Effect of long-term heat stress on some performance traits and plasma enzyme activities in Naked-neck chickens and their F1 crosses with commercial layer breeds

The objective of this study was to investigate the effect of long-term heat stress on performance traits and plasma enzyme activities in Naked-neck (Na) chickens and commercial breeds (New Hampshire, NH and Lohmann white, LW) with their F1 crosses (NaxNH and NaxLW). A total of 240 female chickens were randomly assigned to a completely randomized design in a 5 × 2 factorial arrangement (five genetic groups and two ambient temperatures: high = 30-32 °C; normal = 18-20 °C). Body weights were measured at 20 and 68 weeks of age. Eggs were collected once daily while egg weight and feed intake were determined at 28-d intervals. Blood samples were taken from 12 randomly selected birds per genotype and ambient temperature (12 out of 24 hens) at 22, 38, 51 and 65 weeks age. Levels of glutamic-pyruvic transaminase (GPT), glutamic-oxaloacetic transaminase (GOT) and creatine kinase (CK) were determined in blood plasma. Compared with controls, the commercial hens showed significant (p < 0.05) performance reductions under thermal stress with respect to egg production (33%), egg mass (24.2%), feed intake (15%) and shell thickness (24.3%). At high ambient temperature, the NaxLW cross outperformed all other genotypes with respect to heat tolerance in body weight, egg number, egg weight, feed intake and shell thickness; and thus, appeared to be suitable genetic combinations. Compared with controls, heat stress significantly (p < 0.05) increased CK, GPT and GOT activities in all genotypes. However, the response of heat stressed genotypes at different ages was inconsistent and variable. In conclusion, the present study showed that the Naked-neck chickens and their F1 crosses were more thermo-tolerant compared with commercial layer hens. Although plasma enzyme activities uniformly increased due to heat stress, the response of genotypes with age was inconsistent.