Theory of Wind-Chill Mitigation of Heat Stress in Poultry

Mitigating Heat Stress in Poultry Introduction Try blowing on your hand with it about an inch from your mouth. You should feel a warm breeze. Blowing out of one's mouth in cold conditions may result in what looks like steam - it's easier to watch someone else doing that. The 'steam' is water which came from your lungs as invisible water vapour, condensing due to the temperature of the air being below the dew-point. In your lungs its humidity was nearly 100% but it did not condense in there, as the temperature was above the dew point so fortunately you did not drown ! The warm breeze hitting one's hand come from the warm environment of your lungs - around 36 degrees C so it is generally warmer than the air near your hand before it arrived so it was able to notice the temperature difference. From these two illustrations we know that the air exhaled by a person, and the same is true for a chicken, is generally warmer than the air and contains a significant amount of water vapour. Heat Loss by Convection Breathe in, breathe out - the air going in is warmed by the lungs, thereby removing heat from them, and when exhaled mixes with the air outside. This is heat loss by convection. When the temperature difference between the lungs and the outside is large more heat can be carried out. When the temperature difference is less heat per breath is able to be transferred out. One of the barriers to the transfer of heat to the outside is warm air that is still hanging around outside from the previous breath. That makes the temperature difference even smaller so the transfer becomes even less effective. This shows up as panting in chickens. Sweeping that warm air, near the point it is exhaled, away will increase the temperature difference and the chicken may stop panting. Panting not only helps the chicken to try and get rid of heat more quickly but it also works against it, as the bird has to exercise muscles to pant and this generates additional heat. This makes the situation worse and stresses the bird making it very unhappy - it is called heat-stress. Heat Loss by Evaporation of Moisture Within Lungs The same argument also holds for humidity. About half the heat a chicken loses via its lungs is by evaporation of moisture to water vapour. When the air outside is already heavily laden with water vapour the difference in water vapour levels inside and out becomes small and the process slows down so the chicken loses less heat. Then it tries breathing in and out more quickly to overcome this but if this goes on too long it ends up with a second component of heat stress. Again, the answer is to sweep away the water vapour near the chicken's head to reduce the elevated humidity which occurs when internal water vapour, from the lungs, mixes with already elevated levels outside. Thus sweeping air away with a breeze - natural or artificial, enables both the air convection and evaporative cooling to work much better and it is called wind-chill. In addition to the heat loss via breathing, in chickens they also lose heat by conduction to the air through the rest of their bodies but much of it is insulated by feathers so the losses are retarded. Warmth from the chicken's body warms air beside it reducing the temperature difference which slows the loss of heat by conduction. Sweeping away that layer of warmer than ambient air near the chicken's surface enables the heat loss to speed up. This is a second wind-chill effect,. Summary Overall it is important that as much of the chicken is exposed to the breeze in order to maximise the temperature differences near its mouth and the rest of its body for heat loss by convection and conduction. With its head in a good breeze the chicken will lose heat by evaporation of moisture in its lungs most successfully when the breeze is blowing away the mixture of water vapour exhaled and that which is already in the air.' Items which block the breeze, be they structures or other birds impede the breeze's ability to remove heat - that is reduce the wind-chill effect. 21st June 2018