Simple SummaryCow’s milk is a source of many nutrients and bioactive substances that have a positive impact on consumers’ health. The fat fraction is a valuable source of C18:2cis9trans11 and C18:1trans11, as well as the vitamins A, D, E, and K. The concentrations of these substances in milk can vary depending on both environmental and genetic factors. Good-quality milk with high antioxidant potential is obtained from organic production systems where the animals have permanent access to pasture. However, milk from this form of production system shows seasonal fluctuations in the levels of bioactive components, mostly due to the type of feed intake at end of the grazing season. During the winter season, silage and hay are the main feedstuffs for cows; therefore, the level of bioactive substances in the milk decreases. The aim of this study was to determine the effects of using linseed cake supplementation during the winter period on the levels of some bioactive components in the fat fraction of milk from cows kept on an organic farm.AbstractThe aim of the study was to determine the effect of linseed cake supplementation during the winter period as a factor influencing the level of some bioactive components (milk composition, fatty-acid profile, and fat-soluble vitamins) in the milk fat fraction in cows kept on an organic farm. Forty multiparous (second and third lactation) Holstein–Friesian cows were selected that had 81 ± 12 days in milk and produced 15.08 ± 1.20 kg of milk/day. Two groups were created for the experiment: control (CTL; n = 20) and experimental (LC; n = 20). The experiment was divided into two periods: an initial period lasting 7 days in which the experimental group was habituated to the new supplement in their diet; the proper experimental phase, lasting 6 weeks, in which the cows in the experimental group received an individual daily dose of linseed cake (300 g/day/cow). Linseed cake supplementation had a positive impact on the levels of bioactive components (fatty-acid profile and fat-soluble vitamins) in the milk fat fraction. At the end of the trial, the concentration of C18:2cis9trans11, C18:1trans11, α-retinol, α-tocopherol, and total antioxidant status increased 1.59-, 1.94-, 3.12-, 3.38-, and 3.09-fold, respectively, relative to the control levels. The use of linseed cake in winter on organic farms makes it possible to increase the antioxidant potential of milk, thereby eliminating the disparity in the quality of milk from the summer season compared to the winter season.