The aim of this study was to analyze, on beef lipids and FA and on beef colour, the impact of extruded linseed (rich in n-3 PUFA) given alone or with rapeseed (rich in n-3 and n-6 PUFA and in 18:1 n-9) in association or not with antioxidants given to Normand (fat breed) cull cows during the finishing period. Dietary linseeds significantly increased proportions of 18:3 n-3 (+56 and +36% respectively), total trans 18:1 (+66 and +105%); and of 9cis, 11trans 18:2 (CLA) (+50 and +41%) in LT muscle of which 18:3 n-3 and CLA are known to be beneficial for the human health. Addition of antioxidants (vitamin E plus plant extracts rich in polyphenols, PERP) reinforced the stimulating effect of lipid supplements on proportions of the three considered FA (18:3 n-3, total trans 18:1, 9cis, 11trans 18:2) in total lipids of both muscles. Dietary n-3 PUFA reduced the resistance capacity of plasma against lipoperoxidation (–11%) favouring peroxidized products formations such as conjugated dienes (×1.75) and malonedialdehyde (MDA, ×2). Intensity of beef lipoperoxidation (estimated by MDA level) tended to be higher in meats packaged under modified atmosphere rich in oxygen (70%) in the linseed group (2.96 μg/g of tissue) compared to that of the control group (2.19 μg/g of tissue, P = 0.1). Dietary vitamin E provided with PERP effectively protected beef against lipoperoxidation, even in beef packagings with O2. Beef under air packaging from cows given the linseed supplement had a higher redness value than that from control cows. In the case of animals given lipid supplemented diets and submitted to an emotional stress, beef packaged under air or modified atmosphere rich in O2 had a lower redness than that of the same animals given lipids associated to antioxidant supplements. We concluded that the dietary strategy combining addition of n-3 PUFA and of the antioxidant mixture in diets would avoid major risks of lipoperoxidation and alteration of colour in beef products whatever: 1) the conditions of meat ageing and packaging (especially those with contact with oxygen); 2) the degree of emotional stress of animals just prior slaughter.