Signs and management of heat stress in sheep

Richard Smith, Agriculture Victoria, Tatura

Sheep production in pastoral systems is greatly influenced by climatic effects, which can lead to heat stress. Impacts on animal production from heat include reduced conception rates at joining and lower lamb birthweights.

Managing animals during heat-load events requires proactive planning. A few simple strategies and guidelines can be followed to reduce the impacts of high temperatures on animals.

Factors that can influence heat stress

A complex range of climatic and animal factors influence susceptibility to heat stress. The following tables provide details of climatic and animal factors that may affect an animal’s risk of heat stress.

Climatic factors

Factor Influence on heat stress

Ambient temperature

Temperatures close to or above the sheep’s body temperature will limit its ability to lose heat via convective cooling.

Relative humidity

Increased humidity reduces an animal’s ability to lose heat through evaporation by panting.

Solar radiation

Heat produced by solar radiation can significantly exceed the metabolic heat produced by the animal.

Wind speed

Heat stress is increased in calm air conditions because convective heat loss is dramatically decreased.

Night-time temperature

High night-time temperatures prevent loss of heat gained during the day, increasing the risk of heat stress.

Animal factors

Factor Influence on heat stress

Fleece

Adequate protection from heat is only conferred when the fleece length reaches 30–40mm. In sheep with shorter wool, radiant energy easily passes through to the skin.

Breed

Breeds originating from cool regions, with compact bodies, short legs and necks, and small ears, are more susceptible to heat stress.

Age

Lambs are very susceptible to heat stress because of their high metabolic heat production, higher normal respiration rate, large surface area relative to their mass and limited fleece length.

Pregnancy

The increase in metabolic heat production during late gestation can predispose ewes to increased heat stress.

If a combination of the factors in the above tables are present, heat stress is more likely.

Management

Sheep producers understand the need for forward planning, which can also be applied to heat management.

Forward planning allows development of a management strategy to reduce the heat gain by sheep and facilitate heat loss, rather than just responding to heat stress as it is seen in the sheep. This proactive approach allows sheep to maintain production.

Management strategies

  • Know which paddocks provide the most shade and better air flow, and move animals early to these paddocks.
  • Plant trees or place shade structures in paddocks.
  • Clean troughs or place additional water points to supply access to clean, cool water for all animals.
  • Graze paddocks without shade before hot conditions develop.
  • Use low-stress stock handling techniques, which reduce physical heat.
  • Maximise shade and water for ewes at joining and in late gestation, and for stock under 1 year old. These animals are particularly vulnerable to heat stress.

Strategies during a heat event

Minimise handling and disturbance of animals. Undertake essential activities at the coolest times of the day – usually early in the morning or later in the evening.

  • Do not load or transport stock.
  • Ensure that there is unlimited access to clean, cool water for all animals.
  • Importantly, if affected animals show no signs of improvement, contact your local veterinarian for assistance.

Assessment

In Victoria, sheep can be exposed to a range of conditions that can influence heat load. Therefore, producers need to assess weather conditions and recognise changes as sheep are experiencing heat load.

Two components need to be considered and monitored:

  • Climatic conditions or forecasts – has the Bureau of Meteorology issued a heatwave forecast? (does the location experience high temperatures at a similar time each year? Is the media reporting hot conditions?)
  • Producers can assess panting and sheep behaviour to assess the impact of heat.

Assessment of the sheep needs to occur before the anticipated weather conditions arrive.

Panting assessment

An easy-to-use visual sign that sheep are becoming heat stressed is a change in the way they are breathing. This is because 65% of the heat loss in sheep occurs by panting.

The degree to which sheep are panting is an important indicator of the extent to which they are suffering from heat stress (see Figure 6):

  • Mild heat stress – sheep may show mild to fast panting, but with a closed mouth. Rapid chest movements will be easily observable.
  • Moderate heat stress – sheep show fast panting, progressing to mouth slightly open, but the tongue is not extended beyond the lips. Rapid chest movements will be easily observable.
  • Severe heat stress – rapid, open-mouth panting will be seen, with the neck extended, head held up and tongue extended.
  • Extreme heat stress – open-mouth panting will be seen, with the tongue fully extended and the head often lowered. Deeper breathing will occur, with a reduction in the panting rate for short periods.

A series of photos of sheep showing different scores of panting.

Panting scores to gauge heat stress.

Sheep behaviour

The second assessment method is observing changes in sheep behaviour. Sheep will change or adapt their behaviour to maximise heat loss. These changes can be used together with panting assessment to assess the impact of heat on the animals.

Signs that may be seen in sheep as they are progressively exposed to heat conditions include:

  1. shade seeking
  2. increased standing
  3. decreased dry matter intake
  4. crowding of water troughs
  5. increased water intake
  6. bunching to seek shade from other sheep
  7. changes to, or increased, respiratory rate
  8. immobility or staggering.

Shade availability

The easiest strategy that can be used to reduce heat gain is to ensure that sheep have access to shade. It is important that any shade structure allows enough space for sheep.

Animals grazing under trees have lower body temperatures, less water intake, longer grazing times and shorter times spent walking than those in unshaded areas. Research conducted at Rutherglen in Northern Victoria showed that the most effective shade is provided by a combination of acacias and eucalypts, providing a mixture of shrubby understorey and scattered top storey.

Artificial shade and shelter are a viable option that has been used successfully by several Victorian producers. These include:

  • shade cloth – the height of the structure should allow sheep to move under it and be sturdy enough to withstand animals pushing up against it
  • stacked hay bales (secured and fenced)
  • galvanised sheeting or old hay sheds – all structures need to be engineered to avoid collapse
  • movable shade that is roofed in shade cloth, either on skids or wheels.
Page last updated: 18 May 2021