ORIGINAL ARTICLE Effect of macronutrient ratio of the pre-starter diet on broiler performance and intermediary metabolism Q. Swennen 1,4 , N. Everaert 1 , M. Debonne 1 , I. Verbaeys 2 , C. Careghi 1 , K. Tona 1 , G. P. J. Janssens 3 , E. Decuypere 1 , V. Bruggeman 1 and J. Buyse 1 1 Laboratory of Livestock Physiology, Immunology and Genetics, Department of Biosystems, K.U. Leuven, Leuven, Belgium, 2 Interdisciplinary Research Centre, K.U. Leuven Campus Kortrijk, E. Kortrijk, Belgium, 3 Laboratory of Animal Nutrition, Ghent University, Merelbeke, Belgium, and 4 Centre for Environmental Sciences, Hasselt University, Diepenbeek, Belgium Introduction It is very well documented that the ratio between macronutrients (protein, lipid and carbohydrate) has a major impact on zootechnical performance and on body composition of chickens (Buyse et al., 1992; Nieto et al., 1997; Smith and Pesti, 1998; Bregendahl et al., 2002; Collin et al., 2003; Swennen et al., 2007a). Changing the concentration of one macro- nutrient in the diet has an effect on the level of the other macronutrients, which makes it difficult to ascribe the observed effects to one particular macronutrient. Only few studies have taken this into account (Buyse et al., 2001a). A series of experiments was performed using diets in which the energetic substitution of one particular macronutri- ent was performed by only one other macronutrient while keeping the concentration of the third macro- nutrient constant in broiler chickens from 2 weeks of age onwards. The results showed that changing the dietary protein level affected protein and fat deposition, the intermediary metabolism and endo- crine functioning (Collin et al., 2003; Malheiros et al., 2003c; Swennen et al., 2005, 2006) as well as the total energy and protein balance (Swennen et al., 2004, 2007a,b). On the other hand, isoener- getic substitution of fat for carbohydrate energy with constant protein level had little effect on the Keywords broiler chickens, pre-starter diet, macronutrient ratio, performance, intermediary metabolism Correspondence Prof. J. Buyse, Laboratory of Livestock Physiology, Immunology and Genetics, Department of Biosystems, K.U. Leuven, Kasteelpark Arenberg 30, 3001 Leuven, Belgium. Tel: +32 16 32 85 25; Fax: +32 16 32 19 94; E-mail: johan.buyse@biw.kuleuven.be Received: 9 July 2008; accepted: 1 December 2008 First published online: 11 November 2009 Summary The aim of this study was to investigate the influence of isoenergetic substitution between the three energy delivering macronutrients in pre- starter diets on performance and intermediary nutrient metabolism in broiler chickens. From hatch until 5 days of age, 600 chicks, collected during peak of hatch, were fed one of the three experimental pre-starter diets with isoenergetic (13 MJ metabolisable energy/kg) substitutions between fat (43 vs. 108 g/kg), protein (126 vs. 240 g/kg) and carbohy- drates (391 vs. 510 g/kg). After 5 days, commercial grower and finisher diets were provided. Pre-starter composition influenced body weight until slaughter age, although not statistically verifiable. Broilers fed the low protein (LP) pre-starter had the lowest body weight in relation to chickens on the low carbohydrate or low fat pre-starter diet. After hatch, chicks on the LP pre-starter diet were able to use the residual yolk sac more rapidly to fulfil their protein requirement, which is reflected in small intestine and liver development. Also, plasma metabo- lite levels were influenced mostly by the LP pre-starter, indicating that the main focus for the requirements of newly hatched chicks should be on proteins. Furthermore, optimal nutrition during the first day’s post- hatch should take into account the contribution of the yolk. DOI: 10.1111/j.1439-0396.2009.00918.x Journal of Animal Physiology and Animal Nutrition 94 (2010) 375–384 ª 2009 Blackwell Verlag GmbH 375