Mixing at the beginning of fattening moderates social buffering in beef bulls Luc Mounier a,b, * , Isabelle Veissier a , Stephane Andanson a , Eric Delval a , Alain Boissy a a Institut National de la Recherche Agronomique, Unite ´ de Recherche sur les Herbivores, Adaptation et Comportements sociaux, 63122 Saint Genes Champanelle, France b Ecole Nationale Ve ´te ´rinaire de Lyon, Unite ´ de zootechnie, 69280 Marcy L’Etoile, France Accepted 24 June 2005 Available online 25 July 2005 Abstract Cattle establish strong preferential associations early in their life that determine the cohe- siveness within a group and can diminish fear responses (i.e. social buffering). Mixing beef bulls at the beginning of fattening may lead to less cohesiveness. Moreover, mixing beef bulls induces aggressive interactions, especially when the bulls are of similar weights. This study was aimed at evaluating the cohesiveness and fear responses of 64 bulls that were either mixed versus unmixed and of homogeneous vs. heterogeneous body weights (2  2 factorial design, four groups of four bulls each per treatment). Social cohesiveness was assessed during food competition and a social separation test. Fear responses were observed during the social separation test and pre-slaughter handling. During food competition, unmixed bulls exchanged more non-agonistic interactions (P < 0.05) and were more tolerant with each other than mixed bulls (time spent eating: P < 0.05). Unmixed bulls displayed less fear responses during physical separation (elimination: P < 0.05; startle: P < 0.01) and were less stressed by pre-slaughter handling (blood cortisol: P < 0.05). Very few effects could be attributed to the homogeneity of weights within groups. It is concluded that mixing bulls at the beginning of fattening reduces the strength of positive social relationships and renders social buffering less effective during subsequent fattening. We recommend maintaining www.elsevier.com/locate/applanim Applied Animal Behaviour Science 96 (2006) 185–200 * Corresponding author. Tel.: +33 478 87 27 71; fax: +33 478 87 26 67. E-mail address: l.mounier@vet-lyon.fr (L. Mounier). 0168-1591/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.applanim.2005.06.015