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The possibility for undersowing with subterranean clover of degraded alfalfa seed production stands and used for forage thereafter was studied in a field trial in the Institute of Forage Crops, Pleven, Bulgaria (2007-2013). Three subterranean clover subspecies were tested: Tr. subterraneum ssp. brachycalicinum (cv. “Antas”), Tr. subterraneum ssp. yaninicum (cv. “Trikkala”) and Tr. subterraneum ssp. subterraneum (cv. “Denmark”). The undersowing with clover was performed during the autumn of the fourth year (2010) of using the alfalfa stands with 400 germinated seeds/m2 and interrow spacing of 12 cm. Four cuts were obtained during the first and second year after the undersowing and three cuts during the last. It was found that the subterranean clover contributed to the improvement of the botanical composition, as well as to a two- and three-fold reduction of the weed infestation and it also increased the dry matter yield. Tr. subterraneum ssp. brachycalicinum showed the best development and share in the stands. Undersowing with subterranean clover of degraded alfalfa stands used for seed production is a possible agrotechnical measure.