PRODUCTIVE AND PHYSIOLOGICAL ADAPTIVE RESPONSES OF ETHIOPIAN NAKED-NECK CHICKENS AND THEIR F1 CROSSES WITH COMMERCIAL CHICKEN BREEDS TO HIGH ENVIRONMENTAL TEMPERATURE
The objective of this study was to evaluate the effects of the interaction between chicken genotypes (Naked-neck, Na, from Ethiopia; New Hampshire, NH; Lohmann White, LW; and F1 crosses of Na males with females of NH [Na´NH] and LW [Na´LW]) and ambient temperatures (normal and high) on physiological indicators and performance traits. Two-hundred forty female chickens were assigned to a completely randomized design of 2 × 5 factorial arrangements (2 temperatures and 5 genotypes). Eggs were collected daily while feed intake was determined at 28-d intervals and egg shell thickness at 4 age points. Corticosterone (CS) and 3,5,3¢-triiodothyronine (T3) levels were determined from 480 blood samples taken at 4 age points. Commercial hens reared at high temperature showed significant (p<0.05) performance reductions in egg production (33%), feed intake (15%) and shell thickness (24.3%). The effect of heat stress on T3 levels was significant (p<0.001) and consistent across heat-stressed genotypes resulting in an overall reduction of 29% compared with those reared at normal temperature. Moreover, significant (p<0.05) differences in plasma T3 levels were observed between heat-stressed genotypes. Although the CS levels uniformly increased due to heat stress, the response of genotypes with advancing age was inconsistent. In conclusion, the Na´LW crosses at high temperature outperformed other genotypes and thus, appeared to be suitable genetic combinations. The Na chickens and their F1 crosses demonstrated reduced thyroid gland activity suggesting improved thermo-tolerance to long-term heat-exposure. The present findings suggest that levels of T3 hormone might be considered as reliable indicator of long-term heat stress in chickens.