Quackgrass [Elytrigia repens (L.) Nevski; Agropyron repens (L.) Beanv.; Elymus repens (L.) Gould] is a perennial grass that has colonized much of the world's temperate cropland. Its persistence, perenniality, high nutritional value, and tolerance to several stress factors make it a potentially useful forage crop. This research was undertaken to assess the breeding improvement potential of quackgrass for direct use as a forage crop. Thirty‐five quackgrass parent clones were grown in a series of four greenhouse experiments (growth periods) between November 1981 and March 1984, and in a field experiment from July 1984 to June 1985. A random sample of their polycross progeny was grown in a field experiment from September 1982 to June 1984. Parent means, family means, and progeny means within families varied (P < 0.01) for all traits determined. Greenhouse traits had moderate repeatability (0.50–0.72), high clone × growth period interaction, and low correlation coefficients with field traits (r = −0.50–0.32). Thus, selection should be conducted in the field. There were large amounts of additive genetic variation for all three field traits (forage yield, plant diameter, and maturity). Realized gains from one cycle of selection were between expected gains for disomic and tetrasomic inheritance models. Late maturity was associated with low plant diameter during selection. Forage yield responded to selection, but appeared to be governed primarily by inbreeding because of small effective population sizes. Quackgrass plants that thrive in pastures and croplands still contain hidden alleles for low‐rhizome‐spreading phenotypes.