The effects of feed restriction on plasma biochemistry in growing meat type chickens (Gallus gallus)

Abstract

The effect of feed restriction on plasma hormones (triiodothyronine — T₃, thyroxine — T₄, and corticosterone), protein, lipid, carbohydrate, and mineral metabolism and activity of plasma enzymes (creatine kinase, alkaline phosphatase, aspartate aminotransferase, and alanine aminotransferase) were studied in meat type female chickens (Gallus gallus). Ad libitum fed birds were compared with those subjected to severe and moderate quantitative feed restriction from 16 to 100 days of age. Feed restriction elevated plasma T₄ and corticosterone levels and reduced T₃. A feed restriction-induced decrease was observed for plasma protein and albumin concentrations, but not for uric acid and creatinine. Total plasma lipids, triacylglycerols, cholesterol, high density lipids, and calcium were lower for the feed restricted chickens, in particular during the latter phase of the experiment. Concentrations of glucose and phosphorus were not altered by feeding treatment. Activity of alkaline phosphatase was significantly increased in restricted chicks from day 58. Significant changes of plasma biochemical parameters induced by severe and moderate quantitative feed restriction illustrate that limiting feed intake poses an intensive stress on meat type chickens during the rapid growth period. However, activities of creatine kinase, aspartate aminotransferase, and alanine aminotransferase were significantly higher in ad libitum fed chickens during this period. This elevation in enzymatic activity may be in response to tissue damage, indicating potential health and welfare problems also in ad libitum fed meat type chickens, resulting from selection for intensive growth.

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.