With hot weather heading in the northern hemisphere, it’s time for layer operations to make sure they’re prepared to prevent heat stress. Best-case scenario: We prevent hens from overheating. Worst case: Layers are heat stressed, leading to significant welfare and performance problems.
By Jean E. Sander, DVM
Senior Technical Services Veterinarian
Zoetis, USA
To properly prevent or manage heat stress, it’s imperative to understand how it occurs. A layer’s normal body temperature is about 105° F (40° C). Hens are comfortable with an ambient temperature of 65° F to 75° F (18° C to 24° C). When that temperature gets above 90° F (32° C), the more serious consequences of heat stress occur.1
In one study, heat stress was associated with a 31.6% reduction in feed conversion, a 36.4% reduction in egg production and a 3.41% reduction in egg weight, according to a review article from Purdue University and USDA’s Agricultural Research Service.2,3
The authors of the review point out that heat stress might also affect food safety. “Many recent studies have demonstrated that bacteria, such as Salmonella and Campylobacter, are capable of exploiting the neuroendocrine alterations due to the stress response in the host to promote growth and pathogenicity. Therefore, it is of great importance to be aware that environmental stresses, such as heat stress, can potentially alter the host-pathogen interaction,” they write.4
Other consequences of heat stress include increased water intake that can lead to wet manure and soiled eggs, and reduced immune function, which can impair vaccine efficacy.
Heat-stress basics
When a hen is too warm, she needs to lose heat. She does this in several ways. One is by convection, which is losing body heat to cooler air by spreading her wings.
Another way is by evaporation. When the house is too hot, moisture from the respiratory tract has to be used, causing hens to pant, which increases airflow over mucous membranes.
Panting is what leads to poor shell quality — thin, weak shells — because it disturbs the metabolic acid-base balance. Ultimately, it causes less calcium to be transferred to the shell gland, a deficit that can’t be corrected with diet. Furthermore, if panting is not effective and body temperature further increases, the hen becomes listless and may die.
It’s important to note that humidity is related to the ambient temperature. Higher humidity at any temperature increases bird discomfort, and evaporative cooling is less effective. E...
