Effect of long-term heat stress on key enzyme activities and T3 levels in commercial layer hens

High environmental temperatures are the most important inhibiting factors to poultry production in hot regions. The objective was to test adaptive responses of different chicken genotypes to long-term high temperature and identify suitable indicators of physiological parameters. Forty eight female chickens from each genotype of Lohmann Brown (LB), Lohmann White (LW), New Hampshire (NH), White Leghorn selected for improved feed efficiency (WL-FE) and dwarf White Leghorn (WL-dw) were randomly assigned either to the control group (18 to 20°C) or to the experimental group (30 to 32°C). Blood samples were collected from randomly selected 12 birds of each genotype at three age points. Levels ofglutamic-pyruvic transaminase (GPT), glutamic-oxaloacetic transaminase (GOT), creatine kinase (CK), lactate dehydrogenase (LDH) and 3,5,3'-triiodothyronine (T3) were determined in plasma. The results indicated that compared to controls, the GPT activity in heat stressed chickens significantly increased by 29.2% in all genotypes. The CK activity in heat stressed chickens was only significantly higher at 22 weeks old. Activities of GOT and LDH were variable in all heat stressed chickens. The T3 concentration significantly reduced by 41% in all heat stressed chickens and the WL-dw had the lowest value. We concluded that T3 could be considered as reliable indicator of long-term heat stress. Moreover, LW and WL-dw genotypes demonstrated better heat tolerant.   Key words: Layer hens, long-term heat stress, enzyme activities, T3concentration.