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Minimising the impact of heat stress

Richard Jackson BVMS (Hons) MRCVS

As summer approaches, the risk of heat stress in your flock increases. Heat stress can have huge impacts on bird health, welfare and performance.

All birds will have a given target environmental temperature depending on their age, breed and the relative humidity. Whilst birds are in their thermoneutral temperature range (i.e. their internal body temperature is maintained at a constant level and their body is not stressed) they lose heat through three methods: conduction, convection and radiation. As the temperature increases beyond the ideal level, evaporative cooling becomes important (the evaporation of moisture from the chicken’s airways helps cool the bird. Panting increases the air movement over the respiratory surfaces to increase cooling). From a practical aspect the bird changes its behaviour to help lose excessive heat (the following bullet points are an extract from the Defra Heat Stress Solving the Problem document):

  • Elect to pant slowly
  • Rest to reduce heat generated by activity
  • Reduce feed intake (this reduces the metabolic heat generated by the bird)
  • Increase water consumption
  • Divert blood from internal organs to the skin, which darkens skin colour
  • Begin fast panting (panting causes evaporation of moisture from the bird’s airways leading to cooling)

The above measures by the bird will lead to reduced growth/egg production through reduced feed intake and lower activity levels. Indeed, panting excessively can lead to dehydration and too much loss of CO2 from the bird’s blood. Excessive CO2 loss can cause the blood to become more alkaline. This alters the levels of potassium, chloride, sodium, and phosphates in the blood. This imbalance in a bird’s electrolyte levels can impact bone development causing lameness or in laying hens, the shell quality deteriorates.

Should the environmental temperature continue to rise despite efforts by the bird to cool down then the consequences can be fatal. The normal internal temperature of a chicken is 40-42˚C and when it reaches 45-47˚C the bird will die. The closer to the bird’s body temperature the air temperature is, the less efficient heat loss is. When the shed reaches 27˚C, with fully feathered birds in it, heat loss is considerably impeded and when 37.8˚C is reached the bird is completely unable to cool down.

Using the Heat Index

Humidity has a large impact on the cooling generated by panting. At low humidity birds can lose much more moisture through panting and can therefore tolerate much higher temperatures than at higher humidity. To work out the impact of humidity, a number called the heat index is used.

Heat index = temperature in Fahrenheit + Relative humidity.
If the heat index exceeds 160 then a bird will suffer heat stress.

On a day when the temperature is 27˚C (80˚F), if the humidity is 50% then we have a heat index of 130 and the birds will be able to cope. However, if the humidity is 80% then we have a heat index of 160 and the birds will struggle. Therefore, humidity is very important. As a rule of thumb for every 10% the humidity increases, the temperature the bird perceives rises by 2˚C. Most heat stress related fatalities occur in the evening rather than during the afternoon, because in the evening although the temperature is lower, the humidity is higher.

Mitigating Heat Stress

  • Ensure all fans are working efficiently
  • If necessary, reduce summer stocking (always know your shed’s ventilation capacity)
  • Have an up-to-date heat stress plan
  • Try to move birds showing signs of heat stress, away from other birds – ideally against cooler surfaces or where there’s moving air
  • Reduce insultation by lifting wings away from their body 
  • Maximise air speed over the birds (for broilers 1-3 m/s is recommended in hot weather and in other poultry 0.7 m/s is the recommended minimum. At 1 m/s there is little benefit from air speed, but at 3 m/s the wind chill effect is approximately 6˚C). In most modern houses, tunnel ventilation is used to achieve this. Higher airspeeds remove warmer more humid localised air over the birds and increase cooling achieved through evaporation. It is also important to have an even airspeed over the shed. Note: Be careful in younger birds which are not fully feathered with high air speeds as these can conversely chill them
  • Some farms use misting systems to reduce the temperature. Whilst these can drop the temperature, for every 1˚ drop in temperature, we get an increase in humidity of 4.5%. Always use the heat index as a guide as to whether the misting system will help or hinder your flock. In dry heat, misting systems are helpful but in humid heat they can make heat stress worse
  • Discuss with your area manager about the position of enrichment bales to ensure they are not impeding airflow
  • Some farmers lift the feeders slightly to allow for good air flow under the pans whilst ensuring birds can still access feed
  • Ensure the shed is well insulated
  • Try not to disturb the birds during the hottest part of the day
  • Try to cool the birds more at night so that they can tolerate hotter day time temperatures
  • Due to the dehydration caused by heat stress and the imbalance in electrolytes it is advisable to give the birds Heat Stress Plus. Heat Stress Plus contains electrolytes to help improve the electrolyte balance in the bird’s blood stream thus maintaining leg health/eggshell quality and will encourage the birds to drink. Additionally, it contains antioxidants to help the bird’s body cope with harmful metabolic by-products
  • Heat Stress Plus should be used at 1ml/litre of drinking water prior to, during and immediately after periods of hot weather. If temperatures exceed 30˚c then the dose can be increased to 1.5ml/litre

Other sources of information:

Ross Broiler Management Handbook
Ross Environmental Management in the Broiler House

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