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

338 H Meiss et al.
Relative frequency (mean ± SE)
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
b ab a ab
A
c
g
A
b
A A
B a a ab
a
a
A
a)
Wheat
after
annuals
g
A
a
B
b)
Lucerne
1 year
b
d
C
c)
Lucerne
2–6
years
Group of fields
d)
Wheat
after
lucerne
Impacts on grasses may particularly depend on the exact
crop management history (sowing and cutting dates) of
each individual field, which was not available for this
large scale study.
Results from Indicator Species Analysis (Table 3)
widely agreed with the analysis of functional groups
(Fig. 3). These two methods are complementary: while
ISA allows testing the reaction of individual species,
analysis of FGs allows inclusion of information on all
taxa, including the large number of rare taxa (80 out of
161 taxa). Nevertheless, groups containing only a few
species, such as FG 2, showed clearer differences between
the treatments (Fig. 3). More narrowly defined FG or
trait-based analysis might thus be useful to further
investigate the differences between the weed species.
Weed community changes in perennial lucerne are
most likely due to the absence of soil tillage, reduced
chemical weed control, temporally extended competition
and frequent hay cuttings. However, perennial crops
may also have other, more indirect, impacts on weeds.
The reduced belowground disturbances and the permanent
vegetation may modify the soil characteristics
(organic matter, humidity, and nutrient availability)
and microclimatic conditions (temperature, light quantity
and quality) (Huarte & Arnold, 2003). Perennial
crops may also favour the accumulation of plant litter
on the soil surface, creating a weed suppressing mulch.
Finally, perennial crops may favour weed seed predation
by animals, since weed seeds stay longer on the soil
surface (no soil tillage) and the permanent vegetation
cover may constitute a favourable foraging habitat for
seed predators (Heggenstaller et al., 2006). Perennial
crops may thus correspond to several filters (sensu Booth
A
a
g
a
b
B
B
N° FG (Nb. sp.)
8. Perennial grasses (11)
7. Annual grasses (9)
6. Perennial dicots (42)
5. Intermediate dicots (16) Fig. 3 Relative frequencies of eight
4. Annual dicots, rosette (29) functional groups (see Methods) of weed
species in four groups of fields (a–d)
3. Annual dicots, other (18)
representing key stages of the rotation
(Table 1). The graph shows mean relative
frequencies of each FG and standard errors
inside of each boundary. Dicots,
2. Annual dicots, climbing (3)
broad-leaved species; intermediate,
biennial or facultative perennial species;
upright, erect morphology; climbing,
1. Annual dicots, upright (32)
winding on neighbouring plants; rosette,
circular arrangement of the first leaves near
to the soil; other, all other morphologies;
Nb.sp., numbers of weed species in the FG.
Mean frequencies not labelled by the same
letter are significantly different between
the treatments.
& Swanton, 2002) with varying effects on different
weeds. In contrast, weed communities in most annual
crops are probably selected by a few rather strong and
uniform filters such as herbicides and annual soil tillage.
This is consistent with the increased dissimilarity
(b-diversity) among the lucerne fields (b and c) and
among the wheat following lucerne (d) (Fig. 2).
Using data on expressed weed communities and the
crop rotation histories of a large number of commercial
fields made our study as realistic as possible. This
advantage over local field experiments comes at the cost
of having various uncontrolled factors linked to the crop
management, environmental variables and local weed
species pools, increasing the noise in the data. Nevertheless,
we detected significant differences in weed
species composition associated with the inclusion of
perennial lucerne in the crop rotations. Reduced frequencies
of several weed species that are typical (and
problematic) in annual crops suggest a possible use of
perennial lucerne for preventive weed management.
Increased occurrences of broad-leaved species (perennials
and annuals with rosettes) in and after lucerne are
not likely to be problematic in annual crops. Most of
them already had strongly reduced abundances in wheat
following lucerne (except T. officinale and V. persica)
and were very rare in, or absent from, the established
vegetation in wheat following annual crops (Table 3).
Potential agronomical problems might rather result
from some grasses, even though grasses constituted only
small parts of the weed communities in all treatments
(Fig. 3). Some grass species (B. sterilis, A. myosuroides)
might be favoured both in winter cereals and in mown
perennial forage crops. Therefore, rotations should also
Ó 2010 INRA
Journal Compilation Ó 2010 European Weed Research Society Weed Research 50, 331–340