Effect of cold stress on broilers selected for resistance or susceptibility to ascites syndrome

Genetic selection for an ascites-resistant line of broilers is seen as a permanent solution to the ascites problem. Ascites-resistant and ascites-susceptible lines have been developed using sire family selection based on mortality data taken from siblings reared in a hypobaric chamber (simulated 2,900 m above sea level). The relaxed line is representative of the original commercial pureline stock randomly mated with no artificial selection pressure. The objectives of this study were to evaluate the differences between the lines when reared in floor pens and subjected to an ascites-inducing cold stress. Seven hundred eighty three straight run broilers were reared in floor pens at local elevation (390 m above sea level). Feed and water were available ad libitum. Birds were brooded at 32 C during the first week. The second week birds were maintained at 30 degrees C. Cold stress was applied for the remaining 4 wk at 14 degrees C. Mortalities were necropsied daily to determine cause of death. Birds and feed were weighed weekly. At 6 wk, five birds per pen were bled, and half the survivors in each pen (8 to 15 birds) were killed, necropsied, and scored for ascites. Blood gases, clinical chemistries, and blood cell counts were taken. Liver, spleen, split heart, and lung weights were recorded. Body weights were not different among the resistant, susceptible, and relaxed lines (P < 0.05). Feed conversion was better in the resistant line when compared to the susceptible and relaxed lines (P < 0.05). Ascites incidence, as measured by mortality and lesion score at necropsy, was higher in the susceptible and relaxed lines when compared to the resistant line; 18.8, 12.7, and 1.6% respectively (P < 0.001). Susceptible and relaxed lines showed more right ventricular hypertrophy when compared with the resistant line (P < 0.05). The results show that under severe cold stress at local altitude (390 m above sea level), the ascites-resistant line was growing as rapidly as the other lines and was as resistant to ascites as it had been under hypobaric conditions.