Weed flora shift as affected by cropping systems

Weed composition is dynamic and changes with place, time and management practices. There are numerous factors such as climate, soil, nature of cropping system and management practices that influence the infestation of particular weed flora. The change in the composition or relative frequencies of weeds in a weed population (all individuals of a single species in a defined area) or community (all plant populations in a defined area) in response to natural or human-made environmental changes in an agricultural system is known as weed shift. The reduction in the abundance of a particular weed species due to management practice will lead to increased infestation of other type of weed flora. Agronomic practices recognized as integral part of agro-ecosystem and acts as “filter” that either allow or constrain the growth and infestation of specific weed flora present in weed species pool in a niche. It has been found that weed densities are lower in crop rotational systems than in monocultures. Rotation of crops acts as a key to improve crop yields, soil quality and weed management. Monoculture leads to less diverse and difficult to control weed flora. Phalaris minor and Avena sativa were found to be major weeds I rice-wheat system but their occurrence could be reduced by 30-50 per cent where both or either of the crop were replaced. Echinochloa sp. and Cyperus sp. in rice-rice and Cyanotis axillaris, Euphorbia hirta and Ipomea aquatica in rice-mung+sesame system. Weedy species can easily adapt to changes in production practices in order to take advantage of the available niches. Weed community shifts either increase, decrease, or extinction of weed species occur as there are many change in ecological and agronomic factors along with crops that influence weeds. Variation in weed species in crop rotation could be either due to the direct result of crop rotation or agronomic and weed management practices associated with cultivation of crop.