Abstract Intermediate wheatgrass [IWG; Thinopyrum intermedium (Host) Barkworth & D.R. Dewey] is a perennial grass, dual‐purpose crop that can provide environmental services. In addition to the grain production, assessing its forage potential is crucial. We developed models for near‐infrared (NIR) spectrometry prediction of the chemical composition and digestibility of IWG across various experimental sites. Among these, a Belgian field was used to compare its dual production in pure stands or in mixture with legumes. Good quality NIR predictions were observed, offering an efficient tool to characterize the forage composition of IWG. Its forage parameters were mainly influenced by the phenological stage with an increase of fiber and a decrease of protein, digestibility, and energy content (NE L ) during the growing season. IWG forage at vegetative stages could be used to feed lactating dairy cattle with a NE L of 1625 kcal kg −1 of DM but, its biomass was low averaging 1.8 t of DM ha −1 . At grain maturity, biomass was higher (i.e., 5.3 t of DM ha −1 ), representing 73%–92% of the total biomass production, and could replace straw in high‐starch dairy diets with a NE L averaging 849 kcal kg −1 of DM. Although the mixture of IWG with legumes enhanced some forage parameters, its value as animal feed was not improved. In mixture, we observed a tradeoff between the increase of the forage yield and the reproductive potential of IWG. These insights can inform the on‐going process of breeding and help farmers to design relevant systems to experiment this new crop.