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