Heat Stress and Lost Production

Production limiting regions

It is now well known that heat Stress is the major cause of lost production and lost profits in poultry, swine, beef cattle and dairy cattle in hostile "production-limiting" regions.  

Development of new breeds with faster growth and conformation characteristics has led to virtually all production animals farmed in hot and humid countries being genetically derived from strains originally bred in the more temperate climates of Europe and North America.

However, most farmers in tropical and sub-tropical areas have not experienced the production levels of animals in cool climates and have no concept of how much improvement in productivity is
possible. 

As a general rule the zone of comfort (sometimes called thermal comfort zone) for most animals is between 4° C (39°F) and 25° C (77°F).When temperature exceeds 25° C (77°F) animals suffer heat stress. In severe cases of heat stress the deep body temperature rises, animal cells are affected and production performance is reduced. When relative humidity exceeds 50% The effect is:increased when the relative humidity is greater than 50%. A temperature humidity index (THI) has been devised to assist in determining the level of heat stress experienced. THI graph for cattle THI graph for poultry THI graph for pigs

Animals typically react to heat Stress conditions by eating less food,thereby naturally controlling the rise in deep body temperature caused by digestion. Respiratory rate rises and there is a marked increase in insensible heat loss by evaporation of water from the lungs.They also drink at least 5 times the amount of water they would under temperate conditions, urine output increases and many mineral ions are lost.

The common visual signs of heat stress are:

Stops eating Wing spreading  Seeks shade Wallowing Drooling  Less activity Drinks more

Increased Urine output (and loss of electrolytes) Wet litter Hyperventilation, panting  Frothing at the mouth Panting and gasping respiratory rate increases dramatically  leads to Respiratory Alkalosis

The invisible signs of Heat Stress (detectable by biochemistry)

Gene function is disturbed Resources being diverted to unproductive efforts by the animal (bird) to restore balance (homeostasis)  Heat shock proteins are activated to shut down metabolic reactions and to protect heat-sensitive tissues.  Responses to intercurrent diseases or pathogens decline rapidly.  All production is stopped due to loss of homeostasis

pH of blood plasma rises pH within the cells falls Urine output increases and so does electrolytes Bicarbonate (HCO3) is lost Stress hormones appear in the blood

Better producing animals and birds are at greater risk from heat stress than lower yielding individuals.

See a full explanation of heat stress for each Livestock species.