A previous study by this group demonstrated that a high carbon dioxide tension in venous blood (pvCO2) of juvenile broiler chickens is a reliable predictor for ascites susceptibility.
In a new experiment with five highly selected genetic stocks and two ascites resistant old breeds we studied levels and variability of pvCO2 within each stock at an early age. Effects of different selection traits (principally growth rate) between fast growing sire lines and slower growing dam lines and a commercial hybrid on blood gas (pCO2, pO2) tensions, pH and haematocrit in venous and arterial blood were examined at different ages and compared to values found in ascites resistant breeds. All birds were housed in floor pens in a climate controlled room and subjected to an ascites-predisposing cold environment.
From each stock, 16 birds with the highest (high risk: HRc) and 16 birds with the lowest (low risk: LRc) pvCO2 values were selected at 12 days of age. These birds were marked for future blood sampling to determine changes in haematological characteristics with age and to relate these values to ascites susceptibility. At day 14, eight non-selected birds from each stock were randomly chosen for dissection to determine initial pulmonary arterial pressure index (API) values. Subsequently, all birds were allotted to 8 floor pens (13 birds per pen including two HRc and two LRc birds) per stock. Production performances from 104 birds per stock were measured from 16 to 33 days of age (feed intake (FI); feed conversion ratio (FCR); body weight (BW) at day 33). Mortality was recorded during the complete experimental period. At 5 wk of age, all HRc and LRc birds were necropsied and API values were recorded, which was used to classify the severity of the ascites syndrome.
A convincing effect of pvCO2 values in juvenile chickens on API at 5 wk of age in modern lines confirmed results obtained in the previous study. At an early age, pvO2 values were much less predictive for high pulmonary pressure induced ascites at wk 5 than pvCO2 values. Hypercapnia combined with low blood pH values and followed by hypoxemia (inducing high haematocrit values) provoked a marked high incidence of ascites and high API values in modern breeds.
A total absence of ascitic symptoms within native breeds corresponded with unchanged low API values during ageing from 12 to 33 days of age and with lower pCO2 values in venous and arterial blood compared to modern breeds at all ages. The pvCO2 difference (mean values) between HRc and LRc groups were similar for all modern lines irrespective of age and showed no relationship to growth rate. API, as a reliable indicator for ascites susceptibility, of modern breed chickens correlated with pvCO2 values, but not with growth rate. The high correlation between pvCO2 in juvenile chickens and API values at 5 wk of age indicated that a strong genetic selection pressure on low pvCO2 values at an early age will be an effective method to reduce decisively the occurrence of the ascites syndrome at sea level.