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

height (Bell and Ritchie, 1989), cutting dates (adapted to crop or weed growth dynamics) and
crop species mixtures (see below).
The high temporal resolution of the measurements permitted to follow the weed population
dynamics on a much finer scale compared to the Chizé surveys and other previous studies,
which often conduct only one or two evaluations of the weed density per year. Comparisons of
this experimental study to the large-scale weed surveys on commercial fields showed that the
weed community composition reacted in similar ways (see chapter D.I.1 of the general
discussion for details). This increased the general value of this experimental study.
Four repetitions of each crop treatment on a rather small experimental site with rather
homogeneous chemical soil conditions (except a gradients in the organic carbon and total
nitrogen contents, Fig. 11) gave a sound basis for the comparisons. The enrichment of each
plot with a defined quantity of weed seeds belonging to 17 annual weed species increased the
homogeneity of the repetitions and reduced the number of quadrats where no weeds could be
observed, increasing the statistical power. In future studies, it would be interesting to add also
propagules of perennial weed species to the soil, species that had rather low natural densities
in this experimental site. The plot size was rather small compared to some other studies, thus
requiring specific soil tillage and forage cutting equipment adapted to the experimental plots,
and the restriction to weed species that are not wind-dispersed. Two other facts moved this
experiment away from the current agronomic reality:
• Only monospecific Medicago- or Dactylis- PFCs were included in this study, while both
organic and conventional farmers often use mixtures of two or more grass and legume
species. In our experiment Medicago and Dactylis crops had different temporal growth
dynamics (including the speed of crop establishment, seasons of highest biomass
production, regrowth speed after cuttings and plant longevity). The Dactylis stands showed
a rather slow establishment, were very competitive in the middle of the experiment,
presented very quick regrowth after the cuttings, but also first signs of senescence at the
end. In contrast, Medicago stands were strong during the whole 2.5-years period, but had a
slower post-cutting regrowth (compared to the Dactylis grasses). Other experiments
indicated that resource use efficiencies and competitive abilities of crop species mixtures
may be higher compared to monospecific crop stands due to ‘sampling effects’,
‘complementarity effects’ and ‘facilitation’, which may decrease the invasibility for weeds
107