BONUS SPRING SUMMER 2025

ANIMAL CATEGORY COMFORT ZONE Min temp. ( °C ) Max temp. ( °C ) Day old chicken 32°C 34 °C Broiler day 20 °C 26 °C Layer 20 °C 26 °C INDICATION OF MIN ANDMAX TEMPERATURE COMFORT ZONES • Swollen wattles and combs • Raised wings • Decreased egg production • Decreased egg and eggshell quality • Spike in mortality Why do chickens behave this way? Chickens lose body heat through five main mechanisms: radiation, evaporation, convection, conduction, and vasodilation. Consequences of heat stress While a drop in egg production and an increased risk of fatalities within a flock might seem like the most apparent consequences of heat stress, there are also other concerns: Behaviour In commercial systems, especially, the risk of behavioural issues such as cannibalism, as well as feather and vent pecking increases when birds experience heat stress. This can lead to injuries, decreased welfare, and an increased risk of infection. Immunity Immunosuppression occurs during heat stress due to stress hormones in the blood and activated heat shock proteins. Certain gene functions can be disrupted, and birds become much more vulnerable to different types of diseases. Egg quality and size Heat-stressed hens produce eggs with lower albumen quality (Haugh units decrease). Lower feed intakes and less nutrients allocated for production can lead to reduced egg size, especially during chronic heat stress periods. Shell quality Eggshell thickness and shell strength decrease significantly during periods of heat stress, resulting in a higher occurrence of cracked eggs. How does this happen? Panting and gular fluttering increases the respiration rate of the animal, releasing carbon dioxide (CO 2 ), loss from the body, leading to a change in their blood pH and potentially affecting eggshell formation. This condition is called respiratory alkalosis. The higher the blood pH, the lower the amount of ionic calcium (Ca 2+ ) in the blood. Ionic calcium is the calcium used by the shell gland that produces the eggshell. When feed intake decreases due to heat stress, phosphorous intake also decreases which contributes to the production of poor and fragile shells. Dirty eggs Increased water intake and decreased feed intake result in watery droppings and Table 1: Comfort zone temperature ranges for poultry. Figure 1: Chickens lose body weight through five mechanisms. (Source: PoultryWorld.net) I L Y F RT ZONE www.agribonus.co.za Spring/Summer 2025 BONUS 49