Mycotoxin-deactivating aptamers, especially DNA aptamers, bind to mycotoxins in feed and feed ingredients resulting in the reduction or elimination of toxic and carcinogenic effects of mycotoxins. The invention also discloses a composition comprising a mycotoxin-deactivating aptamer, a binding agent, a biotransforming agent and an antioxidant for detoxifying mycotoxins in feeds. In addition, the invention teaches the methods of preparing the mycotoxin-deactivating aptamer-based composition and also the methods of using it as a feed additive. Furthermore, the invention relates to the use of the mycotoxin-deactivator/s alone, or in a composition comprising the aptamers and other mycotoxin-detoxifying agents, in feeds and feed ingredients for detoxifying the major mycotoxins such as aflatoxins, deoxynivalenol, zearalenone, fumonisins and ochratoxin A.