Influence of Histology and Histomorphometry of Thigh Muscles on Thermal Stress-Related Physiological and Plasma Hormonal Responses in Three Broiler Strains

A study was conducted to compare the histological and histomorphometric observations of thigh muscle and the effects of thermal exposure on rectal temperature and plasma hormone levels in three different broiler strains i.e. white broilers, coloured broilers and native × coloured broilers. Experimental birds were reared up to the target body weight of 1700 g. Histomorphometry of meat samples were studied at the target body weight of experimental birds. The effect of thermal exposure on rectal temperature, plasma cortisol and triiodothyroinine (T₃) levels of experimental birds were studied at 5th week of age. The Iliotibialis lateralis muscle study revealed that native × coloured broilers have larger fascicles (429.38 ± 26.22 µm) with more numbers of muscle fibres (40.90 ± 1.21) and each muscle fibre with a small diameter (24.57 ± 0.69 µm) when compared to the other two broiler strains. The birds were exposed to temperature-humidity index (THI) value of 39.69 °C for 50 min. Thermal exposure did not alter the rectal temperature, plasma cortisol and T₃ levels in native × coloured broilers. On the contrary, there was a significant (P ≤ 0.05) increase in rectal temperature and plasma cortisol level in white broilers (41.31 ± 0.05 to 42.38 ± 0.13 °C and 1.11 ± 0.08 to 1.62 ± 0.09 ng/ml, respectively) and coloured broilers (from 41.18 ± 0.02 to 41.65 ± 0.06 °C and 1.19 ± 0.15 to 1.53 ± 0.05 ng/ml, respectively). The plasma T₃ level was significantly (P ≤ 0.01) decreased after thermal exposure in white broilers (from 2.08 ± 0.07 to 1.54 ± 0.03 ng/ml) and coloured broilers (from 1.66 ± 0.04 to 1.04 ± 0.09 ng/ml). From the present study, it was concluded that native × coloured broilers have higher muscular mass as well as higher metabolic rate rendering these cross broilers to be physically more active and also have better heat tolerance than the other two broiler strains which can be considered as a favourable trait in backyard system of rearing.