Weed communities of winter wheat as influenced by input level and rotation

The influence of cropping systems constituted by the combination of three input levels (low, intermediate and high) and four 2-year rotations between a spring-summer crop and winter wheat (Triticum aestivum L.) on the density and composition of the winter wheat weed flora was evaluated from 1989 to 1992. Spring-summer crops were maize (Zea mays L.), soyabean [Glycine max (L.) Merr.], sugar beet (Beta vulgaris L.) and sunflower (Helianthus annuus L.). Total weed density at the end of the wheat tillering stage ranged from 2.5 to 128.7 plants m-2 and generally decreased with cropping intensity, although to a differing extent, depending upon rotation. Differences among weed communities of the 12 cropping systems were mostly dependent upon input level. The effect of the preceding crop on the weed flora composition of winter wheat was evident only with reduced herbicide use (low-input systems). Weed species could be ascribed to four categories: (a) species sporadically associated with cropping systems (most of the weeds); (b) species associated with a specific input level (e.g. Papaver rhoeas L. to low and intermediate input levels and Veronica persica Poiret to the high input level); (c) species associated with a specific rotation (e.g. Ammi majus L. to sugar beet -winter wheat); and (d) species associated with specific plots that become important every other year (e.g. Convolvulus arvensis L. and Stellaria media (L.) Vill.).