The upper critical temperature (OGT) related to the minimal heat-production was calculated by the relationship between heat-production and ambient temperature in the range of 10 to 38-degrees-C, using 320 hens during the 51st and 60th week of age either adapted to 20 or 34-degrees-C before. Eight breeding types-medium heavy normal (Dw-) and dwarf (dw-) body-size Rhodelander-types without (Dw-nana-ff; dw-nana-ff) or with F-gene (Dw-nana-Ff; dw-nana-Ff) vs. Na-gene (Dw-Nana-ff; dw-Nana-Ff) and combined F- and Na-gene (Dw-Nana-Ff; dw-Nana-Ff) were tested. The single caged birds were kept in respiration chambers in the temperature range 10 to 38-degrees-C with temperature steps of 5-degrees-C vs. 4-degrees-C above 30-degrees-C for 21 to 23 h. The measurements were made counter-rotating from either 10 to 38-degrees-C or 38 to 10-degrees-C. Additionally live-weight, feed intake, egg-production and body temperature were registrated. The OGT of normal feathered and normal body-size hens adapted to 20 vs. 34-degrees-C ambient temperature before, was 20,2 vs. 29,2-degrees-C. The related minimal heat-production was 531 vs. 457 kJ/W(x)/d. The increase of the OGT in heat stressed hens was mainly based on the heat induced depression of production traits and feed intake. Using the genes F- and Na- either single or combined, the OGT of normal body-size types was increased up to 31,9-degrees-C. Dwarf body-size types showed comparable effects of feathering and acclimatization. The values for OGT of birds adapted to 20 or 34-degrees-C before were however 3,8 or 2,9-degrees-C higher than the values for the corresponding normal body-size types. The increased values for OGT using F- and Na-gene were combined independent of body-weight with a significantly higher feed intake and lower rise of body temperature during the measurements in the temperature range 30 to 38-degrees-C compared to normal feathered hens. The acclimatization induced effects of the OGT were deminshed by normal, and less pronounced by dwarf body-size types with decreasing feathering intensity. Using normal feathered hens or breeding types with F-gene however, the influence of the acclimatization before on the OGT was more destinct. This effect was due to the incomplete heat regulation, which was responsible for the depression of production traits. The results confirmed that the OGT turned to be a important parameter for characterizing the productive adaptability of heat stressed hens.
