Effect of cyclic heat stress on thyroidal hormones, thyroid histology, and performance of two broiler strains

摘要

This study determined thyroid hormone (T-3 and T-4) serum levels and investigated thyroid gland histology, as well as two broiler strains performance kept at different periods of heat stress. Broilers (560 Cobb Slow (TM) and 560 Hubbard Flex (TM)) were distributed in control environment (0) reared following the thermal recommendations, and environments 1, 2, and 3 exposed to high temperatures for 1 h, 2 h, and 3 h a day, respectively, from 14 to 42 days old. At 21, 28, 35, and 42 days old, blood samples were collected to determine T-3 and T-4 serum levels. At 42 days old, thyroid glands were removed for histological analyses. The performance was calculated weekly. T-3 levels decreased with age advance, while T-4 serum concentration was higher at 42 days old (23.27 nmol L-1) and lower at 28 days old (15.69 nmol L-1). The histological sections revealed that thyroid morphology was well preserved. Live weight was higher in Cobb Slow (TM) (2.97 kg) than Hubbard Flex (TM) (2.93 kg) broilers at 42 days old. It is possible to conclude that cyclic heat stress exposure for a 3-h period from 14 to 42 days old did not affect performance, thyroid histology, or T-3 and T-4 serum levels. Cobb Slow (TM) broilers had the best live weight values at slaughtering. With increasing age, T-3 levels decreased and T-4 levels increased for both strains.