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The multipurpose cardoon crop (Cynara cardunculus var. altilis) is also exploited to produce bio-intermediates for biorefinery; the press cake composed by partially defatted seed residue after oil extraction represents an important byproduct. This research was addressed at elucidating feeding value and traits of cardoon press cake to be exploited as a novel supplement for feeding ruminants. Specific aims were: to (i) determine the nutritional profile of press cake and (ii) evaluate the effect of anti-tannic agents on methane production, ammonia level, ruminal volatile fatty acids profile, ruminal biohydrogenation of linoleic and linolenic acids, crude protein (CP) degradation and fiber (NDF) digestion. Cardoon press cake shows moderate level of CP (18.5%) and fat (7.9%) with high level of NDF, and represents a valuable source of both essential amino acids and n-6 fatty acids. The content of phenolics reached 32.7 mg g−1 and could positively affect the transfer of n-6 unsaturated fatty acids (−50% of biohydrogenation) in milk and meat ruminant products without negative effects for CP degradability and NDF digestibility. In addition, it is able to modulate the methane release, in particular 48 h after incubation. A moderate level of the polyphenol oxidase was also detected. In vitro results indicate that cardoon press cake is suitable for animal feeding and for improving their performances and products with potential benefits for human consumers, due to its content of unsaturated fatty acids with a proper amount of phenolic compounds.