Thyroid hormones and their ratio determine survival or death during summer and short term starvation stress in broiler hens

Sudden and drastic calorigenic effect of thyroxin due to absence of thyroxin binding globulin in chicken and presence of poor heat loss mechanisms may be responsible for the death of birds during summer stress. Feed withdrawal is a recommended practice during peak summer particularly in breeding broilers. To provide causative experimental evidences, two experiments were conducted to test the thyroid hormones relationship to occurrence of deaths in broiler during summer and change of these hormones on feed withdrawal. Blood serum samples were collected at the onset of summer (May) from healthy broiler adult hens raised under standards management practices in individual cages during first year of lay. The ambient temperature in the shed touched as high as 38°C with the RH of 48% during the summer months. At the end of July month, the samples from the survived or died were grouped into two categories (n=8). The quantitative profiles of T₃ and T₄ hormones were determined following Enzyme-Immunoassay (EIA) protocol. In another experiment, similarly T₄ and T₃ were determined in the serum samples of broiler hens obtained from Zero, 24 and 48 h of starvation with ad lib water supply (n=6). The birds died during summer had significantly (P<0.01) higher T₄ and T₃ levels but lower ratio of T₄ and T₃ as compared to those birds that survived during summer stress. Starvation did not leave any significant (P>0.05) effect on T₄ and T₃ levels though a slight declining trend was indicated in T₄. It is concluded that lower levels and higher ratio of T₄ and T₃ favours the survivability of birds during summer stress.