Heat and Cold Stress of Farm Animals
Farm animals, like humans perform best when the temperature
is neither too hot nor too cold. Research shows that within
a preferred temperature range, animals will not need to expend
any energy keeping themselves warm or cool, are under no temperature
induced stress, and therefore are able to maximise their productivity.
Ideally, farmers should aim to keep all their animals within
this comfort zone all the time. In cases where
animals are housed in sheds this is possible.
The depiction of the typical comfort zones for common farm
animals suggests that animals grazing in an open paddock may
find themselves in less than ideal conditions during much
of their life. This can reduce weight gain, milk production,
birth weights and survival while making stock management more
Comfort Zones For Adult And Newborn Animals
Reference: Reid, R. and Bird, P.B. (1990),'Shelter'
in Trees for Rural Australia, ed. K.W. Cremer, Inkata Press
Melbourne, pp 319-335.
The range of the comfort zone depends upon the
species of animal, its size, genetic condition, health, energy
reserves, age, condition of its coat and any additional stressful
conditions such as pregnancy or lactation. In the graph, the
lower limit (to the left of the grey bar) represents the critical
temperature below which cold stress begins to affect behaviour
and productivity. This causes animals to shiver, consume more
feed and seek shelter. At effective temperatures above the
upper limit (to the right of the grey bar) animals may sweat,
pant, drink more water, reduce their consumption or seek shade.
Under prolonged or extreme conditions, the effect on animal
health can be very significant resulting in irreversible losses
in productivity or even death. Trees have a role to play in
reducing the wind chill factor that dramatically reduces the
effective temperature and by reducing exposure to direct sunlight.
Heat Stress And Farm Animals
Exposure to direct radiation can dramatically increase heat
stress in stock and most will actively seek shade on hot days.
Much of the research into heat stress has occurred within
the dairy industry. High heat loads in cattle lead to depressed
feed intake, decreased milk yield, milk fats and protein %,
elevated somatic cell counts as well as increased risk of
mastitis, weight loss and reduced reproduction. Once the heat
load on a dairy cow becomes severe the animal may show signs
of distress. This can include desperately seeking shade, refusing
to lie down, reducing feed intake, crowding water holes, splashing
water from toughs, laboured breathing, excessive salivation,
convulsions and even death.
Although wool provides good insulation, sheep behaviour and
productivity may also be affected by heat stress. Research
has shown that ram fertility can be affected for many weeks
as a result of a period of intense heat stress, while ewes
are less likely to become pregnant or may be more likely to
abort if stressed. The weight of lambs born to heat-stressed
ewes has been shown to be lower, making them more susceptible
to death from stress, mismothering or predators.
Trees for Stock
Wind Speed and the Windchill
Wind chill is the degree to which wind speeds reduce the effective
temperature. For example, if the temperature is 4°C with
still air, the effective temperature is lowered to -4°C
at a modest wind speed of 20 km per hour.
Reference: Reid, R. and Bird, P.B. (1990),
'Shelter' in Trees for Rural Australia, ed. K.W. Cremer, Inkata
Press Melbourne, pp 319-335.
A shelterbelt that reduces wind speeds by more than 50% may
be capable of dramatically increasing the effective temperature
that an animal experiences. In the above example, if the wind
speed in the open was 20km/hr and the ambient temperature
was 2oC then the animals in an open paddock would be experiencing
an effective temperature of around -6oC. A shelterbelt that
reduced the wind speed by 50% might increase this effective
temperature by 4oC.
Another critical factor influencing cold stress of stock is
rainfall. A small amount of rain can reduce the insulation
value of an animals coat. Goats, for example, dont
have the waterproofing lanolin of sheep wool so when their
hair gets wet it loses much of its insulation value. The same
is true for cattle and freshly shorn sheep. A wet coat also
induces evaporative cooling which further chills animals.
Newborn stock that are unable to dry off are particularly
susceptible to cold stress in windy conditions. A chill index
model developed by Donnelly (1984) relates temperature, rainfall
and wind speed to lamb mortality. Donnelly calculated that
if the chill index exceeds 1100, newborn lambs are more likely
Disregarding the effect of rainfall, the effect of wind speed
on the chill index is shown in the graph below. A chill-index
of more than 1100 is considered a real risk to lamb survival.
If the air temperature is a relatively mild 9oC, the chill
index can reach a critical level if the wind speed is more
than about 25km/hr.
The Effect of Wind Speed on Chill Index
Reference: Cleugh, H. (1997), 'Trees for
Shade and Shelter' in Design Principles for Farm Forestry:
A guide to assist farmers to decide where to plant trees and
farm plantations on farms eds. Abel, N. et al, RIRDC, Canberra,
Trees for Wind
Back to top