Heat Stress on Dairy Animals

Heat stress for the dairy cow can be understood to indicate all high temperature-related forces that induce adjustments occurring from the sub-cellular to the whole animal level to help the cow avoid physiological dysfunction and for it to better fit its environment.

These following are the signs of heat stress observed during summer season

  • Restlessness
  • Crowding under shade or at water tanks
  • Panting (open-mouthed breathing)
  • Increased salivation
  • Increased respiration rate
  • Rates of gut and ruminal motility are reduced
  • Lethargy
  • Increased sweating

In dairy cows two types of sweating can be distinguished: both are involved in heat dissipation. The first type is insensible sweating or perspiration that leaves the body at all times, unless the relative humidity is 100%. The other type, thermal sweating, occurs as the principle evaporative cooling mechanism of the cow when the ambient temperature rises.

  • Rise of rectal temperature

Rectal temperature is an indicator of thermal balance and may be used to assess the adversity of the thermal environment. In severe cases of heat stress the rectal temperature rise. The effect is increased when the relative humidity is greater than 50%. A rise of 1 0C or less is enough to reduce performance in most livestock species.

  • Reduced heart rate

Initial increase in heart rates slows down when the heat stress persists. Reduced heart rate is more typical in heat-stressed cows as it is associated with the reduced rate of heat production as a response to high environmental temperatures.

  • Declined feed intake

Feed intake in lactating cows begins to decline at the ambient temperatures of 25-26 ºC and drops more rapidly above 30 ºC. At 40 ºC, dietary intake may decline by as much as 40%. Heat stress in high producing lactating dairy cow’s results in considerable reductions in roughage intake and rumination. The reduction in appetite under heat stress is a result of elevated body temperature.

  • Increased water intake

Heat stress increases water consumption by at least five times the normal level in temperate zones. Water and macro-mineral needs, influenced heavily by demands to maintain homeostasis and homeothermy are altered for lactating dairy cows during heat stress. Milk contain about 87% water, and contains large concentrations of the electrolytes Na, K, and CI. Therefore, lactating dairy cows have large turnover of water and these electrolytes.

  • Drop in daily milk production

It is accepted that heat stress is the major cause of loss in production of dairy animals in hostile regions. Some authors reported declines in the productions of milk and fat as a direct result of high environmental temperatures. This may be explained by the negative effects the heat stress on the secretary function of the udder. Some authors suggested that milk production is reduced 15%, accompanied by a 35% decrease in the efficiency of energy utilization for productive purposes.

Steps to reduce heat stress:

The following are the management practices proposed to ameliorate the effects of heat stress on animals.

  • Natural shade

Trees are an excellent natural source of shade on the pasture. Trees are not effective blockers of solar radiation but the evaporation of moisture from leaf surface cools the surrounding air.

  • Artificial shade

Solar radiation is a major factor in heat stress. Blocking its effects through the use of properly constructed shade structures alone increases milk production. Two options are available: permanent shade structures and portable shade structures

  • Permanent shade structures

Major design parameters for permanent shade structures (orientation, floor space, height, ventilation, roof construction, feeding and water facilities, waste management system) depend on climatic conditions. In hot and humid climates the alignment of the long-axis in an east-west direction achieves the maximum amount of shade and is the preferred orientation for tied animals, its north-south orientation is better where animals are free to move. Space requirements are essentially doubled in hot climate. Natural air movement under the permanent shade structure is affected by height and width, the slope of the roof, the size of the ridge opening etc. Painting metal roofs white and adding insulation directly beneath the roof will reflect and insulate solar radiation and reduce thermal radiation on animals.

  • Portable or temporary shades

Portable shades offer some advantages in their ability to be moved to a new area in different pastures. Portable shade cloth, as well as light roofing material, may be used on the temporary shades.

  • Cooling by reducing ambient air temperature

Air temperature of micro-environment can be lowered by air conditioning or refrigeration but the expenses of such types of air cooling make these impractical.

High pressure foggers disperse a very fine droplet of water which quickly evaporates, cooling the surrounding air and raising the relative humidity. The typical design incorporates a ring of fogger nozzles attached to the exhaust side of the fan. As fog droplets are emitted they are immediately dispersed into the fan's air stream where they soon evaporate. Animals are cooled as the cooled air is blown over their body and as they inspire the cooled air.

Misters. A mist droplet is larger than a fog droplet but cools air by the same principle. These systems do not work well in windy conditions or in combination with fans in humid environments, where mist droplets are too large to fully evaporate before setting to the ground. The consequence is wet bedding and feed.

  • Enhancing the animals natural mechanism of heat loss

Cooling in hot and humid climates emphasizes shade, wetting the skin, and moving air to enhance the animals major mechanism for the dissipation of heat – evaporative cooling from the skin.

Sprinkler and fan cooling systems (Direct evaporative cooling) Sprinkling uses large water droplet size to wet the hair coat to the skin. Cooling is accomplished as water evaporates from the hair and skin. Upper body sprinkling followed by forced-air ventilation reduces body temperature, increase feed intake and milk yield.