3427 ABSTRACT: The aim of the present study was to estimate genetic parameters for flight speed and its association with growth traits in Nellore beef cattle. The flight speed (FS) of 7,402 yearling animals was measured, using a device composed of a pair of photoelectric cells. Time interval data (s) were converted to speed (m/s) and faster animals were regarded as more reactive. The growth traits analyzed were weaning weight (WW), ADG from weaning to yearling age, and yearling scrotal circumference (SC). The (co)variance components were estimated using REML in a multitrait analysis applying an animal model. The model included random direct additive genetic and residual effects, fixed effects of contemporary groups, age of dam (classes), and age of animal as covariable. For WW, the model also included maternal genetic and permanent environmental random effects. The direct heritability estimate for FS was 0.26 ± 0.05 and direct heritability estimates for WW, SC, and ADG were 0.30 ± 0.01, 0.48 ± 0.02, and 0.19 ± 0.01, respectively. Estimates of the genetic correlation between FS and the growth traits were -0.12 ± 0.07 (WW), –0.13 ± 0.08 (ADG), and –0.11 ± 0.07 (SC). Although the values were low, these correlations showed that animals with better temperaments (slower FS) tended to present better performance. It is possible to infer that long- term selection for weight and scrotal circumference can promote a positive genetic response in the temperament of animals. Nevertheless, to obtain faster genetic progress in temperament, it would be necessary to perform direct selection for such trait. Flight speed is an easily measured indicator of temperament and can be included as a selection criterion in breeding programs for Nellore cattle. Key words: average daily gain, genetic correlation, heritability, scrotal circumference, temperament Genetic associations between flight speed and growth traits in Nellore cattle 1 A. C. Sant’Anna,* M. J. R. Paranhos da Costa,† 2 F. Baldi,‡ P. M. Rueda,§ L. G. Albuquerque# *Programa de Pós-graduação em Genética e Melhoramento Animal, FCAV, Universidade Estadual Paulista 14.884- 900 Jaboticabal, Brazil; †Grupo de Estudos e Pesquisas em Etologia e Ecologia Animal, Departamento de Zootecnia, FCAV, Universidade Estadual Paulista 14.884-900 Jaboticabal, Brazil; ‡Departamento de Nutrição e Produção Animal, FMVZ, Universidade de São Paulo 13.635-900 Pirassununga, Brazil; §Programa de Pós-graduação em Zootecnia, FCAV, Universidade Estadual Paulista 14.884-900 Jaboticabal, Brazil; and #Departamento de Zootecnia, FCAV, Universidade Estadual Paulista 14.884-900 Jaboticabal, Brazil © 2012 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2012.90:3427–3432 doi:10.2527/jas2011-5044 INTRODUCTION Brazilian beef cattle production is characterized by the use of extensive systems and a predominance of Zebu breeds, mostly Nellore and its crosses. The ability of Zebu breeds to adapt to the climatic conditions that prevail in the tropical zone, as well as their moderate growth capacity and resistance to ectoparasite infestations, favors their use in extensive production systems under tropical conditions (Cundiff, 2005). Several studies comparing Zebu and European breeds have shown that Zebu breeds and their crosses demonstrate greater reactivity, defined by a behavioral predisposition to respond to handling by humans (Fordyce et al., 1988; Burrow, 1997). The management of highly reactive animals is more difficult and brings a series of inconveniences, such as an increased stress response in animals (Curley et al., 2006) and risk of accidents (Grandin, 1999), resulting in a poor reputation for these breeds due to their temperament (Fordyce et al., 1988). In this context, it is possible to reduce cattle reactivity, either through the learning process resulting 1 This research was funded by the Fundação de Amparo à Pesquisa do Estado de São Paulo—FAPESP (2009/53609-7). The study was part of the doctoral thesis of the lead author, prepared for the postgraduate program in genetics and animal breeding at Universidade Estadual Paulista Júlio de Mesquita Filho (UNESP/São Paulo State University), Jaboticabal Campus. Appreciation is expressed to Arione Boligon, Désirée Soares, and Tiago Valente, UNESP, Jaboticabal-SP, Brazil, for their assistance during this study. 2 Corresponding author: mpcosta@fcav.unesp.br Received December 16, 2011. Accepted April 10, 2012. Published January 20, 2015