The objective of this study was to evaluate the effects of three different grass silage (GS) based diets on environmental impacts and animal performance of finishing bulls. Experiment 1 (Exp 1) was conducted using 53 Aberdeen angus (AA) and 52 Simmental (SI) beef bulls and experiment 2 (Exp 2) included 69 Holstein (HO) and 36 Nordic Red (NR) dairy bulls. The three experimental diets included three different proportions of concentrates and two different types of forages. Moderate concentrate diet (MC) included GS, barley grain and mineral-vitamin mixture in proportions of 600:385:15 on dry matter (DM) basis while the respective proportions on low concentrate diet (LC) were 850:135:15. No concentrate diet (NC) included GS, whole crop barley silage and mineral-vitamin mixture (685:300:15). The bulls were fed total mixed rations ad libitum. Environmental impacts of the dietary treatments were estimated using a gate-to-gate life cycle assessment approach with the following key impact categories; global warming potential (GWP), eutrophication, and acidification potential. In Exp 1 the carcass gain, carcass weight and carcass conformation of the SI bulls were 10, 8 and 23% higher, respectively, compared to the AA bulls (P < 0.001) but there was no difference in feed conversion (kg DM/kg carcass gain) between AA and SI. In Exp 2 the NR bulls had 12% better conformed carcasses compared to the HO bulls (P < 0.001) but the carcass gain of the HO bulls tended to be 4% higher (P = 0.06) compared to NR. There was no difference in feed conversion or carcass weight between HO and NR. In both experiments, LC and NC diets impaired growth performance, feed conversion and carcass conformation compared to MC. In Exp 1 the GWP was 19.1 kg CO₂ eq/kg of produced beef during the experiment with MC, 20.7 kg with LC and 25.9 kg with NC. Eutrophic emissions were 17.1, 21.3 and 21.8 g PO₄ eq/kg of produced beef and acidifying emissions 38.6, 49.9 and 53.7 AE eq/kg of produced beef, respectively for MC, LC, and NC treatments. In Exp 2 the GWP was 20.7, 21.7 and 26.3 CO₂ eq/kg of produced beef, eutrophic emissions 16.4, 18.9 and 20,8 PO₄ eq/kg of produced beef and acidifying emissions 39.4, 50.6 and 59.2 AE eq/kg of produced beef for MC, LC, and NC treatments, respectively. It can be concluded that impaired feed conversion in LC and NC diets led to the higher environmental impacts compared to MC.