Complex pasture mixtures are advantageous, but little information exists on the best forage species and nitrogen (N) management in eastern Canada. We compared under mob stocking four complex mixtures of one of two legume species [alfalfa (Medicago sativa L.) and birdsfoot trefoil (Lotus corniculatus L.)] plus one of two grass mixes [No. 1 — timothy (Phleum pratense L.), meadow fescue (Schedonorus pratensis (Huds.) P. Beauv.), reed canarygrass (Phalaris arundinacea L.), and Kentucky bluegrass (Poa pratensis L.); No. 2 — tall fescue (Schedonorus arundinaceus (Schreb.) Dumort., nom. cons.), meadow bromegrass (Bromus biebersteinii Roem. and Schult.), reed canarygrass, and Kentucky bluegrass] under three N application rates at three sites over the first three post-seeding years. Legume species had little effect on most forage attributes mainly because of their low contribution to forage dry matter (DM) yield (<30%) in second and third years. The grass mix with tall fescue and meadow bromegrass (No. 2) yielded similar or slightly better than the one with timothy and meadow fescue (No. 1) but tended to have a greater neutral detergent fibre concentration, and lower N and total digestible nutrient concentrations. Nitrogen fertilization increased forage DM yield only in second and third years when the legume contribution to forage DM yield was <30%, and it increased N concentration and decreased nonstructural carbohydrate concentration. These results highlight the challenge of maintaining legume species in rotationally grazed pastures of eastern Canada and confirm recommendations of applying no or little N fertilizer on grazed legume–grass mixtures when the legume contribution to forage DM yield is >30%.