Acclimation to heat during incubation: 4. Blood hormones and metabolites in broilers exposed to daily high temperatures.

The objective of the present experiment was to study the effects of parental age and heat acclimation of embryos on blood metabolites and hormones of broilers exposed to daily cyclic ambient temperatures from d 21 to 42. Eggs obtained from 32 (younger), 42 (middle-aged), and 65 wk (older) breeders were divided into 2 groups. One group of eggs was incubated at control incubation temperature (ITCONT), whereas the second group was heat-acclimated at 38.5 degrees C for 6 h/d from d 10 to 18 of incubation (ITHA). Chicks were reared at standard brooding temperatures from 1 to 21 d. From d 21 to 42, half of broilers/incubation temperature/parental age was kept as the control (ATCONT), whereas the other half was exposed to daily cyclic heat treatment (ATHIGH) to impose a stress response. There was a reduction in plasma triiodothyronine (T3) levels in ITHA broilers. On d 28, plasma T3 levels were similar regardless of parental age of eggs incubated at ITCONT, whereas ITHA resulted in lower levels of T3 in broilers from 65 wk parents. At the same age, ATHIGH reduced plasma triglycerides with the effect greater for ITHA than ITCONT broilers. Plasma uric acid was also lower for ITHA than ITCONT broilers for the offspring of 65 wk parents on d 28. There was an increase in plasma creatine kinase activity on d 42 in ATHIGH broilers regardless of parental age and incubation temperature. Plasma corticosterone was consistently lower for the ITHA than ITCONT treatment, being significant on d 21 and 42. It was concluded that these changes in blood metabolites and hormones may enhance the thermoregulatory ability of ITHA broilers when exposed posthatch to daily high temperatures.