Performance and behaviour of calves reared in groups or individually following an enhanced-growth feeding programme Marta Terre ´ 1 , Alex Bach 2,1 * and Maria Devant 1 1 Grup de Nutricio ´ , Maneig i Benestar Animal, Unitat de Remugants, Institut de Recerca i Tecnologia Agroalimenta `ries (IRTA) 08193 Barcelona, Spain 2 Institucio ´ Catalana de Recerca i Estudis Avanc ¸ats (ICREA), Barcelona, Spain Received 10 January 2006 and accepted for publication 16 May 2006 Nineteen calves were reared in individual pens (IP), and twenty calves were grouped in four pens of five calves each (GP) to study the effects of grouping on behaviour and performance. All calves were on an enhanced-growth feeding programme and had unlimited access to starter. Total dry matter intake (DMI) body weight (BW), serum urea, non-esterified fatty acid (NEFA), cortisol, insulin, and glucose concentrations, and immune response to vaccination were measured. Behaviour patterns categorized as : non-nutritive oral behaviour, cross-sucking, inter- sucking, and self-grooming, were monitored 20 min following the morning and afternoon milk replacer consumption, and at the same times after weaning. There were no differences in final BW and starter DMI between treatments. The decrease of average daily gain (ADG) during the week following weaning was more pronounced in GP than in IP calves. Serum NEFA concentration increased at the beginning of the study in IP calves but serum urea concentrations were greater in GP than in IP calves the week after weaning. Self-grooming increased in both treatments throughout the study but GP calves increased non-nutritive oral behaviour and decreased cross-sucking and inter-sucking behaviours throughout the study. Rearing calves in groups following an enhanced-growth feeding programme did not increase starter intake. Furthermore, cross-sucking and inter-sucking behaviours in calves reared in groups did not seem to cause increased metabolic or immunological distress. Keywords : Calves, behaviour, grouping, weaning. In recent years, enhanced-growth feeding programmes for dairy calves have been proposed to increase growth rate during the preweaning period. The aims of these pro- grammes are to achieve the full expression of the genetic growth potential of suckling calves, raise calves more healthfully, and reduce the age at first calving without compromising the structural development of replacement heifers. The most common enhanced-growth feeding pro- grammes are based on feeding calves milk replacer (MR) ad libitum ( Jasper & Weary, 2002), providing dry matter (DM) from MR at 4% of body weight (BW) (Diaz et al. 2001), leaving calves suck the dam freely (Bar-Peled et al. 1997), or gradually feeding increasing amounts of MR (Terre ´ et al. 2006). However, a common pitfall reported in all studies using enhanced-growth feeding programmes is that calves struggle at weaning, and average daily gain (ADG) during the week after weaning is reduced by more than half compared with the ADG achieved before wean- ing (Bar-Peled et al. 1997; Terre ´ et al. 2006), probably as a consequence of the low starter intake during the pre- weaning period. Lambs seem to learn eating behaviour by watching the adult behaviour (Phillips & Youssef, 2003). Similarly, calves reared in groups increased fresh grass intake com- pared with calves reared individually (Phillips, 2004) ; the author suggested that the sight of feed being taken into the mouth was the relevant stimulus to learning the feeding behaviour from each other. The objective of the present study was to evaluate the potential role of rearing calves in groups on starter intake and performance following an enhanced-growth feeding programme. Materials and Methods Forty Holstein male calves (11 . 5±1 . 92 d old, and weigh- ing 40 . 6±3 . 83 kg) were purchased from commercial farms and raised in the facilities of IRTA (Prat, Spain) in spring *For correspondence ; e-mail : alex.bach@irta.es Journal of Dairy Research (2006) 73 480–486. f Proprietors of Journal of Dairy Research 2006 480 doi:10.1017/S0022029906002019 Printed in the United Kingdom