The aim of this study was to investigate an association between aggressiveness and high level of feeding in a half-open feedlot production system. An experiment was conducted on 72 head of beef cattle of different breeds. The animals were at about 10 months of age. Medium quality silage was offered ad libitum and supplemented with high (HE) and low level (LE) of barley (2.5 and 1.5 kg/day/head, respectively) and supplemented without (nil) or with (+) extracted soybean meal (0.45 kg/day/head). Several types of animal behaviour were observed such as those parameters that are categorized to be main aggressive behaviours, butting, being butted, mounting and being mounted. Significant differences (P < 0.05) were found in butting, being butted behaviours between HE and LE treatment groups. Mounting and being mounted behaviours were significantly different (P < 0.05) in steers and heifers and between the seasons as well. Steers performed more incidents of mounting behaviour than heifers and it was the same for spring, during which animals had more mounting behaviours. It was concluded that there was a close relationship between high-energy diets and aggressive behaviour, which necessitates some management measures to be taken in order to ensure better animal welfare and beef production.