1. Male broilers of 5 genetic stocks (A, B, C, D, E) selected in different ways for fast growth and low food conversion ratio (FCR) and differing in ascites sensitivity were subjected to 2 different ambient temperature step down regimens: high temperature (HT: 33 to 20 degrees C over 33 d) and low temperature (LT: 30 to 15 degrees C over 17 d). 2. Ascites incidence was recorded daily. Food intake and body weight gain were measured weekly and FCR was calculated. Heat production (Hp) was calculated using the comparative slaughter method. At 28 d venous samples were taken for blood gas analysis and haematocrit and relative heart, lung and liver weights were recorded. 3. Populations A and C showed the highest growth rates combined with a low FCR and a higher ascites incidence. A low FCR in these stocks was attributable to low values for Hp. These stocks also had low pO(2) and high pCO(2) in venous blood at low ambient temperature compared with other stocks. Stock B, which exhibited the slowest growth rate and the highest FCR, was not susceptible to ascites and showed higher Hp and PO2 and PCO2 at low ambient temperature. Populations D and E were intermediate for almost all variables. Heart and lung weights were both increased at LT, while Liver weight did not differ between temperature regimes. 4. Our results show that a high haematocrit is not necessarily linked with an increased susceptibility to ascites.
