The last 4 weeks of pregnancy and the first 4 weeks of lactation, known as the transition or periparturient period, is a highly critical period for dairy cows. Because their defense mechanisms are often insufficient against the intense action of numerous physiological and environmental factors, especially high-yielding dairy cows are the most susceptible to oxidative stress in this period. The increased intensity of processes related to the metabolism of large amounts of energy, accompanied by the emergence of a large quantity of free radicals and reactive oxygen species (RFT) causes that the secondary metabolic products are not completely neutralized. An increased level of RFT adversely affects metabolism, gene expression, and modification of proteins. Free radicals cause the reduction of the level of polyunsaturated fatty acids in tissues, increasing the production a keto- and hydroxy-acid and lipid peroxides, which are secondary radicals. Long-term oxidative stress causes permanent changes in the structure of biological active macromolecules in cells (DNA, proteins, sugars, and others). Abnormalities in metabolism cause various diseases, e.g. stopping estrus after calving, udder edema, mastitis, retained placenta, and the premature dying of embryos. In order to minimize these negative phenomena, dairy cows should be kept in proper conditions to reduce the deficiency of energy after calving. The diet should be balanced carefully according to its protein-energy ratio. A very important aspect is the provision of optimal quantities of macro- and microelements and vitamins, which are the active components of antioxidative system.
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