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.

Visible signs of Heat Stress in Dairy Cattle

  • Decreased activity.
  • 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  temperate climates).
  • Increased urination (with heavy electrolyte loss).
  • Crowding over the water troughs.
  • Excessive salivation.
  • 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 digestive tract.
  • 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.
  • Bicarbonate (HCO3) 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  balance (homeostasis).
  • All production is stopped due to loss of homeostasis.
  • 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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