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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332 H Meiss et al.<br />
communities (Booth & Swanton, 2002). Growing the<br />
same or similar <strong>crop</strong>s in consecutive years would thus<br />
favour the same type of weed species in every year. This<br />
may reduce weed species diversity, while abundances of<br />
a few, well adapted weed species may increase and<br />
become problematic. In contrast, <strong>rotations</strong> of dissimilar<br />
<strong>crop</strong>s should favour different weed species types in<br />
subsequent years and lower the risk of <strong>de</strong>nse populations<br />
of problematic weeds, as suggested by several,<br />
mostly experimental, studies (Liebman & Dyck, 1993;<br />
Sosnoskie et al., 2006; Smith & Gross, 2007). However,<br />
such positive effects may be limited if the <strong>rotations</strong> are<br />
too short or if the inclu<strong>de</strong>d <strong>crop</strong>s are too similar in terms<br />
of weed growth conditions.<br />
Besi<strong>de</strong>s the introduction of other annual <strong>crop</strong>s or<br />
cover <strong>crop</strong>s, <strong>rotations</strong> may also be diversified by<br />
perennial <strong>crop</strong>s lasting several years on the fields<br />
including various legumes such as lucerne (Medicago<br />
sativa), clovers (Trifolium sp.), and vetch (Vicia sp.);<br />
various grasses (Dactylis glomerata L., Lolium sp.,<br />
Festuca sp.) or legume–grass mixtures (Freyer, 2003).<br />
Such <strong>crop</strong>s are usually grown to produce livestock<br />
forage in mixed farming systems and for improving soil<br />
fertility and <strong>crop</strong> yields of the following annual <strong>crop</strong>s<br />
(Freyer, 2003). They are known as Ô<strong>temporary</strong> <strong>grasslands</strong>Õ,<br />
ÔleysÕ, Ôsod <strong>crop</strong>sÕ, Ôfod<strong>de</strong>r <strong>crop</strong>sÕ, Ôhay <strong>crop</strong>sÕ or<br />
even Ôcleaning <strong>crop</strong>sÕ (Liebman & Dyck, 1993; An<strong>de</strong>rsson<br />
& Milberg, 1996; Teasdale et al., 2004). While the<br />
need and profitability of livestock forage has <strong>de</strong>creased<br />
in some regions, perennial legume or grass <strong>crop</strong>s might<br />
increasingly be used for producing energy or raw<br />
materials for industry (Tilman et al., 2006).<br />
Several studies suggest that perennial forage <strong>crop</strong>s<br />
have strong impacts on weeds (e.g., An<strong>de</strong>rsson &<br />
Milberg, 1996; Clay & Aguilar, 1998; Ominski et al.,<br />
1999; Schoofs & Entz, 2000; Bellin<strong>de</strong>r et al., 2004;<br />
Teasdale et al., 2004; Albrecht, 2005; Heggenstaller &<br />
Liebman, 2006; Hiltbrunner et al., 2008). Most of these<br />
studies report reduced seed or plant abundances of some<br />
major weed species at the end of the perennial <strong>crop</strong>,<br />
<strong>de</strong>spite reduced or no herbici<strong>de</strong>s used in perennial <strong>crop</strong>s<br />
(Bellin<strong>de</strong>r et al., 2004), while other species sometimes<br />
profited. Several characteristics of perennial <strong>crop</strong>s may<br />
contribute to these weed community shifts: (i) the<br />
absence of soil tillage for long periods may prevent<br />
weed seed germination (Huarte & Arnold, 2003),<br />
although it may favour established weeds, especially<br />
perennials, (ii) frequent mowing operations (1–5 per<br />
year) may reduce weed growth, survival and seed<br />
production (Meiss et al., 2008), (iii) however, direct<br />
curative weed control actions are often reduced or<br />
omitted (Bellin<strong>de</strong>r et al., 2004) and (iv) <strong>de</strong>ep and <strong>de</strong>nse<br />
rooting systems and intense canopy closure during the<br />
whole vegetative period may create high levels of<br />
interspecific competition (Schoofs & Entz, 2000). The<br />
impacts and relative importance of these mechanisms<br />
are largely unknown.<br />
Nine of the 10 available studies (cited above) investigating<br />
the impacts of perennial forage <strong>crop</strong>s on weeds<br />
are based on field experiments and are therefore limited<br />
in space and time. Typically, 2–4 year <strong>rotations</strong> were<br />
analysed <strong>with</strong> only 1–2 years of perennial <strong>crop</strong>s. Only<br />
one study was based on a larger number of commercial<br />
fields from a whole region in Canada (Ominski et al.,<br />
1999).<br />
The aim of the present study was to analyse how the<br />
insertion of perennial <strong>crop</strong>s into cereal-based <strong>rotations</strong><br />
affects the weed composition in a realistic situation. We<br />
test the hypothesis that weed communities follow a<br />
temporal trajectory during the <strong>crop</strong> <strong>rotations</strong>, owing to<br />
the insertion of perennial corps. We therefore compared<br />
the weed species composition, diversity and frequency of<br />
functional groups between four key phases of such a<br />
long <strong>crop</strong> rotation: (a) annual <strong>crop</strong>s following annual<br />
<strong>crop</strong>s, (b) young perennial <strong>crop</strong>s (year 1), (c) ol<strong>de</strong>r<br />
perennial <strong>crop</strong>s (year 2–6) and (d) annual <strong>crop</strong>s following<br />
perennial <strong>crop</strong>s using a space-for-time-substitution<br />
<strong>de</strong>sign. One strength of our <strong>de</strong>sign is the use of weed<br />
surveys of a large number of commercial fields and a<br />
data set that allowed reconstruction of the rotation<br />
history of sampled fields over the last 10 years.<br />
Materials and methods<br />
Study area and sampling <strong>de</strong>sign<br />
The study comprised 420 fields randomly distributed in<br />
an area of 450 km 2 (containing about 18 000 fields),<br />
located in the Plaine <strong>de</strong> Niort, a region of intensive<br />
agriculture dominated by cereals <strong>with</strong> rather fertile and<br />
calcareous lime and clay soils in central-western France<br />
(46°11¢N, 0°28¢W). Mean annual precipitation is<br />
779 mm; mean temperature is 12.3°C (5.6°C in winter,<br />
18.9°C in summer). Since the start of the study in 1995,<br />
land use (<strong>crop</strong> species) has been recor<strong>de</strong>d annually and<br />
mapped in a Geographical Information System. These<br />
data were used to compile the history of the <strong>crop</strong><br />
sequence of each field.<br />
Four groups of fields were chosen to represent four<br />
key stages of a <strong>crop</strong> rotation including perennial <strong>crop</strong>s,<br />
namely: (a) winter wheat following at least 5 years of<br />
any annual <strong>crop</strong>s (representing annual <strong>crop</strong>s before the<br />
perennial phase), (b) 1-year-old lucerne following several<br />
years of annual <strong>crop</strong>s (representing young perennial<br />
<strong>crop</strong>s), (c) 2–6 year-old lucerne (representing established<br />
perennial <strong>crop</strong>s) and (d) winter wheat following pluriannual<br />
lucerne (representing annual <strong>crop</strong>s after the<br />
perennial phase). The four groups (treatments) thus vary<br />
Ó 2010 INRA<br />
Journal Compilation Ó 2010 European Weed Research Society Weed Research 50, 331–340