Effect of tillage systems, N, and cover crop on the composition of weed flora

The adoption of conservation tillage systems has been challenged by concern s of potential weed species shifts. A 9-yr study from 1988 to 1996 was cond ucted at Delhi, Ontario, on a loamy sand soil to evaluate the effect of til lage systems (conventional [CT] and no-till [NT]), cover crop Secale cereal e and nitrogen (N) rate (0, 50, 100, 125, 150, and 200 kg N ha(-1)) on mono crop Zea mays L. (corn) yield and changes in the composition of the weed fl ora. CT consisted of spring moldboard Flowing followed by cultivation with a tooth cultivator. Weed counts were taken in the last 3 yr of the study (1 994, 1995, and 1996) prior to postemergence herbicide application and then again 2 to 3 wk after herbicide treatment. Composition of the weed flora wa s analyzed by canonical discriminant analysis (CDA). The relationship betwe en weed density and tillage system was not consistent. Weed species composi tion differed between CT and NT systems. Chenopodium album and Amaranthus r etroflexus were associated with CT and Digitaria sanguinalis with NT. N rat e and cover crop did not affect weed density or species composition. Proper management of weeds with herbicides appeared to minimize any long-term eff ect on the weed flora resulting from varying N rates. Zea mays yields did n or differ between CT and NT systems but were greater in both systems with a cover crop at the higher N rates. Disturbance caused by tillage was more i mportant than N rate and cover crop as a mechanism influencing composition of the weed flora.