Broilers of 4 different commercial strains were kept during winter at an altitude of 1 350m. The birds were slaughtered at weekly intervals and their relative right ventricular mass (pulmonary arterial pressure index = API values) determined. In addition, the incidence of ascites was recorded. Two of the strains suffered high losses from ascites, while the other 2 showed a certain degree of resistance. The former 2 groups had a higher mean API and a greater percentage of high API values in clinically normal birds than the other 2 groups. The group with the lowest incidence of ascites achieved the highest mean live mass at 51 days. There was also a decrease in mean API with age in clinically normal birds. There was no difference in the incidence of ascites between males and females.
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