Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology
The effects of feed restriction on plasma biochemistry in growing meat type chickens (Gallus gallus)
Introduction
Genetic selection for growth parameters in meat type chickens gives rise to a parent stock (broiler breeders) that tends to lack the ability to self-regulate feed intake. As such, their high body mass is associated with excessive fat deposition, lameness, and high mortality rates (often due to skeletal and/or cardiovascular disease). In order to regulate weight gain, limit health risks, and also maintain high fertility, husbandry practices for the parent stock of broiler chickens encompass a high degree of feed restriction (Renema and Robinson, 2004).
Chronic feed restriction represents a permanent stress for any organism. This is of particular concern for young chicks in a phase of rapid growth with relatively high metabolic requirements (Mench, 2002). As a result, the entire spectrum of metabolic processes (e.g. a shift from anabolism to catabolism) occurs. Many metabolic hormones mediate adaptive changes to physiological stress. Previous research in poultry showed that feed restriction modified the plasma levels of hormones that modulate energy metabolism and growth, such as T3, T4, growth hormone (GH), insulin-like growth factor-I (IGF-I) (Bruggeman et al., 1997, Buyse et al., 2000, Decuypere et al., 2005), and corticosterone (Hocking et al., 1993, Savory and Mann, 1997, De Jong et al., 2002). Elevated plasma corticosterone is an accepted indicator of stress in birds (Siegel, 1995).
Basic anatomic and functional differences between birds and mammals impact biochemical values and their assessment. Interpretation of the results of standard biochemical analyses may then serve for comparative and/or diagnostic purposes (Harr, 2002). Evaluation of plasma biochemistry in birds allows for the identification of metabolic alterations due to a realm of factors, including genetic type, husbandry conditions, age, physiological state and pathology (Meluzzi et al., 1991, Harr, 2002, Gayathri et al., 2004, Juráni et al., 2004, Alonso-Alvarez, 2005).
Total plasma proteins are a common endpoint utilized to estimate avian body condition. Albumins serve as a protein source under conditions of subnormal intake; and both albumin and total proteins are known to decrease in young chicks maintained under feed restricted conditions (Yaman et al., 2000a). Uric acid (UA), the major avian nitrogenous waste product (Harr, 2002), plays a critical role as a potent regulator of oxidative stress. Increased plasma UA is associated with fasting and feed restriction (Mbugua et al., 1985, Alonso-Alvarez and Ferrer, 2001). Creatinine is a metabolic byproduct of the phosphocreatinine breakdown in skeletal muscle (Wyss and Kaddurah-Daouk, 2000). Its levels are directly proportional to muscle mass and inversely related to age. There is some evidence of an increase in creatinine excretion in response to reduced feed intake or other stressors (Work et al., 1999). Creatine kinase (CK) catalyzes the conversion of ATP and creatine to phosphorylated creatine, which serves as a high-energy phosphate for use in muscle activity (Wyss and Kaddurah-Daouk, 2000). Increased plasma CK throughout the rearing phase in broiler breeders fed ad libitum (Hocking et al., 2001) suggests that genetic selection for rapid growth rate induces myopathies and elevates the efflux of intracellular enzymes (Mitchell, 1999). Slowing the growth rate via feed restriction reduces muscle damage and, consequently, decreases the efflux of intracellular enzymes (Hocking et al., 1998).
Hepatic lipogenesis tends to be prevalent in birds, and is modulated both nutritionally and hormonally. This physiological state is highly responsive to dietary modifications (Kersten, 2001). For example fasting decreases adipose tissue lipogenesis and increases the rate of lipolysis (Kersten et al., 1999). In contrast to lipids, the blood glucose levels of birds seem markedly resistant to prolonged feed deprivation (Sartory et al., 1995). However, plasma calcium (Ca) and phosphorus (P) levels are diet-dependent, and reflect an intense osteoblastic activity and bone metabolism required for somatic growth (Viñuela et al., 1991). Modifications in Ca and P may be closely related to the activity of alkaline phosphatase (ALP), an enzyme closely associated with the metabolism of these two minerals and may function as a key regulatory mechanism for avian growth (Viñuela and Ferrer, 1997). Plasma aspartate aminotransferase (AST) may reflect changes in hepatic function in addition to alterations in muscle membrane permeability (Coles, 1986). Alanine aminotransferase (ALT) is a cytoplasmic enzyme whose increase in blood plasma frequently signals either liver or muscle damage (Lumeij, 1997).
Although feed restriction enhances many aspects of broiler breeder production, it may also be stressful, in particular for young chicks undergoing rapid growth with high metabolic requirements. The stress induced by feed restriction may be reflected in dramatic changes to the plasma biochemistry. In particular, young broiler breeders undergo dynamic muscle growth, skeletal size fixation and the development of many important physiological systems. On the other side, it is clear that the major welfare problems in broilers are those which can be regarded as side effects of the intense selection mainly for growth and feed conversion (SCAHAW, 2000) and therefore not only influences of feed restriction, but also the effects of ad libitum feeding regime are of interest. The objective of this study was to compare the impact of moderate and severe quantitative feed restriction with an ad libitum regime on plasma biochemistry during the first 100 days of development in broiler breeder hens.
Section snippets
Materials and methods
One hundred and eight 1-day-old female broiler breeder chicks (Gallus gallus) Ross 308 (Xaveross, Prague, Czech Republic) were used. From day 1 to day 16 they were kept in plastic boxes (12 chicks per box), with feed and water provided ad libitum. At day 16, the chicks were divided into three feeding treatment groups (36 individuals per treatment) and transferred to six floor pens (18 individuals per pen) measuring 1.35 × 2.10 m with deep litter (wood shavings). The photoperiod during the first
Results
Asymptotic body mass estimates obtained from the growth curves of the individual feeding treatments (Fig. 1) reached 5443 g in the AL group, while mean 2R and R weights were 3881 g and 3235 g respectively. Similarly, the growth rate constant decreased with graded restriction (0.032, 0.03 and 0.023 respectively). The inflection point of the growth curve was delayed in R birds.
The ANOVA showed a significant overall effect of treatment on T3, T4, and corticosterone levels. Age had significant
Discussion
Feed restriction resulted in a decrease of T3 and increase of T4 concentrations, similar to previous findings for broiler breeders (Darras et al., 1995, Bruggeman et al., 1997, Kubíková et al., 2001). Bruggeman et al. (1997) showed that T3 concentrations decreased and T4 concentration increased in both restricted and ad libitum fed chicks during ontogeny (2–24 weeks). However, the nutrition effect was more marked than the age effect. The changes in plasma TH levels in feed restricted birds
Acknowledgements
The authors thank Xaveross (Prague, Czech Republic) for the kind gift of the broiler breeders, Helena Rojčíková for technical assistance, Boris Bilčík, Ľubica Kubíková and Sabrina Brougher for the critical reading of the manuscript. This work was supported by grants from the Slovak Grant Agency for Science VEGA (2/5127/25), VEGA (1/1294/24) and the Science and Technology Assistance Agency (APVT-51-024904).
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