Heat Stress Home
Heat Stress Explained
How to recognize heat stressed dairy cattle
Dairy Cattle not capable of reducing heat load
Dairy cattle are unable to dissipate their heat load efficiently. Their sweating mechanism is poor and they rely on respiration to cool themselves. A further disadvantage is the fermentation process within the rumen generates additional heat that cattle need to disperse. As they cannot get rid of heat effectively they accumulate a heat load during the day and dissipate heat at night when it is cooler. When cows cannot dissipate their body heat, in addition to depressed feed consumption, heat stress has also been shown to have
an effect on milk production and composition including milk protein
and butterfat content.
Heat stress has a dramatic impact on feed consumption and milk production.
It is not only related to ambient temperature but also associated with humidity
and air movement. When the humidity increases the cows evapo-transpiration
is reduced and the animal cannot cool itself. This inability to cool itself
increases the core body temperature and greatly depresses feed intake.
Dairy Cattle Temperature Humidity Index
Temperature alone is not a good way of measuring heat stress. Various
indexes have been developed which take into account such factors as
ambient temperature, relative humidity and evaporation rate. These are
known as THI (
Temperature Humidity Index
The most severe heat stress occurs when both ambient temperature and relative humidity are high and night temperatures do not decrease sufficiently to allow cows to dissipate their body heat.
to see if you live in area where night temperatures and humidity is high.
Dairy Cattle Zone of Comfort
Zone of Comfort
” of Bos Taurus cattle range between a range of 4ºC - 24ºC (39° F- 75° F). Within this temperature range dairy cows are most efficient.
Zone of Comfort
” of Bos Indicus e.g. Zebu, is 10°C (50 °F) to 27°C
Each genotype has a different and characteristic “zone of comfort” Science
has established that under heat stress conditions, Bos Indicus breeds and
their crosses have better heat regulatory capacity than Bos Taurus breeds,
due to differences in metabolic rate, food and water consumption, sweating
rate, and coat characteristics and colour.
Dairy production in these climatic conditions demands different management standards from those of conventional temperate zone dairy farming.
Heat Load in Dairy Cattle
When the temperature increases above 24ºC (75° F) the deep body temperature
of the cow rises and production is depressed.
Research has shown that this effect is aggravated if relative humidity is in excess of 45%. Once the
Temperature Humidity index
(THI) reaches 78 there is a significant decline in milk yield.
Evaporative heat loss through sweating and panting is the primary mechanism for heat loss at high environmental temperatures.When cows cannot dissipate their body heat serious production and reproduction losses occur.
Experimentation has shown that exposure to 40°C (104°F) for 7-10 hours will
induce severe panting, gasping and drooling, sweating and drastically
increased water consumption.
When the relative humidity exceeds 50%, the dissipation of heat by
evaporative cooling becomes much more difficult and signs of heat
stress develop sooner.
Any deviation from optimal (
Zone of Comfort
) conditions results in
physiological behavioural stress adaptation which is made at the expense
of production – decreased live weight gains and reduced feed conversion
efficiency are the readily-measured outcomes.
As this stress adaptation develops, diversionary activity within cells and
in their membranes escalates, to protect their structures and productive
machinery from heat damage. Cellular metabolism is disturbed and
membranes close down some of their operations altogether.
Energy is almost exclusively diverted to prevent a rise in the deep body temperature.
As a result of these physiological responses there are changes in acid-
base balance, there is loss of
and the frank loss of key
(Sodium, Potassium and Bicarbonate).
If the severity of the stress progresses, lethargy, lassitude, weakness,
stupor, staggers and death may result.
Cows capable of producing more milk are at greater risk from heat stress than lower yielding cows.
Visible signs of Heat Stress in Dairy Cattle
Animals seek shade and/or wind.
Refusal to lie down.
Increased respiratory rate (open mouth panting).
Open mouth and laboured breathing.
Sweating and excessive drooling.
Reduced food intake.
Agitation and restlessness.
Thirst is increased. Drinking water intake increases markedly (5 times
Increased urination (with heavy electrolyte loss).
Crowding over the water troughs.
Milk yield drops by as much as 50% or more. (Milk Yield - at 35ºC there
is up to 33 % depression and at 40ºC, as much as 50 %.).
Loss of milk quality - fat and protein content declines.
Loss of body weight.
The incidence of milk fever increases.
Metritis is more widespread.
Uterine prolapse is more common.
Mammary gland infections increase.
There are increased uterine infections.
Udder oedema is more severe.
Laminitis is more frequent.
Keto-acidosis is a recurring problem.
Fertility is lowered - insemination success rate falls.
Increased somatic cell counts and risk of Mastitis.
Embryo mortality increases.
Calves are often premature and small.
Growing animals have markedly reduced weight gains.
Inability to move.
Collapse, Convulsions, coma.
Invisible signs of heat stress in Dairy Cattle
Ruminal pH is typically lower in heat stressed cattle. Rates of gut and
ruminal motility are reduced, thus slowing passage of feed through the
Increased peripheral blood flow.
Some indigestibility of feed.
The huge water flux resulting from increased water consumption also
causes heavy loss of electrolytes. Potassium (K+ ) loss from the skin
increases by 500% in unshaded cattle. In attempts to conserve K+,
cows increase urinary excretion rates of sodium (Na+).
Alter the production of reproductive hormones essential for pregnancy.
Changes the balance of developing follicles in the ovary.
Embryonic development is affected.
) is lost.
Stress hormones appear in the blood.
Gene function is disturbed.
Heat shock proteins are activated to shut down metabolic reactions and to
protect heat-sensitive tissues.
Responses to intercurrent diseases or pathogens decline rapidly.
Resources being diverted to unproductive efforts by the animal to restore
All production is stopped due to loss of
The animal has done all that it can do to stop deep body temperature from
rising and assistance is needed to restore, and retain, electrolyte balance
to the optimum range.
Heat stress is acidogenic and on the acid side of pH 6.8 there is probably widespread cell damage.
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