Foraging mechanics and their outcomes for cattle grazing reproductive tropical swards Marcelo A. Benvenutti a,b,d, * , Iain J. Gordon a , Dennis P. Poppi b , Robert Crowther c , Warwick Spinks c a Sustainable Ecosystems, CSIRO, Davies Laboratory, Queensland, Australia b Schools of Animal Studies and Veterinary Science, University of Queensland, St Lucia, Queensland, Australia c Institute of Sport and Exercise Science, James Cook University, Townsville, Australia d INTA EEA Cerro Azul, Misiones, Argentina Accepted 25 October 2007 Available online 28 November 2007 Abstract For grazing animals an important determinant of animal performance is the rate of nutrient intake (RNI) which depends on diet quality and instantaneous intake rate (IIR). In turn, diet quality and IIR are the outcome of the interaction between the morphology and behaviour of the animal and the structure of the sward. Using artificial microswards of Panicum maximum we evaluated the effect of four levels of the tensile resistance of stems in tropical swards on the grazing behaviour of cattle of two age classes (1- and 3-year-old steers) faced with a stem barrier either in a vertical (experiment 1, E1) or horizontal plane (experiment 2, E2). The animals did not select against low tensile resistance stems (LTRS) but did avoid high tensile resistance stems (HTRS) which resulted in a significant difference (P < 0.001) between diet DM (dry matter) digestibility and forage DM digestibility in swards with HTRS. IIR decreased (P < 0.001) on average 62% in E1 and 67% in E2 as stem tensile resistance increased, 144% and 177% in E1 and E2, respectively. This led to a reduction (P < 0.001) in digestible DM IIR in both young and mature cattle respectively of 56% and 68% in E1, and 45% and 79% in E2, as stem tensile resistance increased. The decline in IIR was due to an increase in time per bite and a reduction in bite dimensions that were the result of different mechanisms in the two experiments: when leaves were taller than the stems (E1) the decrease in bite area was associated with an asymptotic increase (P < 0.001) in bite force in mature animals, and a maintenance (P= 0.456) of bite force in young animals, suggesting that maximum bite force regulated bite area in E1. This was not the case in E2 because both bite area and bite force decreased (P < 0.05) with the increase in stem tensile resistance suggesting that the animals were not able to gather enough plant material with the tongue in order for bite force to regulate bite area. We conclude that bite force and tongue force www.elsevier.com/locate/applanim Available online at www.sciencedirect.com Applied Animal Behaviour Science 113 (2008) 15–31 * Corresponding author at: Sustainable Ecosystems, CSIRO, Davies Laboratory, PMB PO Aitkenvale, Queensland 4814, Australia. Tel.: +61 7 47538664; fax: +61 7 47538600. E-mail address: Marcelo.Benvenutti@csiro.au (M.A. Benvenutti). 0168-1591/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.applanim.2007.10.005