Diversifying crop rotations with temporary grasslands - Université de ...
Diversifying crop rotations with temporary grasslands - Université de ...
Diversifying crop rotations with temporary grasslands - Université de ...
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ANNEXE 2: WHAT IS A WEED?<br />
The term ‘weed’ may be used for plants <strong>with</strong> a variety of attributes including: ‘growing in an<br />
un<strong>de</strong>sirable location’, ‘useless, unwanted’, ‘competitive and aggressive’, ‘persistence and<br />
resistance to control’, ‘appearing <strong>with</strong>out being sown or cultivated’, and ‘unsightly’. In this<br />
thesis, ‘weed’ <strong>de</strong>signates any plant, that has not <strong>de</strong>liberately been sown or planted but grows<br />
spontaneously at a place and time where it is not wanted (by farmers). (The French ‘mauvaise<br />
herbe’ and the German ‘Unkraut’ and ‘Ungras’ are directly reflecting the aspect of<br />
un<strong>de</strong>sirability). Due to the <strong>de</strong>finition of weeds as unwanted plants at a given place and time, it<br />
is not possible to classify any plant species per se as a weed. Several characteristics of weeds<br />
are briefly discussed in the following.<br />
Harmfulness vs. utility:<br />
Weed plants may be consi<strong>de</strong>red as<br />
un<strong>de</strong>sired<br />
including competitive yield losses and harvest contamination (see Ch. A.II.1)<br />
or toxicity to<br />
201<br />
because of obvious negative effects on <strong>crop</strong>s<br />
man and animals. On the other hand, species commonly <strong>de</strong>signated as weeds may also have<br />
positive effects on the agro-ecosystem: They may e.g. provi<strong>de</strong> soil cover, which may reduce<br />
soil erosion and nutriment leaching at places and times where the <strong>crop</strong> is absent. They may<br />
also provi<strong>de</strong> food resources and habitat for animals (see Ch. A.II.3). Several plant species<br />
frequently consi<strong>de</strong>red as ‘weeds’ may also be used as feed for animals or for producing<br />
human food or pharmaceutics (e.g. Amaranthus retroflexus, Chenopodium album, Sonchus<br />
oleraceus, Spergula arvensis, Solanum nigrum). Weeds might also be exploited for their<br />
genetic resources: by crossing <strong>crop</strong>s <strong>with</strong> related weed species, plant bree<strong>de</strong>rs may introduce<br />
some <strong>de</strong>sired traits of the weed species (e.g. resistances to pathogens, tolerances to climatic<br />
conditions). The same species may thus be consi<strong>de</strong>red ‘harmful’ and ‘un<strong>de</strong>sired’ in some<br />
circumstances and ‘useful’ in others.<br />
Cultivated vs. wild species:<br />
In agro-systems,<br />
all plant species other than the actual <strong>crop</strong>(s) may be called weeds, thus<br />
including even ‘volunteers’ of other <strong>crop</strong> species or varieties sown in the past or on adjacent<br />
fields. Some weed species are very similar to <strong>crop</strong> species (weeds mimicking the <strong>crop</strong>), or<br />
even i<strong>de</strong>ntical in the case of <strong>crop</strong> volunteers. Crop and weed species of the same families may<br />
sometimes cross and form fertile hybrids, that are well adapted to the <strong>crop</strong> management and<br />
difficult to control. This may be particularly problematic for crossings between weeds and