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Male broilers of two genetically related stocks with divergent growth rates and feed conversion ratios were used to study metabolic backgrounds on the occurrence of pulmonary hypertension, heart failure, hypoxemia, and ascites in poultry. An experiment with a 2 x 2 x 2 x 2 factorial split-plot arrangement of treatments with 96 groups of 12 broilers was performed. Effects of stock and environmental factors such as ambient temperature, dietary fat, and dietary energy on performance, energy metabolism, oxygen consumption, hematocrit values, and mortality were investigated in broilers from 1 to 5 wk of age. Dissimilar responses of the two stocks to environmental factors reflected genotype by environment interactions and revealed metabolic disorders related to heart failure and ascites. The results indicated that in the stock with the lower feed conversion ratio, a fast protein accretion was achieved together with a reduced ability to convert chemical energy to metabolic heat and to deposit body fat directly from ingested fat. Birds with a low feed conversion ratio show less flexibility in metabolic adaptation to a changing environment, which can account for the development of ascites.

1. Male broilers of two different genetic stocks, a pure broiler sire line (A) and commercially available Ross broilers (B), were used to study the effect of haematological characteristics in juvenile chickens on the development of clinical ascitic signs. Production performance (body weight gain (BWG), feed intake (FI), feed conversion ratio (FCR)) from 448 birds per stock was measured from 2 to 5 weeks of age. Mortality was recorded from 2 to 6 weeks of age. The birds were housed at a low ambient temperature to stimulate the incidence of ascites. 2. From each stock, 32 birds with the highest (high risk: HRc) and 32 birds with the lowest (low risk: LRc) carbon dioxide tensions (pCO2) in venous blood were selected at 11 d of age. These birds were marked for future blood sampling to determine changes in pCO2 with age to relate these values to ascites susceptibility. 3. At 2 weeks of age all birds (including HRc and LRc birds) were allotted to 32 floor pens (one HRc and one LRc in each pen) per stock. Venous blood samples were collected weekly from HRc and LRc birds for blood gas analysis and haematocrit, and at week 5 also for thyroid hormone (T3, T4) concentrations in plasma. At 5 weeks of age all HRc and LRc birds were examined post-mortem, relative heart, lung, and liver weights and arterial pressure index (API) values were recorded. 4. Birds from stock A showed a lower BWG and FCR and notably higher ascites mortality compared with stock B. An effect of pCO2 tensions at d 11 was found on the incidence of ascitic signs in selected birds of both stocks up to week 5. From the HRc groups 30% of the birds showed ascitic signs, whereas this was only 8% in the LRc group. LRc birds of stock B in particular showed constant low API values (20 +/- 3%) and none of these birds showed signs of ascites. 5. Our results suggest that the ascites problem in Ross birds can be eliminated by selection for low pCO2 tensions in venous blood. Stock effects on API, liver weight, lung weight, and plasma thyroid hormone independent of pCO2 showed a more complex picture of the ascitic signs in stock A compared with B. 6. We concluded that in this experiment a high pCO2 tension in venous blood measured at d 11 was a reliable predictor for ascites susceptibility observed at 5 weeks of age. A low pCO2 tension provides an appropriate criterion for genetic selection, whereas a high pCO2 tension emphasises the necessity for intensive management in poultry houses.

SUMMARYA continuously increasing production level in poultry breeding has resulted in changes in metabolism.Selection procedures in breeding programmes are focused on an increase in growth rate and on a decrease in feed conversion ratio (less feed intake per unit of deposited tissue). These procedures do not pay attention to the maintenance requirements of birds. Imbalances between production (protein and fat deposition) and supply of energy for maintenance requirements lead to homeostatic dysregulation and to diseases of organs which supply the energy for production and maintenance. The alarming increase in metabolic diseases, such as heart failure syndrome, ascites, and oedema in the lungs and heart, can be directly related to an insufficient oxygen supply. A low oxygen consumption and heat production is one of the mechanisms by which a low feed conversion ratio can be achieved, as is induced hypothyroidism by which physical activity and thus heat production is reduced.Other diseases, such as liver cirrhosis, malabsorption syndrome, sudden death syndrome in broilers, and fatty liver-hemorrhage syndrome, which is nowadays the most important disease in laying hens in the Netherlands, can be related to an imbalance between the production rate and maintenance requirements. A continued selection on the basis of retained energy (in protein and fat) without paying attention to the maintenance requirements of birds will be detrimental for the health and welfare of poultry. These undesirable devellopments in poultry husbandry should be a challenge for sciences focused on welfare and stress in animals. Such a scientific approach to animals suffering from dysgenic changes in metabolism is needed to solve serious problems in poultry breeding. INTRODUCTIONA number of diseases which are related to undesirable changes in metabolism are increasingly threatening the health and welfare of our farm animals. Frankenhuis, et al. (9) pointed at the relationships between a continuously increasing production level in livestock breeding programmes and the appearance of particular pathological changes in poultry and

A previous study by this group demonstrated that a high carbon dioxide tension in venous blood (pvCO 2 ) 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 pvCO 2 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 (pCO 2 , pO 2 ) 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) pvCO 2 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 pvCO 2 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, pvO 2 values were much less predictive for high pulmonary pressure induced ascites at wk 5 than pvCO 2 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. Ascites and venous carbon dioxide tensions: C.W. Scheele et al.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 pCO 2 values in venous and arterial blood compared to modern breeds at all ages. The pvCO 2 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 pvCO 2 values, but not with growth rate. The high correlation between pvCO 2 in juvenile chickens and API values at 5 wk of age indicated that a strong genetic selection pressure on low pvCO 2 values at an early age will be an effective method to reduce dec...