
• 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. 

Managing EHL 

Feedlot operators can minimise the heat load burden placed on animals during hot conditions by implementing a range of management strategies. These must be planned in advance and implemented in unison as activities undertaken in isolation, and not as part of a broader approach, are rarely effective. 

There are three main components to an effective excessive heat load (EHL) management plan: 

1. A pre-summer review of the feedlot's preparedness for an EHL event. This should include: 

• an examination of the feedlot environment including the site characteristics, infrastructure and condition
• upgrades to key elements such as shade and water can be implemented as needed
• a management review of the feedlot's preparedness for an EHL event
• the preparation of a summer nutrition program that considers the risk of EHL 
• the preparation of an EHL event strategy. 

2. The development of a summer management program to reduce the risk of an EHL event and allow early detection of an event. 
3. The preparation and implementation of an EHL event strategy when an EHL event is forecast or occurs. 

A proactive approach to the management of EHL is more effective than a reactive response once an EHL event has occurred.