WEED COMMUNITY AND SPECIES RESPONSE TO CROP-ROTATION, TILLAGE, AND NITROGEN FERTILITY

Producers in the northern Great Plains are exploring alternative crop rotations, with the goal of replacing spring wheat-fallow. We characte rized the weed associations occurring with tillage system and nitrogen level in two rotations, spring wheat (SW)-fallow (F) and SW-winter wh eat (WW)-sunflower (SUN). Weed density was measured 10 yr after initia tion of the study. With both rotations, weed community density was hig hest with no-till. For SW-F, green foxtail, yellow foxtail, and fairy candelabra comprised 99% of the weed community, whereas 13 species wer e observed in SW-WW-SUN. Fairy candelabra, a rangeland species, was ob served only in the no-till system of SW-F In SW-WW-SUN, no-till favore d kochia, Russian thistle, and foxtails, whereas common lambsquarters and annual sowthistle were more common in tilled systems. Nitrogen fer tilizer increased crop competitiveness in SW-WW-SUN with no-till, subs equently reducing weed density. Cultural strategies that disrupt weed associations will aid producers in managing weeds.