Sheep have a natural insulation to extreme weather with their fleece. In cold, wet and windy conditions, sheep shiver, huddle together in the mob and seek shelter behind windbreaks to produce and conserve heat. However, these mechanisms have limits. If weather stress is excessive or prolonged, the sheep’s capacity to maintain a stable body temperature may be exceeded and cold stress will result. Hypothermia most commonly occurs in freshly shorn, light condition sheep during wet and windy conditions at any time of the year.
As outlined in Western Australia's Animal welfare codes of practice, all reasonable precautions should be taken to minimise the effects of weather that produce either cold stress or heat stress in sheep. Sheep should be attended to promptly in the event of fire, flood, injury or disease.
Factors contributing to hypothermia
Young animals are more susceptible to hypothermia as they have less fat reserve to mobilise. Losses can be substantial in newborn lambs following cold weather if precautions are not taken.
Time of shearing and acclimatisation
Newly shorn sheep are also prone to hypothermia. The shorter the period after shearing in which the cold weather occurs, the greater the risk of death from hypothermia. Recently shorn sheep may only have about three millimetres of insulating wool remaining, which can cause up to a three-fold increase in heat loss. Although a couple of weeks of wool growth does offer some protection, high mortalities have occurred in mobs up to four weeks after shearing.
Sheep shorn in late winter or spring have been exposed to cold weather prior to shearing and are acclimatised. This acclimatisation takes about two weeks to develop and lasts about two months. Once acclimatised, sheep are less likely to die from hypothermia, even if a cold spell occurs immediately after shearing.
Sheep that have been shorn in summer are conditioned to hot weather, and if cold weather occurs shortly after shearing, they are at greater risk of dying from hypothermia.
Sheep weather alerts
High rainfall and high winds combined with temperatures below normal will cause mortalities in young animals, especially newly shorn sheep without shelter. The impact of the cold weather will depend on its duration, rainfall, wind speed and temperature—the ‘wind chill’ factor can double heat loss.
Rainfall causes heat loss in two ways. First, any water evaporated from the skin will cool the body in the same way as sweat evaporation. Second, rain falling on the sheep, lodging briefly in the fleece, and finally dripping off will remove warmth from the skin.
The loss of a sheep’s insulating fleece combined with the evaporative and thermal conductivity of rain falling onto skin, and finally the chilling influence of wind, all result in rapid hypothermia.
The type of weather most likely to cause catastrophic sheep losses due to hypothermia is usually associated with the passage of a strong cold front or a rain-bearing depression.
Summer rain events (typically associated with tropical depressions) are the most problematic.
Sheep weather alerts are issued when a combination of rain, wind and low temperatures reaches a critical level. It is not necessary for all three readings to be extreme to create an alert. For example, a heavy rainfall event with high winds and moderate temperature precipitates a warning as does a low temperature with some rain and wind.
Sheep suffering from hypothermia often die as a result of their own behaviour and their attempts to cope. Sheep move in the direction of the wind until they are stopped by a barrier such as a fence, gully or creek. At this point they may pile on top of each other leading to suffocation or drowning. Sheep may be reluctant or unable to move when wet and cold.
- Be vigilant for cold weather and sheep weather alerts for at least four weeks after shearing. After shearing, sheep need to be fed about 40% above the maintenance level to cope with cold stress, so if a sheep weather alert is given, start feeding before the storm arrives. Supplementary feed should be continued for up to one week after bad weather as rain causes feed to become less palatable, and without supplements, sheep may not receive adequate nutrition.
- If a weather alert is received during shearing, discontinue shearing if it is not possible to shed all shorn sheep. If a weather alert has been received at the end of shearing, shed as many sheep as possible and provide hay for the duration. Once the bad weather has passed, move the sheep to a paddock with adequate shelter and continue to provide supplementary feed.
- Have feed stores available for supplementary feeding before, during and after a storm.
- Be prepared to relocate animals to a shed or land on higher ground with shelter in the event of very heavy rainfall and likely flooding. Sheep may be reluctant to move once they have become wet and cold.
- Prioritise your animals, giving shelter to the most vulnerable such as the ewes and lambs and those newly shorn.
Layered shelter belts are effective in moderating wind speeds and providing direct shelter for livestock. Tall-growing open plantings will reduce wind speeds downwind for distances of 25 to 30 times the height of the barrier. A two or three-row shelter belt with eucalypts as the taller component and acacias and casuarinas providing the lower cover combines well to provide an all-purpose shelter belt.
Lambing and off-shears paddocks should be small and well protected from cold winds by relatively impermeable shelter belts. Artificial shelter such as round bales may not be helpful as sheep will often pile on top of each other around the bales and suffocate. The same can occur if sheep are placed in a paddock with only trees and no dense scrub at sheep height. However, long belts created with straw bales have proved beneficial in lambing paddocks.
Clinical signs of hypothermia
Initially sheep will try to maintain their body temperature by:
- shallow breathing in order to reduce the rate of respiration (that is, rapid respiration or panting causes heat loss)
- seeking shelter
- huddling together.
If the cold persists, body temperature will drop and sheep become lethargic, recumbent (down on their chest or on their sides), the mucous membranes (pink lining of the mouth) will turn pale to white and the legs feel cold.
Treating hypothermic sheep
Early action is very important.
- Attend to sheep that are standing and mobile first, since relocation and treatment of moribund sheep (those lying down and unable to stand) may not be successful.
- Relocate the sheep to a sheltered paddock or the shearing shed as soon as possible and provide supplementary feed.
- Start feeding sheep immediately even if they cannot be relocated, and keep feeding them in the days after the storm. Offer oats and good hay. The oats contain high levels of carbohydrate (energy), which will increase heat production, and the hay stimulates the rumen to start working.
- If dealing with smaller numbers, bed them down in thick hay and throw hay over them.
Your private veterinarian can offer advice on appropriate oral or injectable treatments for individual sheep. These may include calcium borogluconate injections and propylene glycol or glycerine drenches so long as sheep are not moribund.
If dealing with a small flock, consider applying garbage bags as coats for the sheep. Trials have shown that properly fitted plastic bags can decrease the loss of body heat even in severely hypothermic sheep.
Factors determining treatment or humane destruction
Moribund sheep should be humanely destroyed immediately as they are unlikely to respond to treatment. These sheep will immediately collapse when assisted to stand but you can also look for:
- decreased eyelid or corneal reflexes when lightly touched with the finger
- very cold skin and extremities
- pale to white mucous membranes (gums and lips)
- body temperature at or below 37 degrees Celsius.