Abstract:

The overall weed infestation indexes and density of all weed populations occurring in both the rice_cotton crop rotation and continuous dry-cropping systems of the main cotton belts in Jiangsu Province were surveyed over five years continuously. The succession patterns of weed communities and weed occurrence were studied through statistical analysis in space and time. The results showed that there was a close relationship between the structure of weed community and the level of weed infestation and cropping systems. Under the rice-cotton crop rotation system, the density of hydrophilous weeds such as Echinochloa crusgalli, Alternanthera philoxeroides and Malachium aquaticum was the highest, while that of xerophilous weeds (e.g.，Digitaria sanguinalis) was lowest. However, there was marked linear relationship between abundance and time, with abundance of hydrophilous weeds (e.g.，E. crusgalli) decreasing, and that of xerophilous weeds as (e.g．，D. sanguinalis) increasing with time, when cotton was grown continuously in the fields with the rice_cotton rotation system. After four years continuous cotton growing, the structure of weed community seemed to tend to stable condition. In the cotton fields with cotton cropping system, there were dense density of xerophilous weeds as dominants and no marked change of weed community structure between different years. Moreover, the marked difference of weed community structure and weed density occurred between two different cropping systems. The influence of cropping systems on weed community was much more sign ificant than that of geographical factors.