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 charactenrzed the weed associations occurring with tillage system and nitrogen level in two rotations, spring wheat (SW)-fallow (F) and SW-winter wheat (WW)-sunflower (SUN). Weed density was measured 10 yr after initiation of the study. With both rotations, weed community density was highest with no-till. For SW-F, green foxtail, yellow foxtail, and fairy candelabra compnsed 99% of the weed community, whereas 13 species were observed in SW-WW-SUN. Fairy candelabra, a rangeland species, was observed only in the no-till system of SW-F In SW-WW-SUN, no-till favored kochia, Russian thistle, and foxtails, whereas common lambs- quarters and annual sowthistle were more common in tilled systems. Nitrogen fertilizer increased crop competitiveness in SW-WW-SUN with no-till, subsequently reducing weed density. Cultural strategies that disrupt weed associations will aid producers in managing weeds. Nomenclature: Annual sowthistle, Sonchus oleraceus L. #1 SONOL; common lambsquarters, Che- nopodium album L. # CHEAL; fairy candelabra, Androsace occidentalis Lunell; green foxtail, Setaria viridis (L.) Beauv. # SETVI; kochia, Kochia scoparia (L.) Schrad. # KCHSC; Russian thistle, Salsola