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

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