Ranks and number of weed species in a defined crop

The weed rank and the number of different weed species of a defined crop are correlated. The number of the most frequent weeds within a given crop on a continent, in a cropping area or even around the world is usually relatively small. Echinochloa species, primarily E. crus-galli, and Chenopodium species, primarily C. album, are the dominating weed species in maize all over Europe. Echinochloa species are the most frequent weed species in rice globally. Man has modified a number of crops such as cereals through directed breeding so that they can grow in very different environments. Monocot weeds seem to be more problematic in monocot crops. Rice is a crop in which monocot weeds are much more important than dicots. The number of dicot weed species in cereals can, however, be rather high as, for instance, in European cereals.