This study compared the nutritional composition, bioactive compounds, and in vitro ruminal fermentation characteristics of ribwort plantain (Plantago lanceolata; PLA) with Italian ryegrass (Lolium multiflorum; LOM) and sainfoin (Onobrychis viciifolia; OVI) cultivated under arid conditions. Herbage samples were analysed for nutrient composition, amino acids, fatty acids, minerals, carotenoids, and secondary metabolites. In vitro ruminal fermentation was evaluated using rumen fluid incubation, and differences between forages were assessed statistically. PLA showed significantly higher concentrations of non-fibrous carbohydrates (NFC), α-linolenic acid, and several amino acids, including aspartic acid, glutamic acid, glycine, isoleucine, leucine, and phenylalanine, compared to LOM and OVI (P < 0.05). It also contained higher levels of minerals (Ca, Fe, and S) and carotenoids (lutein, zeaxanthin, β-carotene, γ-carotene, lycopene, and β-cryptoxanthin), resulting in increased total carotenoid and provitamin-A contents. The concentrations of serine, histidine, arginine, threonine, and tyrosine in PLA were similar to those of LOM but higher than in OVI (P < 0.05). Saponin content in PLA was comparable to OVI and higher than in LOM (P < 0.05). Although PLA produced lower herbage yield under arid conditions, it demonstrated superior nutritional quality and a richer bioactive profile. These findings suggest that PLA can be considered a valuable alternative forage source in arid environments due to its high content of essential nutrients, omega-3 fatty acids, carotenoids, and secondary metabolites.