Diversifying crop rotations with temporary grasslands - Université de ...

ANNEXE 2: WHAT IS A WEED?
The term ‘weed’ may be used for plants with a variety of attributes including: ‘growing in an
undesirable location’, ‘useless, unwanted’, ‘competitive and aggressive’, ‘persistence and
resistance to control’, ‘appearing without being sown or cultivated’, and ‘unsightly’. In this
thesis, ‘weed’ designates any plant, that has not deliberately been sown or planted but grows
spontaneously at a place and time where it is not wanted (by farmers). (The French ‘mauvaise
herbe’ and the German ‘Unkraut’ and ‘Ungras’ are directly reflecting the aspect of
undesirability). Due to the definition of weeds as unwanted plants at a given place and time, it
is not possible to classify any plant species per se as a weed. Several characteristics of weeds
are briefly discussed in the following.
Harmfulness vs. utility:
Weed plants may be considered as
undesired
including competitive yield losses and harvest contamination (see Ch. A.II.1)
or toxicity to
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because of obvious negative effects on crops
man and animals. On the other hand, species commonly designated as weeds may also have
positive effects on the agro-ecosystem: They may e.g. provide soil cover, which may reduce
soil erosion and nutriment leaching at places and times where the crop is absent. They may
also provide food resources and habitat for animals (see Ch. A.II.3). Several plant species
frequently considered as ‘weeds’ may also be used as feed for animals or for producing
human food or pharmaceutics (e.g. Amaranthus retroflexus, Chenopodium album, Sonchus
oleraceus, Spergula arvensis, Solanum nigrum). Weeds might also be exploited for their
genetic resources: by crossing crops with related weed species, plant breeders may introduce
some desired traits of the weed species (e.g. resistances to pathogens, tolerances to climatic
conditions). The same species may thus be considered ‘harmful’ and ‘undesired’ in some
circumstances and ‘useful’ in others.
Cultivated vs. wild species:
In agro-systems,
all plant species other than the actual crop(s) may be called weeds, thus
including even ‘volunteers’ of other crop species or varieties sown in the past or on adjacent
fields. Some weed species are very similar to crop species (weeds mimicking the crop), or
even identical in the case of crop volunteers. Crop and weed species of the same families may
sometimes cross and form fertile hybrids, that are well adapted to the crop management and
difficult to control. This may be particularly problematic for crossings between weeds and