Relative frequency of species, diversity and dominance indexes of the weed community in three wheat crops grown in field of 40 ha where wheat, corn or potato were grown int he previous season, were analyzed. The study was performed in Mar del Plata, Province of Buenos Aires (Argentina). The ocurring species were registred in each of forth 1 m2 samples distributed in a regular arrangement at each field. This was done previous to application of herbicides. Small differences between communities in floristic composition and richness were observed. But, important changes in the relative frequency of some species were observed. Stellaria media was the most frequent specie in wheat -wheat and corn-wheat sequences, and was only exceeded by Solanum tuberosum in potato-wheat sequence. The relative frequency of some species such as Chenopodium album, Zea maíz, Solanum tuberosum, Polygonum aviculare, Ammi viznaga, Veronica persica and Taraxacum officinallis was increased in wheat crop cultivated on corn or potato. On the other hand, the relative frequences of other species such as Apium leptophyllum, Polygonum convolvulus and Matricaria chamomilla decreased in the same sites, The specific changes in the relative frequences did not affect the structural indexes of weed communities, which showed a similar and high diversity index and low dominance.
Weed flora; crop sequences; relative frequency of species; species diversity